GLASS article in 12th Edition 1922 Supplement Volumes to 11th Edition [Page 284]
[Proofing: document is mostly in excellent condition. Some formulas need subscripts corrected. Page numbers are in square brackets thus [287]. British spelling (mould) generally retained. 2006-02-09]

GLASS (see 12.86).— During 1910-20, and more especially during the period of the World War, very considerable developments in the glass industry occurred, both in the glass produced and in the varieties of glass manufactured. In the following article attention is, of necessity, given to the British glass industry. In certain respects the art of glass-making has for long been at a very high level in Great Britain. The products of leading manufacturers in the London, Stourbridge and Manchester districts, so far as table-ware, ornamental glassware and colored glasses for windows are concerned, have, for many years, been of as fine a quality as any obtainable elsewhere. Indeed some of the ornamental glassware made in England has long been unrivalled. There is no need to amplify what has been said in the earlier article on these matters. When the war drew attention to the British position in respect of glass generally, it was in the direction of scientific glassware and special glass for certain industries, that the deficiencies were realized. Glass for scientific purposes may be taken to include optical glass and all glassware used in laboratories.

Laboratory Glass.— To deal with laboratory glassware in the first place. Before the war it may be said that nearly all the glass and glass apparatus used in laboratories throughout the United Kingdom was obtained from abroad. The main kinds of glass required for laboratory purposes may be grouped thus: Soft glass for tubing, and for a number of articles and vessels where the highest resistance to chemical action is not required, glass highly resistant to chemical action; very hard glass for combustion tubing; glass for thermometers.

Soft Glass.— Such a glass must be soft enough to be readily worked in a flame, and must stand prolonged heating without showing the changes in appearance and working qualities generally described as devitrification. Before chemical glassware of foreign origin became practically universal in laboratories, vessels and apparatus in great variety were made from lead glass. Many examples have survived long and continued usage. Their appearance at the present time shows how good this glass was in respect of its general resistance to chemical change, and their survival probably is to be ascribed largely to the property of such glass, when well made, of withstanding changes of temperature. Lead glass for chemical use has certain disadvantages. It may contaminate solutions with lead, and some varieties of it are specially prone to show surface darkening when exposed to solutions of alkaline sulphides. Again, in working lead glass in the flame, the care needed to avoid reduction of the lead with consequent blackening made the introduction of a workable glass free from lead a very welcome change to those who had not mastered the art of working lead glass. Experience has shown that the many advantages claimed for non-lead glass as a material for laboratory apparatus have been proved, and there is no likelihood of a return, nor adequate reason for a return, to lead glass. Common custom has, however, directed the attention of laboratory workers so markedly away from lead glass that it may be worth mentioning here that this glass can be made of such high resistance to the action of water, and of many solutions which also abstract alkali from glass, that in some special cases vessels made from it are only surpassed by silica in resistance to chemical change. As one instance, certain colloidal preparations can be kept far longer in vessels made from a suitable lead glass than is found to be possible with any of the chemical resistance glasses of the non-lead type. The durability of well-made lead glass is a matter of common experience in table glassware, many examples of which have been in constant or occasional use for years, exposed to variable atmospheres and all the processes incidental to cleaning, without showing any noticeable disintegration or discoloration of their surfaces. It is not intended here to advocate the use of lead glass for general scientific other than optical purposes, but only to suggest that it has certain properties which are useful, and which might advantageously be more fully considered than has been the custom in dealing with glasses for laboratory use.

In the early days of the war it was recognized that there would be a serious shortage of laboratory vessels. A simple sodium-calcium-silicate glass was known to be unsuitable since the readiness with which it devitrifies in a flame makes it impossible to produce from it any articles which have to be lamp-blown, and tubing made from it is practically useless to workers in laboratories. The immediate advance made was the addition of alumina, either as such, or preferably in the form of feldspar. The use of alumina for retarding devitrification and for rendering a glass workable in the flame was known in Great Britain, and at least three British manufacturers had for some years produced glasses containing various percentages of alumina up to about 10%.

It is unnecessary to go in detail through the stages of development of the so-called soft soda glass, but one or two points may be mentioned. Quite early in the production of this kind of glass it was recognized that a sodium-potassium-calcium-aluminum-silicate type of glass had most satisfactory general properties, that arsenic was not permissible, and that the only constituent other than those indicated which might be added was a small amount of manganese dioxide, to disguise the green color due to the presence of iron in the material used. The formula given here shows the approximate composition of a batch mixture expressed in percentages of silica and oxides of the metals in the various ingredients of the batch:— SiO₂ 68; Al₂O3, 4; CaO, 7; K₂O, 6.5; Na₂O, 14.5. Manufacturers vary [285] the proportions somewhat, either to suit their furnaces or through preference for some particular set of proportions, but the formula given is an example of one yielding a good glass, soft enough for flame-working while possessing good durability.

Glass of this type was made in the early part of the war, and would have been continued, as one meeting many scientific and industrial requirements, had it not been for the necessity of conserving supplies of potassium compounds, of which the amounts that could be apportioned for use in glass manufacture were sufficient only for the production of certain optical glasses. Without potassium compounds these optical glasses could not be made having the constants required by the optical industry. Manufacturers of other scientific glass ware had, therefore, to search for methods of producing soft work able glasses without employing quantities of manufactured potassium compounds. To some extent nitre was available and was used. Potash feldspar, which for long had been an ingredient in certain glasses, was a convenient form of aluminum compound for introducing alumina. The amounts of this material employed varied between wide limits, and glasses of good working qualities were obtained. Good examples of potash feldspar contained about 10% of K₂O. If in the above formula all the alumina be introduced in the form of such feldspar, about 2 % of K₂O is also introduced into the glass. Glasses having many good qualities were made with enough feldspar to yield from 3 to 4 % of K₂O in the resulting glass, but the amount of alumina introduced rendered the glass too stiff, and liable also to give a roughened surface if long worked in the blowpipe flame. Such roughening could be removed by heating to a higher temperature, but its occurrence was a decided objection, and, moreover, flame-workers were placed at some disadvantage in respect of the time occupied in the production of blown vessels and apparatus. To remedy these defects varying proportions of borax were employed, and in this way sodium-potassium-calcium-aluminum-boro-silicate glasses of good working qualities and of marked durability were produced, which met many of the requirements of laboratory workers. Some investigators and manufacturers of scientific glassware, however, looked upon these glasses as temporary expedients, and only awaited supplies of potassium compounds to return to the earlier type.

It is not found convenient by glass-makers to have to produce a very great variety of glasses. Unless a glass is generally suitable for the needs of laboratories and of industries where ready and kindly working in a flame, along with good durability, is required of it, the glass fails to fulfill the requirements it may reasonably be expected to meet. For example, the boro-silicate glass referred to possessed many desirable properties, and articles made from it in the flame, and also by blowing into moulds, left little to be desired when the glass was well made and the necessary technical skill had been acquired. It failed, however, when used for X-ray tubes. Good and workable bulbs and tubing could be made from it, but experience showed that X-ray tubes of this glass took longer to exhaust, and that there was a lack of stability in the vacua obtained. Investigation left little doubt that the glass parted with water vapor under electrical bombardment, and the results of numerous experiments proved that borax was an undesirable ingredient in glass intended for X-ray work. A glass of the general type indicated in the formula above is quite suitable for such work, and hence X-ray bulbs and tubing can be made from it in the course of working a pot for a variety of other articles. It may be mentioned here that, unless manganese in small quantity be present in the glass, an X-ray tube in use does not exhibit the green phosphorescence with which workers with X-rays appear to have become accustomed. As manganese dioxide is generally added as a so-called decolorizer, only one type of glass need be made for practically all the scientific purposes and many of the industrial purposes to which a comparatively soft glass is put. Experience so far appears to show that the best type is on the lines of the formula given, and that the presence of notable proportions of the oxides of aluminum and potassium are essential. It is unnecessary to go into details about the form in which each ingredient of the glass is introduced in the batch mixture. Potash feldspar has been mentioned as a convenient source of alumina, and part of the alkalis may be usefully added as nitrates. In general, all the materials of the batch mixture should be as pure as can be obtained commercially, so that the composition of the glass may depart as little as possible from that which it is intended to have, and which has been proved to give satisfactory results.

Before proceeding to other types of scientific glassware which were called for during the war, one or two remarks which are relevant for almost all glasses may be made here.

As far as it is possible to obtain and to store them, all the sub stances for a batch mixture should be free from water. In several instances it has been shown that a glass made from anhydrous materials differs from one calculated to give the same composition finally, but produced from a wet batch, or from one containing an ingredient having a notable proportion of combined water. In addition to some lack of general stability, the glass from a wet batch may show, and in many instances has shown, a greater tendency to devitrification when heated in a flame or by radiation. The amount of water left in a glass may be very small, but it has been shown to be sufficient to affect the behavior of the glass. The only reservation to the statement that to produce the best glasses the materials should be dry is that the action of water to effect change in glasses either during their production or on subsequently heating them is, if not imperative, at least an advantage in respect of the production of certain colored glasses and apparently of some opal glasses.

The other remark is about homogeneity. Apart from optical glasses, which must have the same composition throughout, all glasses for laboratory use should be made in such a way as to secure the greatest possible homogeneity. It is a matter of experience that glass which has been kept heated for some time, even after it is apparently " fined " and ready for working, is more resistant to heat changes and is also more generally stable than the same glass less well founded. Attempts to secure the thorough incorporation of all the ingredients by making the glass at a very high temperature were not altogether satisfactory, since there was greater attack of the pot, and, in many cases, too much loss of some of the more volatile constituents. Some glasses require very high temperatures, and problems connected with them led to investigations on materials for pots and furnaces to improve their refractory nature and so to make the production of such glasses possible. The remark about long heating applies to these glasses as well, but the attempt to substitute heating through a relatively short period of time at a very high temperature, for long-founding of glasses which only needed a moderately high temperature, led to uncertainty of composition and failed to secure the homogeneity aimed at. It is perhaps unwise to dogmatize on this matter, having in mind certain exceptions, but as a general rule it may be said that in the present state of our knowledge the long-founding so much insisted upon by many experienced glass manufacturers cannot be dispensed with if the nicest possible refinements of a good glass are to be realized.

Stirring to secure homogeneity is a necessary operation in making optical grass. It is not customary to stir glass for laboratory use, but this is not to say that such glass would not be improved by being stirred if it were economically possible to do so. Although it is out side the range of scientific glasses, the opportunity may be taken here of drawing attention to an instance in which perfect homogeneity in glass does not appear to all eyes as an advantage. The instance is that of colored glasses used for decorative purposes, such as windows. Some of the charm of old glass seems to be associated with a marked lack of identity of composition, and, therefore, of regularity of optical properties throughout the glass. From a glass-maker s point of view it was an imperfect manufacture, but those who find depth and life in the less perfect production may ask, "Would it be imperfect manufacture to take advantage of the possibilities in a glass-melting to secure a more perfect fitness and suitability for the purposes for which such a glass is designed?" Certainly the control which modern manufacturers have over glass, and the knowledge and experience which they possess, would make it possible to secure a great variety of pleasing results.

The subject of annealing has, in recent years, been given much attention, and several investigations have been carried out. Results of much interest and importance have been obtained, dealing with the conditions for removing strain in glass and with the problem of annealing, both from the theoretical and a practical point of view. Consideration of these results serve to emphasize the importance of thoroughly annealing any glass articles which are required to with stand marked changes of temperature, and of arranging that any vessels, etc., which in the course of production are re-heated locally, shall be re-annealed. Tubing is not customarily annealed as part of the process of manufacture, but for certain purposes, notably with tubes which are to be ground, it is an advantage to anneal them.

"Resistance" Glass.— Laboratory glassware, to deserve this description, must possess great stability, and must part with only minute traces of any of its ingredients when it is exposed to the action of the majority of solutions and liquids used in a chemical laboratory. In the early days of the war the production of such glassware was undertaken by British manufacturers. The chief varieties made can be included in two types: one containing compounds of zinc and the other free from this metal. In neither type is the inclusion of arsenic or antimony considered to be permissible.

The following formulae, illustrative of these two types, give approximate proportions for batch mixtures expressed in percentages of the oxides contained in the various ingredients of a batch:—

(A)	Si02	.... 66	(B)	SiO2.	66
	B2O3	.. .. 8		B2O3	9
	Al2O3	.... 9		Al2O3	2.5
	CaO	.... 5		ZnO	8
	MgO	1		MgO	5
	Na2O	... . 8		Na20	9.5
	K2O	.... 3

It is to be understood that adjustments of the proportions given can be made to suit different furnaces and also to fit in with the amount of broken-up glass from previous meltings (cullet), which is incorporated in the batch. The addition of cullet is customary on [286] economical grounds, and also because it is a matter of experience that, with a number of glasses, the desirable properties are more easily realized when notable proportions of cullet are used. Of these two types of resistance glass (A) requires a rather higher temperature in the making and on the whole presents more difficulties than (B). It has also a somewhat higher coefficient of expansion, and on that ground is less liable to withstand sudden changes of temperature. Previous remarks on the influence of long-founding for securing homogeneity and stability apply in a marked manner to such a glass as (A), and this type has been made of very high chemical resistance and of satisfactory behavior when quickly heated or cooled through a greater range of temperature than it would usually be exposed to in a laboratory. In comparison with glass (B) it is generally more reliable for working in a flame. Several examples of the type (B) tend to show reduction of zinc in a blow pipe flame, but glasses of type (B) can be, and have been, made by British manufacturers, which exhibit none of this reduction even in a very hot flame. The general resistance of glass (B) to chemical action is good, but with hot strong solutions of caustic alkalis it does part with some zinc, and, to a very small extent, this is true of its behavior with strong acids. Good examples of glass (A) are more resistant in the sense that they are less soluble in such reagents, but the slight action which does occur causes a roughening of the surface of the glass which is noticeable, while in the case of glass (B) the surface is left polished even though the solvent action on it may have been much greater.

Balancing the evidence of the advantages and disadvantages of the two types in their general applications to laboratory work, it is probably fair to give preference, on the whole, to glass of type (B). It is inherently more capable of withstanding sudden changes of temperature, and because it is the easier glass to make, there is less likelihood with vessels made from it of mishaps due to imperfect manufacture of the glass. Whichever class of glass is chosen, all vessels and apparatus made from it need thorough annealing.

Combustion Tubing.— Tubing of very hard glass is essential for many laboratory experiments, and since it is largely used in the analysis of carbon compounds by combustion it has come to be known specially in this connection, but in tubing of various diameters it is required for a number of other purposes. Most of these preclude the use of any compounds of arsenic or antimony in the composition of batch mixtures for making the glass. Before the introduction of a new type of tubing from Jena, combustions and other operations at high temperature were carried out in a potassium-calcium-silicate glass, the best known form of which was Kavalier's combustion tubing. The general composition of this glass is indicated by the following percentages to the nearest whole numbers:— SiO₂, 78; CaO, 8; K₂O, 12; Na₂0, 2. Glass of this kind served many useful purposes in laboratories, but it was difficult to use in a blowpipe flame, considerable skill being needed to work it quickly enough to avoid devitrification to an extent sufficient to roughen the surface and bring about a pasty condition which prevented the glass from flowing under heat. The Jena glass which took its place possessed greater plasticity over a longer range of temperature, and was stiff enough to allow of tubing being usable at a temperature at which the older kind tended some what suddenly to collapse. During the war very hard glass tubing was much needed, and as the result of experiments on a laboratory scale and in glass works, tubing of a type similar to the Jena combustion tubing was produced fully equal to any obtained before the war. With regard to hardness and suitability for working in the flame it fulfils its purpose most satisfactorily. It differs advantageously in one respect from the pre-war glass, in that it does not show anything like the same tendency to become opal when heated for a long time. The following is the composition for a batch mixture, given as for other glasses in the percentage of oxides:— SiO₂, 68.5; B₂O₃, 5.5; Al₂O₂, 6; CaO, 8; BaO, 6.8; Na₂O, 3.2; K₂O, 2. Remarks made about formulae for batch mixtures of glasses previously mentioned apply to this formula in respect of adjustments for addition of cullet and for some modifications to suit different furnaces. With this glass, however, there is not much latitude allowable if the full hardness of the glass is to be realized and difficulties in manufacture are to be avoided. The glass is one requiring a high temperature for its successful production, and is another example of the need for such glasses calling for investigation of refractories in order to make their production possible.

Thermometer Glass.— The manufacture of thermometers of all kinds has been carried on in Great Britain for many years, and British capillary tubing of high quality and technical perfection has long been available for their production. The tubing has been made both from lead glass and from various other types of glass, and has been in constant demand. An ideal glass for thermometers, in addition to being a good durable and workable glass, must be of such a nature that bulbs blown from it are constant, in that after being heated they rapidly return to their original volume. Thermometers made from such a glass would not show any change in their zero points after use. Jena thermometer tubing has gained a high reputation for close approximation to this ideal, and large quantities of it have been used by British thermometer makers. Mention should be made of the fact that at least one British glass manufacturer produced tubing also near to this ideal some years before the war. During the war very great numbers of thermometers were called for, the greater proportion being for medical purposes, but many also for scientific and industrial use. The production of these drew attention to the subject of glass for thermometers generally. Guided by their own knowledge and experiments, and assisted, in some instances, by other investigations, manufacturers of glass produced tubing to meet the demand, not only in lead glass, for the production of which they were ready and pre eminent, but also in other varieties of glass having properties closely similar to two Jena glasses of high reputation. One of these can be used for thermometers, capable of standing high temperatures up to about 500° C., and the other is for more general application. The following formulae, given as for other glasses in percentages of oxides and with similar reservations, indicate the nature of batch mixtures for these types of glass:—

Vessels and Apparatus.— If we turn from the character of the glasses themselves to the vessels and apparatus made from them, scientific glassware may be broadly classified as furnace-made and as lamp-blown. The former is for the most part produced by blowing into moulds molten glass gathered from the furnace on a blowing-iron. When the variety in shapes and sizes of flasks, beakers and other apparatus used in laboratories is considered, it will be realized how great a development had taken place in this direction after the war in a British industry in which, for several years, practically none of this type of apparatus had been made. So also in the lamp-blown apparatus had there been a remarkable extension in development. Before the war, lamp workers for laboratory apparatus were few in number in Great Britain and were chiefly engaged either in making a comparatively small amount of apparatus to special design or in repair work. During the war numbers of workers of both sexes were trained in lamp-blowing generally, and in 1921 those making scientific glassware were producing practically all the varieties of this kind of apparatus needed in laboratories, the best examples comparing favorably with any obtained in the past from abroad. Glass for such apparatus is supplied to the lamp blower in the form of tubing, in the production of which, there fore, there had also been a great development.

The production of scientific glassware arising out of the needs of the war was one of the most noteworthy extensions of glass manufacture in Great Britain. Since so little of this kind of glassware had been made there for such a long time, the manufacturers were not, in the majority of instances, equipped with the knowledge and experience necessary to start at once. The deserved reputation of most of the scientific glassware of foreign origin made it natural at first to attack the problem of its reproduction. It is only justice to foreign manufacturers to acknowledge indebtedness to them for a number of types to work to. At the same time it would be injustice to British manufacturers of glass to give the impression that, among the great [287] number of varieties which they had to make during the war for all kinds of purposes, there were not very many which were produced de novo, as the result of the work of a number of investigations outside, as well as inside, the industry, and of ready enterprise on the part of the manufacturers. For several of the glasses needed there were no available data to go upon, and the knowledge and experience required for guidance in the earlier stages of their production had to be gained by research.

The best examples of glasses for scientific purposes, of British manufacture, are now fully equal to any pre-war glass, and some are superior. The glasses already mentioned are the chief ones required for the production of laboratory apparatus, but they do not, by a long way, exhaust the list of glasses called for during the war for special scientific use or for industrial purposes. Examples are here briefly referred to.

For glass for miners' lamps, a glass withstanding rapid changes of temperature exceptionally well was necessary, since the lamp glasses are thick and the flame of the lamp may often touch them. There was an urgent demand for them early in the war. It was successfully met, and such glasses of British make are now produced in large quantities. Another glass on similar lines, but differing somewhat in composition, was prepared for the production of chimneys for incandescent and high-pressure gas illumination, paraffin lamps, etc. In addition to withstanding heat changes well such a glass must be markedly resistant to the chemical action of hot products of combustion. Both these glasses consist chiefly of alkaline boro-silicates having a high percentage of boric anhydride. They need a high temperature for their successful production in a homogeneous state. When well founded their low coefficients of expansion render articles made from them highly resistant to sudden variations in temperature over a long range.

Glass rods for half-watt electric lamps were required, to hold the thicker tungsten wires which support the filament of this metal. They had to be made specially, since no existing glass of British make capable of withstanding heat changes was also sufficiently reliable in respect of not cracking round the sealed-in wires. This glass in most cases involved also the production of special rods to join with it and with the stem of the lamp.

Other glasses were needed which, while making safe joints with ordinary laboratory tubing, etc., would hold platinum, copper, iron or nickel wires. Such glasses are often described as sealing-in enamels. Several of these have been made, and, generally speaking, they arc of the type either of a soft glass containing a high percentage of lead, or of one free from lead and containing a notable proportion of a fluoride, such as cryolite. The coefficient of expansion of the glass, in relation to that of the metal wire used, has to be taken into account, but it is not the only factor, as may be just indicated here by the mention of a sealing-in enamel which is successful with platinum and copper but cracks with iron, nickel or tungsten.

Other glasses, and glassware from them, which had to be made during the war will be mentioned very briefly. They were of great importance, but, generally speaking, they were familiar to British manufacturers, and their manufacture did not need the extensive preliminary investigations and trials which the production of most of the foregoing glasses involved.

Bulbs for Mating Ordinary Electric Lamps.— Before the war some what less than a quarter of our requirements of these was made by British manufacture. A very great extension of this part of the industry during the war was urgent. In 1918 bulbs were being made at the rate of about 1,000,000 per week.

Jars for Preserving Fruit and Meat.— Though numbers of these had long been made in Great Britain, about 80% of the total number used had been obtained from abroad. Great increase in the production of these vessels was required to meet the needs, enhanced as they were by the war. Bottles for a great variety of purposes had always been made by British manufacturers, but not in the great quantities which were required when sources of supplies from abroad were cut off or were inadequate. The extension of this part of the glass industry was very great even on the older lines of manufacture, but the necessity for more economic production led to a review of methods and to the adoption of new machinery.

Glassware for Medical Purposes.— Some of this has for many years been made in Great Britain, but not in sufficient quantities to supply the demand, and much of it was obtained from abroad. The war caused a great increase in the demand, and very large quantities of vials, tubes, syringes, graduated measures, etc., had to be made. Most of these could be produced from glass, and by methods familiar to manufacturers, but some requirements had to be met by investigation and experiment before suitable glass was produced. In connection with medical glassware, artificial human eyes may be mentioned. For their production there are required opal glasses to suit variations in the tint of the sclerotic; bright clear glass for the lens; black glass for the pupil, and a great variety of colored glasses for the iris; a clear glass containing fine embedded

threads of opal used for imitating the irregularly radiated appearance of the iris, and a red glass for the veins of the eye. Artificial eyes had for many years been made in Great Britain, but many were imported. Most British makers of them are used to working in lead glasses, and many of their products will bear comparison with the best of foreign origin, which, as a rule, are made from glasses free from lead. Experiments for the production of such glasses as the latter furnished the data for their manufacture.

There was considerable increase during the war in the production of colored glasses, e.g. for spectacles to protect the eyes of the great numbers of men working at steel furnaces. Colored glasses in considerable variety were also wanted for other purposes, but in comparatively small amounts. Some of them needed investigation and a number of experiments before the conditions for their production could be determined.

From what has been stated already it may be gathered that a great advance had to be made in glass manufacture through needs arising out of the war, and sufficient has, perhaps, been said to indicate that the knowledge and experience gained in meeting them had placed the British glass industry in this respect in 1921 in a very different position from that of 1911.

Optical Glass.— Of none of the glasses already mentioned can it be said there was more imperative need for their production than for the variety of glasses required to make the numerous optical instruments used during the war. Early in it there was no doubt that the supplies of optical glass existing in England would soon be exhausted. For about three quarters of a century, Messrs. Chance Brothers of Birmingham had produced optical glass. They were enabled greatly to extend their facilities for production, in order to meet the demands which rapidly arose and were very urgent. The change which was brought about in the production of optical glass in England will be gathered from the following comparative figures. For a year or two before the war, out of the total amount of optical glass used in Great Britain, approximately 60% was imported from Germany, 30% from France, and 10% was of English manufacture. In 1916 81% was English and 19% was obtained from France, while the total quantity supplied by Messrs. Chance Brothers was about 18 times as much as they sent out in 1913 and over three times as much as the total quantity of optical glass from all sources used in Great Britain in that year. About the middle of 1917 the Derby Crown Glass Co., which was formed in the autumn of 1916 for the manufacture of optical glass, was sup plying it. Figures for the first quarter of 1918 show that 96% of the optical glass used in Great Britain was made at home, while France supplied only 4% of the British requirements. In that first quarter the output of optical glass made in England was about nine times as much as the total quantity of English manufacture produced in the whole of 1913, and it was being made at the rate of an annual production of more than six times the total quantity of optical glass from all sources used by British optical instrument manufacturers in 1913. This great increase in production was due entirely to the war, since during it very little optical glass was used for purposes other than the manufacture of instruments for the fighting services. The compulsory extension of manufacture called mostly for development in quantity production rather than increase in the number of types manufactured. A few types not hitherto made in England have been produced; but Messrs. Chance Brothers for some years have manufactured a number of glasses having properties similar to several types of Jena optical glass. Both this firm and the Derby Crown Glass Co. have been called upon for glasses having pre-determined optical constants, and the meeting of these demands has involved a considerable amount of investigation and experiment. No completely new type of optical glass has been manufactured; but in some instances the requirements of the optician have necessitated a departure so marked as to constitute an extreme variety very like one.

It is not easy to suggest a strict definition of " type " as applied to optical glass. The two types of earlier days were " flint " and " crown " the former containing lead oxide and the latter calcium oxide along with alkalis and silica. The names are convenient, as their connotations are understood and they have become conventional; but a glass free from lead may be [288] used as the flint, and one containing lead may be employed as the crown, in some optical combinations of lenses. For practically all the optical glasses other than the old flints and crowns the optical industry is indebted in the first instance to the re searches and manufacture carried out at Jena. Many substances not used before in the production of glass enter into their composition, and it would seem preferable to restrict the expression types to such glasses as have markedly different chemical compositions. It is not necessary to elaborate this point here. It is mentioned only to indicate a distinction between the comparatively few distinct types of optical glasses which have been made and the large number of varieties of them which are needed to provide for the many differences in optical constants asked for by opticians.

Investigation and experience have enabled the English manufacturers of optical glass to go far in satisfying the demands of the manufacturers of optical instruments of all kinds; but there is still room for much experimental research for glasses and transparent media nearer to the ideals present in the minds of designers of optical systems.

With regard to homogeneity, freedom from color and durability, optical glasses made in England have reached a high level. During the war, in spite of the fact that production had to be so largely increased, the good qualities of the glass generally were not only maintained, but in many instances of glasses somewhat difficult to make a high quality was reached, at least equal to the very best which was available before the war. With the experience gained in recent years improvements of manufacture are possible which point to a greater percentage of yield of high-quality glass than has hitherto been obtained from any melting. The use of more efficient mechanical means for handling pots of glass, the production of pots more highly resistant to the chemical action of molten glass, increase of the durability of some of the less stable glasses, ready production of large homogeneous masses of glass, and the production of new glasses, are the lines along which future progress may be expected, and are the developments indicated during the great activity in the British optical industry owing to the war.

No attempt can be made here to discuss the compositions of the various glasses which have been produced, or to deal in any detail with the range of optical constants now available. Before leaving the subject, however, the relevancy of the problems connected with optical glass to the production of some other glasses may be mentioned. It is a matter of experience that the numerous researches required for the production of various types of optical glass have a considerable value, not only in arriving at the immediate end in view, but also because the knowledge obtained of the properties of glasses of very varied chemical composition is of the highest importance in pointing the way for designing many glasses for other scientific purposes, and also for certain industrial use. This applies not only to the actual glass, but also to several considerations in respect of furnaces and refractories.

Acknowledgment must be made here of the work organized in England during the war by the Department of Scientific and Industrial Research; of work and investigations carried out by the National Physical Laboratory; of the practical investigations by a committee of the Institute of Chemistry; of the work and investigations undertaken by the glass technology department of the university of Sheffield, and of the guidance and stimulus given by the department of the Ministry of Munitions, which, concerned at first with optical glass only, soon became responsible for supplies of glass and glassware of all descriptions.

Progress in the Use of Machinery.— It is probable that between 1910-21 the greatest advance in the economic production of certain types of glassware was in the direction of the introduction of machinery and minor labor-saving devices in substitution of the older hand methods employed in production. Naturally this substitution was only rendered practicable by concurrent improvements in the means for assuring a continuous supply of molten glass in a suitable condition to permit of the machines being run continuously. It will, therefore, be understood that whereas the common practice in the past has been to found the

glass in pots in direct-fired furnaces, there has been a gradual tendency for tanks, some of them being of very large capacity, holding as much as 300 tons of molten glass, to take the place of the older pot furnaces. It is scarcely possible that the pot furnace will entirely disappear from practice, inasmuch as those glasses which are only required in comparatively small quantities or of absolute purity, as in the case of optical glass, certain colored glasses, and those liable to contamination from furnace gases, will still have to be pot-founded.

The development of machinery in glass manufacture has been by gradual evolution. In general the earlier efforts were directed towards imitating by mechanical means the sequence of the operations performed by the skilled glass worker;

and we find, therefore, that skilled labor was not suddenly displaced. The earlier machines were partly automatic or semi-automatic, and required a gatherer, a human link between the furnace and the machine, and also a boy to take and trans port the finished article from the machine to the annealing lehr. The human links have now been dispensed with in many American factories; neither the raw materials nor the glass is handled at any stage during the progress of manufacture. Conveyors transfer the raw material from the trucks to the storage bin; automatic weighers discharge the requisite quantity of material from the storage bin to a rotary mixer mounted on a trolley; another conveyor transfers the mixed batch to the batch storage bin in close proximity to the charging end of the tank, for the ready release of the batch down a chute at periodic intervals into the tank.

Although machinery has entered so largely into glassware production, there are still some few operations where man has not been displaced. This is more particularly in evidence in the production of many types of chemical glassware produced by the glass-blower with the aid of a blowpipe, the beautiful specimens of cut table-ware, the handiwork of the craftsman skilled in the use of the grinding wheel and polishing pads, and other ornamental ware.

The types of machines may be conveniently divided into the following groups:— pressing machines for the production of tumblers, meat and jelly jars, bull's-eye lenses, tableware and pavement lights; press and blow machines for all types of bottles, and many kinds of food containers; blow machines for electric lamp bulbs, lamp chimneys and similar articles; rolling machines for plate glass, figured and ribbed glass and reinforced sheet for sheet and window glass, and for drawing tubes and rods.

In addition to the glass-forming machines there are many other types for miscellaneous purposes, including cracking-off machines for severing the fashioned article from the waste glass, employing multiple fine jets of flame which impinge on the line of severance. This line is usually started by a short diamond cut .at the predetermined point. Calibrating machines for accurately dividing measuring devices such as thermometers, burettes, pipettes and cylinders; grinding and polishing machines for preparing and finishing the surface of plate glass; machines for forming the stoppers of bottles and for grinding the seating in the neck of the bottle; flowing devices and feeding machines, to take the place of the gatherer and his operation of withdrawing from the pot or tank, by means of a gathering-iron, a sufficient quantity of molten glass to make the article required.

It will be readily appreciated that in common with certain other industries the development of glass manufacture has made remarkable strides on the engineering side. The advance is the more marked, inasmuch as the progress made in other essentials has not been commensurate with mechanical progress.

Press Machines.— No very marked advance has been made in recent years in this type of machine so far as principle of operation is concerned, but there has been constant improvement in detail. It will be appreciated that a very limited number of types of article can be made with a solid mould. Only such as have both an internal and external taper, the diameter being reduced in the direction of the movement of the plunger, are suitable. In all other cases where the ware has external shoulders or ornament, the mould must be hinged. In the semi-automatic and fully automatic machines the movements of the plunger and mould are operated by compressed [289] air. It is in the control valves and mechanism for operating the moulds that improvements have been effected. In the earlier press machines the plunger was directly connected with the piston, resulting in an equal pressure throughout the stroke. The later types in which the power is transmitted through toggles are much more efficient, since a slower motion and increased pressure are obtained as the plunger nears the end of its stroke. This arrangement con forms to what has been found to be the best practice in using the hand-lever press. If too much glass be fed into the mould, less pressure is required to form the article than if there is a comparative shortage of glass when a heavy pull is required on the lever. The toggle machine, therefore, is adapted to compensate for variations in the quantity of glass fed into the mould.

In another type of press machine a series of moulds is arranged on a rotating plate, and by means of air pressure the mould containing the molten glass, when it reaches a point immediately beneath the plunger, is forced upwards to meet the fixed plunger. This type of machine is employed in making deep pressed ware such as tumblers and is usually adapted only to solid moulds. Press machines naturally vary very much in size according to the ware to be produced. Probably the largest of its kind is one operated in America for the production of glass burial caskets, which measure up to 6 ft. 3 in. in length. The machine weighs about 6 tons, and is capable of developing a pressure of about 700 tons.

Press and Blow Machines.— This type of machine is used in greater numbers and of more varied forms than any other class of glass machinery. It is essentially the bottle machine. It may be interesting to note that the first attempt to produce a machine for making bottles was the work of an Englishman, although it is to American skill and enterprise that the successful development of this complex machine is due. A press and blow machine is designed to perform two distinct operations. In the first the neck of the bottle is formed by pressure in what is known as the parison mould, and in the second the body is blown and finished in the blow mould. In general the machine consists of two circular plates, either disposed one above the other and capable of a step by step rotation about a central pillar, or arranged side by side, the second being driven from the first by an intermediate pinion or gearing also by a step by step motion. At regular intervals near the periphery of one plate (the upper one in the first case) are situated the parison moulds for forming the neck, and on the other plate similarly situated are the blow moulds for forming the body. All the necessary movements, including the rotation of the plate, the actuation of the plunger, the automatic opening and closing of the moulds, and the transfer of the blank from the parison mould to the blow mould are performed by air pressure. In certain types of machine the last-mentioned operation is not performed automatically, but by hand.

The sequence of operations in making a bottle on such a machine is as follows. A supply of molten glass is fed into the blank mould, either by one of the various types of automatic feed or by a gatherer using the usual form of punty rod. A blowing head then makes con tact with the mould and blows the glass down on to the plunger, thus forming the neck of the bottle. The plunger is withdrawn, and the table advanced to the next position. A puff of air is blown into the orifice, to blow the glass to the full size of the blank mould. Again the table advances, and at the third position the mould opens and the glass blank is transferred to the blow mould situated on the second table. This mould closes round the glass, and the blowing head takes up the correct position to supply the necessary air pressure to blow the bottle to the required shape. The mould again advances and opens at the next position when the finished bottle is removed and transferred to the annealing lehr. Considerable difficulty was experienced in producing satisfactorily small narrow-necked bottles, but within the last few years the difficulties have been overcome and satisfactory results have been obtained on fully automatic and semi-automatic machines. Naturally the speed of production depends on the type of machine and the size of the ware being produced, but the capacity of some of the later types of machine may be gathered from the fact that eight or nine bottles of one quart capacity can be produced in one minute, and smaller even faster.

Blow Machines.— The use of this type of machine is not nearly so common as that of the press and blow machine. They are principally used in the production of electric lamp bulbs, lamp chimneys, tumblers and similar light hollow ware. The main difference and special feature of these machines is that the moulds are coated internally with carbonized material, and the article is rotated in the mould during the period of blowing. It is therefore obvious that ware produced in such a mould must have a smooth regular surface, and any kind of figuring or ornamentation is out of the question. These machines may be semi-automatic, in which case a gatherer is necessary to feed the required quantity of glass to the machine, or they may be fully automatic, in which case the machine sucks up the molten glass from the tank. The former type of machine comprises generally four vertical frames mounted on a cast-iron base frame. A horizontal shaft carrying four circular discs runs in suitable bearings situated at the head of the vertical frames. Mounted loosely on the shaft and close to the discs are four arms, which, by means of suitable cams and tripping devices, can be rotated between a horizontal and vertical position. Each arm is provided with a small air pump at its extremity. On a level with the arm when in the [10] horizontal position is a small disc, which serves the purpose of a marverer. This disc, mounted in a bearing attached to the back of the frame receives its motion from the vertical disc. A suitable mould disposed at the base of each vertical frame is mounted on a horizontal spindle capable of movement through 90°. When in the horizontal position, the mould is submerged in a trough of water situated in the base casting. The moulds are hinged, and the opening and closing movements are effected by a rod at the back.

The sequence of operations in making an electric bulb is as follows:— A gatherer having withdrawn from the pot or tank a mass of glass on his blowpipe secures the latter in contact with a rubber washer, forming part of the air pump at the end of the machine arm when the arm is in a horizontal position. The mass of glass on the end of the blowpipe, which is slowly rotating, comes into contact with the marverer, which is also rotating in the same direction. After a short period a cam comes into operation, causing a slight puff of air to be given to the glass through the blowing iron. The arm now assumes a vertical position with the glass at the bottom, and at the same time the mould takes up a vertical position, and the operator by pulling a lever causes the mould to close over the glass. Whilst the glass still attached to the end of the blowpipe is kept rotating in the mould, a puff of air is admitted through the blowpipe and expands the glass into the desired bulb. The operator then opens the mould, withdraws the blowing iron from the machine, and places it in an adjacent stand, when another operator severs the bulb from the iron with shears; the iron is cleaned off and is again ready for use.

A brief description of the other type of machine, which is fully automatic, is as follows:— The machine rotates by a step by step movement about a vertical axis at the rate of about two revolutions per minute, and is provided with six double arms actuated by cams disposed on a vertical drum. The first operation is the projection of the cantilever head into the furnace, when glass sufficient for two bulbs is sucked up by vacuum into the blank moulds; the cantilever head then withdraws and the glass blanks are released and deposited in cups. The cantilever heads are capable of being rotated about a horizontal axis, and the cups are in their top position when receiving the glass. At this point a rod is forced up into the glass to form a hole for the blowing operation. The machine is rotated to the next position and the head moves into the horizontal plane when the glass blank receives the first puff of air; at the next rotation of the machine the head assumes a vertical position with the partly formed bulb hanging downwards. At this stage either a reciprocating or a swinging motion is imparted to the head in imitation of a hand worker's movements. The mould is then raised, and closes over the partly formed bulb, and a final puff of air is given. The next rotary movement of the machine opens and lowers away the mould, and the finished bulb is removed by hand. From the time that the glass is fed into the cups of the head until the bulb is blown the glass is kept rotating. Before the mould is again brought into operation it passes through a mixture of soap and water.

Sheet or Window Glass Machinery.— The earlier attempts to manufacture window glass by machinery better illustrate the tendency to imitate the methods which had proved by long practice to be best suited to production by hand. The objective of all the earlier machines was the production of as perfect a cylinder of glass as possible. Patents and improvements related rather to modification of detail than variation of first principles. The general method employed in this type of machine is to bring a ring or circular bait of metal into contact with the molten glass, to raise the bait by mechanical means, and at the same time supply air under a low but increasing pressure into the cylinder of glass so formed.

The following will give in brief the outline of a machine which is being successfully worked at the present time:— A pot or receptacle about 3^ ft. in diameter, and of a depth sufficient to hold the quantity of glass required in making a cylinder is charged by means of a ladle with molten glass taken from a tank furnace. A structure alongside the pot is so arranged as to permit of a bait being raised vertically by means of a motor to the full height of the cylinder to be drawn. The bait, which consists of a short hollow cylinder about 1 ft. in diameter, furnished with an internal lip at its lower end, is lowered into the molten glass contained in the pot, which has been left standing for a short time until the glass has attained the correct drawing temperature. As the bait is lowered the glass flows over the lip and solidifies, thus forming a starting point for the cylinder. An operator standing on a platform well above the pot level starts the motor, which raises the bait, and at the same time air under pressure is admitted through the top of the bait. The cylinder of glass quickly increases in diameter, and the pressure of air is arranged to give the desired dimension. In order to ensure a uniform thickness of wall, both the speed of drawing and the pressure and volume of air are increased to counteract the increased viscosity of the glass due to falling temperature. When the full cylinder, 40 ft. long and weighing about 1,000 lb., has been drawn, it is cracked off from the pot; the lower portion is swung out and the cylinder lowered into a horizontal position; the top portion or cap is cracked off and the remainder is divided into convenient lengths for handling; these are usually about 5 ft. long. The remaining processes of slitting and flattening are similar to those followed in hand-made cylinders.

In a later, and not yet so widely used, type of machine the sheet is drawn directly from the tank and requires no subsequent flattening [290] treatment. The tank is furnished with an extension at the refining end into which the glass flows and cools sufficiently to be drawn. When in a proper condition, an iron bait in the form of a narrow iron plate is lowered into the molten glass, which welds to it. By means of a hand-actuated device the bait is raised; the sheet of glass following it is drawn through a pair of narrow water-cooled rollers arranged at each side of the sheet, which assist in maintaining its width, and then over a hard and highly polished roller situated about 30 in. above the drawing pot. Here the glass assumes a horizontal position. In the neighborhood of the bending roller, additional heat is applied to the sheet to prevent any possibility of cracking ; the sheet of glass then passes over a flattening plate and enters the annealing lehr. At this point a caterpillar drive pulls the sheet along and furnishes the power for the automatic drawing of the sheet. It will be seen, therefore, that the process is continuous so long as a supply of glass is available. By this process sheet-glass can be produced which may be of any predetermined thickness within wide limits, the governing factors being the speed of drawing and the temperature of the glass in the draw pot. The width of the sheet is about 6 ft., and the speed of drawing for the thin variety is about 2 ft. 6 in. per minute. At first there was a tendency for the glass manufactured by this process to be somewhat cordy, probably due to the surface cooling in the drawing chamber, but this has been overcome and the product is now of very good quality.

In the Belgian method of drawing sheet-glass the space above the glass in the tank is divided into two parts by means of a brick curtain which depends from the roof to a short distance below the surface of the glass; by this means the flame and hot gases are restricted to the melting end. Subsidiary ports are provided in the refining end to regulate temperature. In the refining end of the furnace two further similar brick curtains are arranged parallel and comparatively close together; between them floats a debiteuse, a hollow vessel made of fireclay or similar refractory material, rectangular in plan and with rectangular ends, but having a section to within a short distance of each end somewhat like an inverted M with the apex of the central angle cut off, thus leaving a long narrow slit giving access from the outside to the inside of the receptacle. This device has a specific gravity slightly less than that of glass; it floats, therefore, in such a position that the narrow central slit is just below the surface of the glass. Above the refining or drawing end of the tank, an erection in the form of a square tower about 13 or 14 ft. high, made of sheet iron lined with refractory material, is provided. On opposite sides of this tower are sets of double resilient rollers disposed vertically, and so arranged that when the sheet of glass is being drawn the edges of the sheet will pass between, and be gripped by, the rollers. The rollers on one side are driven by suitable gearing from an electric motor.

In drawing a sheet of glass a bait consisting of a narrow flat woven iron sheet of a length equal to the length of the slit in the debiteuse is lowered within the lips forming the slit. When the glass has welded to the bait the latter is raised, lifting with it a sheet of glass. By means of a water-circulating system the glass is chilled sufficiently to retain its form and then passes up between the rollers. When once gripped by the rollers the upward draw is continuous so long as the motive power is applied to the rollers. The bait is removed when the sheet reaches the top of the tower. The tower is provided with a series of inclined iron diaphragms, the upper part of each of which is flush with the rollers. These diaphragms serve the double purpose of preventing broken glass from falling into the tank, and of preventing the heat from the tank ascending the tower. By this means a rough annealing is performed, since the ascending sheet of glass is subject to a gradually falling temperature. When the sheet reaches the top of the tower it is cut to size and packed.

Plate Glass.— No special innovations have been introduced in recent years in the methods of manufacturing plate glass, with the exception of the means for annealing the plates. In the older method the plates are placed on the floor of a kiln when the latter is at a dull red heat; the opening is then built up and luted with fireclay. The heat is shut off, and the kiln allowed to cool gradually over a long period. Recently, however, a plant has been installed in the United States for annealing the plates in a continuous lehr, and it is claimed that the glass is equally well annealed as in the old process. The time saved is considerable, being five hours as against three days by the kiln method.

After the glass has been melted in a pot, the latter is taken bodily from the furnace, and the glass poured on to the rolling table, about 28 feet x 16 feet. This consists of a large cast-iron bed, usually made up in segments, carefully bolted together so as to give an even smooth surface and cooled by a water circulating system. A large roller extending the full width of the table, and weighing from 5 to 6 tons, is mechanically driven forward and spreads the glass out into a sheet. Guides are provided at each side of the table upon which the roller bears; the height of the guides governs the thickness of the sheet formed. The plate having been rolled is moved forward into the first section of the lehr, which is maintained at a temperature of about 600°C., and then progresses by an intermittent motion through the other sections of the lehr. The floor of the first sections of the lehr is made up of fireclay slabs, and, in the cooler sections, the glass moves forward on wooden slats or battens, the total length of the lehr being about 400 feet. As a fresh plate is rolled about every ten minutes, this fixes the period during which a plate remains in any one section of the lehr. After leaving the lehr the plates are carried by a traveling crane to the grinding and polishing shop.

Tube Drawing Machine.— There are two types of machine, the semi-automatic and the fully automatic. In the semi-automatic machine the mass of glass on the blowing iron is prepared as in the case of drawing by hand. The drawing machine is installed in a tower about 170 ft. high, in the basement of which is a motor-driven winding drum. A steel wire rope connected to the drum runs straight to a fixed pulley at the top of the tower and down again to the blow pipe carriage. The carriage is therefore raised or lowered when the drum is operated. The carriage is provided with means for securing the blowpipe, and also with four rollers which permit it to move freely between vertical guides. The glass having been prepared on the blowing iron, a punty is secured to a socket between the vertical guides; the glass, still on the blowing iron, is lowered on to the upper face of the punty and adheres to it; the blowing iron is then locked in its carriage and the motor started. The speed of the draw governs the size of the tube, which may be regulated by means of a variable speed on the motor. The tube having been drawn, it is parted from the punty, and by means of a band brake gradually lowered and cut up into lengths. Practically any type of tubing can be drawn on this machine, inasmuch as the finished product depends upon the form imposed upon the glass by marvering and blowing prior to being put into the machine.

In the case of the fully automatic machine only tubing having a circular section can be drawn. Glass is ladled from the melting furnace into a specially constructed pot, heated by a system of burners and provided with a baffle extending from the top of the pot down into the glass, and also with an orifice from which the glass flows regularly into a rectangular clay trough. From a small opening in the trough, the size of which can be controlled, the glass flows, in the form of a ribbon, on to a revolving cone. The cone is hollow and made of fireclay, and varies in size according to the tube to be drawn. Longitudinally through the centre of the cone is a steel tube with a nichrome steel cap. This tube is for supplying air to the interior of the glass tube being drawn, and also serves as a means for rotating the cone. The speed of revolution can be governed by the motor. The axis of the cone is inclined so that the apex is depressed. The ribbon of glass from the pot flowing on to the larger diameter of the cone tends to flow by gravity towards the apex, and soon after starting the whole cone is covered with molten glass; the flow continues beyond the end of the cone and maintains its form of a hollow cylinder owing to the air under pressure which is admitted to the central tube. At this stage the glass tube is much larger in diameter than the finished tube, but by the time it has reached a series of pulleys in line the diameter has been reduced to the desired size, and it has cooled sufficiently to retain its form. It continues to pass over the series of pulleys until at about 150 ft. from the pot the tube passes between, and is gripped by, two endless chain belts faced with asbestos sheet pads. As soon as the tube is gripped by the belts a steady pull is maintained. The speed at which the belts travel, combined with the temperature of the glass at the cone, determines the size of the tube. After passing the belts the tube is cut into lengths automatically; they fall into a tray of a rotary conveyor, where they are automatically sorted into separate racks.

Automatic Flowing and Feeding Devices.— Various forms of feeds have been devised for delivering a pre-determined quantity of molten glass from a tank furnace to the glass-forming machine. In all of these it is essential that the supply of glass should be maintained at a constant level, and it follows, therefore, that these devices have been applied either to tank furnaces or subsidiary containers which are constantly replenished.

The most primitive form of feed consisted of a simple overflow from a spout or lip with a stream of glass cut at periodical intervals by means of a pair of blades actuated automatically and water cooled, and an improvement on this form of feed provides for the substitution of one of the blades by a series of cup-like devices fixed at the ends of radial arms rotating from a common centre, in such a way as to bring the cups in succession under the stream of glass. Each of these cups rotates about its own horizontal axis.

At the moment when a mould has been sufficiently charged the edge of a cup meets the moving blade, scissor fashion, in the line of the stream of glass. The glass now falls into the cup which is gradually rotating about its axis. During this part of the operation a new mould has taken up its correct position, the cup continues to rotate and pours the accumulated glass into the mould, into which the now unimpeded stream of glass also falls. The next following cup again intercepts the stream and so completes the cycle.

In another type of feed a spout is provided at the working end of the furnace. On the under-side of the spout there is a hole capable of being varied in size. A pair of shears automatically cuts off the glass as it flows from the hole, after which a timing device retards the flow of glass. This governs the quantity of glass delivered.

In another type of feed the glass is delivered by a reciprocating paddle working in a specially devised extension to the tank. The mass of glass which is forced over the lip of the receptacle by the paddle is severed by suitably actuated blades. The mass of glass falls on to an inclined chute, water lubricated, and is delivered into the glass-forming machine. [291]

Refractories and Pots.— With the introduction of more efficient pot furnaces and tanks in which higher temperatures were reached it soon became evident that the question of refractories would have to be investigated in order that the pots, tank blocks and furnace parts, would stand up to the increased strain which was being put upon them. During the war a large amount of experimental and practical work was undertaken with a view to improvement in this direction. As a result of some of these investigations a provisional Specification was drawn up for the help of users and makers of refractories, and it was hoped by this means to standardize the types of clay, percentage and size of grog, porosity, shrinkage and other factors necessary in production of the refractory articles used.

Evidence would appear to point to the fact that in so far as concerns the majority of types of optical glass, and, if it were an economical proposition, for other glasses also, pots of a porcelain nature or of a composition approximating in relative proportions of alumina and silica to kaolinite have given the most satisfactory results. In connection with the manufacture of optical glass in America for war purposes it was found as a result of considerable experiment that a pot of the porcelain type was the best suited to their purpose. In other directions considerable experimental work has been devoted to the production of pots by the ordinary casting and by the vacuum casting processes, and also of pots from osmose clay. In each case results of great promise have been obtained.

Furnaces.— There is no doubt that the exigencies of the war in relation to glass production caused British manufacturers to consider very seriously the equipment at their command in respect of its efficiency and quality of output. Although in some instances a reasonably efficient type of furnace had been installed, the general run of furnaces, although satisfying the type of work performed in Great Britain before the war, were unable to attain or maintain the necessary temperatures for producing, certain essential kinds of glass for the supply of which the British consumer had previously relied on foreign sources. In factories existing at the outbreak of war the more efficient furnaces were of the regenerative type, but in the recently erected pot furnaces the tendency has been to adopt the recuperative principle. In this type of furnace there are two sets of channels or passages, one for the air supply and the other to carry away the hot products of combustion; the temperature of the air is raised due to interchange of heat by conduction through the common party wall of the channels. As the flow of the secondary air and hot flue gases are constant in direction, there are, therefore, no reversing valves to be operated as is the case in the regenerative type, and it is claimed that the furnace can be maintained at a more even temperature in the former than in the latter.

The recuperative type of furnace is producer-gas-fired. The furnaces, according to the designer, differ in respect of the disposition of their elements; in one type both the producer and recuperators are situated immediately below the furnace, and both the air pas sages and hot flue gas passages are disposed horizontally, whereas in another type the recuperators are at the sides, and the air passages are vertical whilst the flue gas passages are horizontal. It is claimed for the latter type that glass from a broken pot can be more easily dealt with and is not likely to cause so much damage.

Oil Fired Furnaces.— Oil firing has not been installed to any extent in the glass industry in England and in very few, if any, cases has the furnace been designed specifically for oil fuel. But the coal strike in 1921 and consequent high price and irregular supplies of coal caused attention to be more particularly directed to oil as a fuel, and during the first half of 1921 some optical glass furnaces in England were fired with heavy oil.

From experience gained so far it would appear that better results were obtained with the heavier grades of oil, and that tank furnaces lend themselves more readily to this type of fuel. In the case of pot furnaces the objection is raised that the pots are liable to suffer on account of the irregular heating due to localized combustion.

The prospect of increased consumption of oil as fuel has led naturally to efforts being made to improve the existing types of oil burners in the direction of better efficiency, etc. Several oil burners are now on the market, in some of which atomization is effected by steam under pressure. In others mechanical means and air pressure are utilized. Although more complete atomization is obtained by the former means, yet it appears to be generally admitted that the burner utilizing air pressure with some mechanical means for assisting atomization gives more complete combustion and higher temperatures. The virtue in this method of firing is increased cleanliness and the absence of discoloration or deterioration of the glass, due to the effects of the flame coming into contact with it. Moreover, with oil fuel the temperatures can be more easily controlled.

Annealing.— Prior to the war a very wide gulf separated the methods in use for annealing optical glass from those practiced by the makers of other types of glassware. In the former an efficient system of well-lagged electrically heated towers ensured a satisfactory result. In the latter, however, a primitive, straight-through tunnel (usually coke heated) formed the lehr. It was open to the objection that it was exceedingly draughty and the glass was hurried through in all too short a time. .During the war, however, when new types of ware had to be produced in which the annealing needed to be above suspicion, close attention was devoted to the subject of improved lehrs. In many of the factories considerable care was exercised to ensure efficiency in this operation. The site was well chosen, the system of heating was considered in relation to the necessity of a variation in the maximum temperature according to the class of ware being annealed, and of a gradual fall in temperature after passing the hottest zone. Precautions were taken to prevent draughts sweeping through the lehr and so defeating the object of the operation. (H. JN.; S. W. M.)
Sir Herbert Jackson, K.B.E., F.R.S. Director of Research, British Scientific Instruments REsearch Ass., Emeritus Professor of Chemistry, Univ. of London.
S.W.Morrison, Board of Trade London