Making Tubes & Sheets of Glass

Rev. 03/03/02 2003-08-31, 2004-12-17, 2005-09-19,
 2007-03-17, -04-20, 2008-01-26, -04-27, 2009-03-24, -06-21

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Broad Glass
Crown Glass
Cylinder Glass
1920's Advances
Float (1950's)
Curved Sheet
Two skills that were very important in the history of glass have been downgraded to almost nothing as (usually much better, certainly cheaper) mechanical methods have replaced them - the making of glass tubing and flat sheets of glass.


Handmade tubing is made by gathering glass, forming a cylinder with a uniform bubble in it and pulling, like making stringer, but with the glass blower applying pressure to add volume to the air inside and the puller working to make the glass wall as uniform as possible. Mechanically, tubing is made by extrusion - molten glass is forced around a mandrel through an opening and air is added inside while the glass is pulled.  Tubing is used in a number of obvious and non-obvious places including thermometers, fluorescent lighting, neon lighting, and scientific glassware including test tubes.

Here is a link to a page with a description by a neon worker of David Wilson making tubing

"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 fonil 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 onto 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."



Sheet or Window Glass

Making sheets of glass (window glass, stained glass) has been a preoccupation of glassblowers down through the centuries.    Sheet glass is made mechanically these days by floating molten glass on molten tin under a nitrogen atmosphere (to avoid oxidation) until it is annealed.  Previously, sheet glass was extruded, sort of; a sheet was started by pulling a leader up from the surface and curving the soft sheet to horizontal before annealing, so there are waves in the glass where the curve was straightened out.

There have been four phases of making flat glass - pouring the glass out and rolling it (cast glass), blowing a cylinder and cutting the side while it is hot then flattening, blowing a rondel disk cut into panes, and blowing a cylinder then cooling to break off ends and length, then sagging it flat.

"Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. According to the website of the London Crown Glass Company [], broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glassmakers and others were making broad sheet glass earlier than this." Wikipedia Broad Sheet
MF Note: Broad glass made in a cylinder is cut hot and pushed down on the plate, cylinder glass (below) is cut cold and reheated in a kiln to sag it flat, producing better quality glass. 2009-03-24

Drawing of steps in blowing crown glassFor a long time, flat glass was only made by spinning a circle of glass, several feet in diameter at the height of this activity, which was then annealed and cut into panes. The center point, called a bullseye, was used as a decorative piece where looking through the glass was not vital; it has now become a design feature of imitation old windows. Panes cut from the circle were relatively small. If the piece is big enough it may be possible to see curved strain lines or bubble tracks. Stained glass in cathedrals was made this way and one characteristic of spun glass is that it is usually thicker near the center, so pieces are tapered. Window makers put the thicker part at the bottom, because it produces a better effect, and this led to the modern legend that the glass (being "fluid like") had flowed downward over the years. Rarely is it pointed out that if the flow had occurred there would be a gap at the top, which is not there. More Below



Crown Glass - Blowing a rondel

Making a disk of glass in the old fashioned way involves making a special shape before spinning and having an annealer big enough to hold the disk. The shape is a cylindrical walled vase with inward sloping upper walls - the thicker glass near the opening and thinning to provide glass for the rim. The spinning to a rondel is done on the punty. Spinning must be kept up until the glass is rigid.
In one citation, NEGG p81, reports on a factory that is required as a condition of their monopoly, to make tables (disks) of glass at least 38" in diameter and to make 4000 minimum a year.
"The quality of glass made by the cylinder method was not so good as that made by the crown-glass method, it was more economical to produce and in general superseded crown glass production in America by about 1840."

Cylinder Glass - Blowing a bottle for sheet

Drawing of steps in blowing cylinder glassAfter the disk method of making sheets came the step of blowing enormous thin bottle shapes (eight or ten feet long finally, a foot or more in diameter) on a special vertical stand, cutting off the ends of the bottle and splitting the cylinder then flattening (sagging) the sides in a kiln to form a sheet, usually with slight straight ripples in it. "Old" window glass from the 1800's and early 1900's was made this way and can still be seen in houses. More Below YouTube demo showing blowing and  flattening

Here the problem is blowing a big enough bottle with uniform flat sided walls, having a big enough annealer to hold the bottle(s), and having enough floor or shelf space in an annealer that can be heated to the sagging point of the glass. Not much care is taken with forming the bottom of the bottle, which is just scrap, all the effort goes in making as uniform sides as possible.
In the book NEGG, there is a discussion of making window glass ca. 1755 which discusses an "oven" holding 208 rolls which are reported as producing 360 "foot" of glass window pane in specific sizes. I take "foot" to mean square feet. Thus each roll is yielding an average of about 1.7 square feet. (At another point he estimates at least 1.5 foot per roll.) Since most of the glass is cut 10 by 8 or 9 by 7, I will assume the goal is to make the former, each being 0.56 sq.ft.. A bottle with walls 10" tall and 10.2" in diameter would give four panes having an area of 2.24 sq.ft. which allows for a 24% loss on original production in cutting and handling.
A drawing (NEGG p.46) reports the cylinder is 5 feet long and 1 foot in diameter, which would yield about 15 square feet. These large cylinders are often shown in drawings with a single cut down one side, but I doubt that. The bottle example above would lose nothing if the cylinder were cut directly to panes. And a cutting to at least a half cylinder would make flattening much more reliable and save space in the flattening oven. A cylinder shape would almost certainly tend to fold rather than lay out, I believe. [Web discussion has convinced me that full cylinders were flattened, the larger sheet of glass being worth the extra problems of manipulating the glass in the sagging kiln. 2003-08-31]
In pictures from the late 1800's, rigs are shown where a rising platform with the blower pulls the completed cylinder up while heat is added at the bottom to extend the lower walls. Photos exist of women carrying the long thin annealed cylinders on their shoulders for lengthwise cutting and sagging to flat.
"The union allowed workers to blow only 40 feet of single strength glass a week or 30 feet of double strength (it took 11 pains [sic] of single strength and 6 pains of double to equal an inch. ... Blowers here worked 7-8 hours then rested for 16 while the next batch was melted [until continuous furnaces were introduced.] Cylinders of glass were sometimes 70 inches long." GGNJ p. 101

Window Glass cylinder flatteningDescription: "In the arrangement of the flattening and annealing ovens numerous improvements have been effected, which have resulted in greatly increased smoothness and uniformity of the glass, and in considerable economy of time and labor in the operations. This shows a section of a flattening (L) and annealing kiln (M) in common use. The split cylinder O is introduced and gradually pushed forward so as to be uniformly heated till it reaches P, the flattening stone or table, mounted on a movable waggon N. On this waggon after it has been flattened it is carried into the annealing arch M, as shown by the dotted outline. Here in a less heat is gradually stiffens, till it is ready to be moved by a forked tool to a horizontal position on the bed of the annealing oven." [and then raised to vertical R] — Encyclopedia Britannica, 1893
Source: The Encyclopedia Britannica, New Warner Edition (New York: The Werner Company, 1893)
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1920's Sheet and Plate Window Glass

The following was taken from the 1922 added volumes to the 1911 Encyclopaedia Britannica.  Reproduced here because it shows the state of the art between hand blown glass and the modern float method (below) More 1911 EB Glass More EB22 Glass

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 I 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 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 (S 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.

Pilkington's float The information below is from the book Inventing in the 20th Century and shows the actual illustration page from the patent for Pilkington's float glass along with a good description

Float glass
Using a bath of molten tin to make plate glass cheaply
Lionel Alistair Pilkington, Rainhill and Kenneth Bickerstaff, St Helens for
Pilkington Brothers, Liverpool, all in Lancashire, England
Filed 10 December 1953 and published as GB 769692 and US 2911759

Traditionally there were two ways of making plate glass. 'Window glass' involved forming a sheet by stretching molten glass either by blowing or by pulling. Distortion tended to occur, but the product was cheap to make. 'Plate glass' was made in various ways but all involved casting a plate of glass, grinding it flat, and then polishing it to make it transparent. This was expensive and there was a loss of glass
when it was polished.

Float glass involves a ribbon of glass moving on rollers from a furnace at 1,000°C and floating along the surface of a bath of molten tin. The surface of the tin is completely flat and so the bottom glass surface as well as the top surface is also flat. The ribbon is then cooled and passed through an annealing oven. The result is glass of uniform thickness and with a bright, fire-polished surface. The process is cheap and effective, particularly as it is a continuous surface, and variations in the width of the glass can easily be carried out.

Pilkington Brothers was (and is) the major British glass manufacturer. Alistair Pilkington was a member of the family which controlled the private company. Born in 1920, a mechanical engineer, he thought of the basic concept. The first 'pilot' production plant started in May 1957. Poor-quality glass was produced for 14 months. Then suddenly good-quality glass began, and continued, to appear. The process was publicized. Now it was decided to replace the worn-out equipment and to the amazement of everyone poor-quality glass again came out from the plant. It took three months of investigation to realize that the good glass had only occurred because of a broken part. Once the set-up was reproduced, with the broken part, good-quality glass again began to be produced.

Another piece of good fortune was that the glass as originally produced was the right thickness for half of the market. Glass was produced for sale while research was carried out into how to make thinner or thicker glass (this is done by controlling the speed with which the glass ribbon is drawn off from the furnace). The process took £7 million to develop, a huge sum for the time. Pilkington Brothers were able to license it to many foreign countries during the l960s. Today 90% of all window glass is made in this way.
float process original patent drawings

Flat Window Glass - Modern

Sir Alastair Pilkington  formerly

"When he started work on his process, the target was to make, more economically, the high-quality glass essential for shop windows, cars, mirrors and other applications where distortion free glass was necessary.  At that time this quality of glass could only be made by the costly and wasteful plate process, of which Pilkington Brothers had also been the innovator. Because there was glass-to-roller contact, surfaces were marked. They had to be ground and polished to produce the parallel surfaces which bring optical perfection in the finished product.

Sheet glass - glass made by drawing it vertically in a ribbon from a furnace - was cheaper than polished plate glass because it was not ground or polished, but it was unacceptable for high-quality applications because the production method imparted some distortion. It was suitable for domestic and horticultural glazing, but could not replace polished plate. Many people in the glass industry had dreamed of combining the best features of both processes. They wanted to make glass with the brilliant surfaces of sheet glass and the flat and parallel surfaces of polished plate. Float glass proved to be the answer.

In the process, a continuous ribbon of glass moves out of the melting furnace and floats along the surface of a bath of molten tin. The ribbon is held at a high enough temperature over a long enough time for the irregularities to melt and for the surfaces to become flat and parallel: because the surface of the molten tin is flat, the glass also becomes flat. The ribbon is then cooled down while still on the molten tin, until the surfaces are hard enough for it to be taken out of the bath without rollers marking the bottom surface: so a glass of uniform thickness and with bright, fire-polished surfaces is produced without the need for grinding and polishing. "


Curved Window Glass

How are large curved corner glass windows in big shops made?
The type you may see on the corner of a department store. Huge and perfect. How is it done?
Answer: It is done by sagging the glass in a large kiln over a stainless steel sheet (clay in the old days) that is coated with a material called kiln wash which keeps the glass from sticking and being marred. Window glass starts to sag of its own weight if supported at the edges at about 1050F and will settle fairly quickly by 1250. At these temps the surface is fairly tough and will not usually be marred by the smooth mold surface. Once to shape, it is cooled fairly quickly to about 1000F and then cooled slowly to room temp. 2008-04-27

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