Old Glass Miscellany

Rev. 2002-10-24; 2003-01-08, 2004-05-22, -07-08, 2007-04-20, 2009-06-27

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Melting before 900 AD
It is my habit to look on the library shelf for books related to one I am looking up.  Recently, I brought home the 2002 volume 44 of Journal of Glass Studies of the Corning Museum. Therein I read a lot of forgettable detail and a couple of amazing, to me, items.  One article discussed the excavation of four furnaces for making glass found in close conjunction on Tyre an island off Lebanon.  They were built using some of the existing wall in the ruins of a Roman colonnade that fell in an earthquake in 551. Glass composition suggests 8th century usage.  From evidence, each furnace had been fired several times, each time the glass broken out, the bottom re-mortared, the (missing) arched roof replaced.  The astounding figures for me were that amount of glass per firing was at least 37 tons in the largest,  16, 16 & 13 tons in the smaller.  Based on processes still used in Africa and India (ethnographic evidence) the firing would have taken 30 days at a temperature of 900C (1652F).  At this time, glass was made from sand, etc., at separate locations from where it was melted and worked - glassmaking and glassworking were sharply different occupations.
In terms of processing, the factory below melts 3 tons of glass at a time, mostly cullet from broken light bulbs and previous production, taking 36 hours to do it, then the glass is used over 6 days, with 2 tons of firewood and 1 ton of coal being used each day.  Consider the pile of wood needed to maintain cooking temperature for 30 days for 15 tons. 2004-07-08
Mirrors - Curved with lead coating
From the same Journal, a discussion of mirror making in medieval times in Southern Scandinavia is tied to a technique still done at a single factory in India.  The reflective backing is lead which is poured molten into a large thin glass globe while it is still hot, the excess being poured out.  The glass is so thin annealing is not required.  The mirrors have a distinctive curve - that of the globe.  The individual mirrors are shaped with scissors and mounted in frames or clothing.  The old framed mirrors are 5-15 cm (2-6 inches) while the mirrors embroidered into fabric are much smaller.
Most modern glass shops provide some kind of viewing area with seats or bleachers, but NEGG cites a notice in the August 23, 1753 Boston News-Letter which states that so many people have been visiting the glasshouse that damage has been done and the work "very much retarded", so a shilling admission will be charged and visitors will be limited to 3 or 4 questions which may not be answered anyway. p.42
In 1814 or so, to run a glass operation took "four gatherers, four blowers, two flashers, two punty stickers, three teasers (who took care of the furnace), one man to take care of the flashing oven and two wood driers." NEGG p.81
Size of operation
The Chelmsford Glassworks, ca 1820, NEGG p85, had a building 124 feet long by 62 wide with 40 men, to make 330,000 feet of window glass per year. They had 2 furnaces, 3 flattening ovens, 2 tempering ovens, 6 ovens for drying wood, cutting, mixing and pot making rooms, a kiln for burning brick, a mill house and a sand house.
Bottle Making Team
Mold blown, gimmick to hold bottle, tool to shape neck, 3 guys/gals -
1 gathers glass, 2 blows into mold then place in gimmick held by 3, and snap neck, 1 brings hot gather for neck shape then goes to gather for next, 2 shapes neck, 3 to deliver piece to annealer.
Slow: a bottle every 2 minutes, 25 an hour with break
Fast: one every minute, 50 an hour.
200-400 per 8 hour day.
Source: reading various history books and being astounded at production. Of course, they worked 12 hour days and got a break every 2-3 hours, not on the hour.
Melting Glass
I have been bugged by the contradiction of a wood burning furnace with several pots for melting and blowing glass when modern recipes make it clear that cooking the glass must be done at a higher temperature and fairly quickly to not burn off the fluxes, yet I know that blowing glass at that higher temp makes for a thin mess.  I think the solution is the mention of "fritting" furnaces or ovens, which I take to be smaller hotter ovens in which the chemicals are turned into "rough glass" and then moved to the main pots. So I looked on the Internet.
FRITTING FURNACES N A furnace for the calcining and roasting of a mixture of sand and fluxes as a preparatory stage in glass-making.

fritting glazes
-To reduce melting temperature and improve melt predictability
Since frits have been pre-melted to form a glass, re-melting them requires less energy and lower temperatures. Frits soften over a range of temperatures (in contrast to crystalline raw materials that melt suddenly) and lend themselves very well to production situations where repeatability and ease-of-use are necessary.
Frit"ting (?), n. [See Frit to expose to heat.] The formation of frit or slag by heat with but incipient fusion.
Frit (?), n. [F. fritte, fr. frit fried, p. p. of frire to fry. See Far, v. t.]
1. (Glass Making) The material of which glass is made, after having been calcined or partly fused in a furnace, but before vitrification. It is a composition of silex and alkali, occasionally with other ingredients. Ure.

2. (Ceramics) The material for glaze of pottery. Frit brick, a lump of calcined glass materials, brought to a pasty condition in a reverberatory furnace, preliminary to the perfect vitrification in the melting pot.
Both “white” and “red” lead are used. The lead is added to other ingredients, which have been “ fritted “ or fused together and then ground very fine in water, making a thick creamy liquid into which the articles are dipped.

Ben Solwitz
Registered User Join Date: May 2008  Location: Chardon, OH  Posts: 220
The wood fired furnace was built at Corning Community College mostly as a proof of concept. Steve Gibbs said a bunch of times that the most common question at the hot glass shows is, "how did people do this two hundred years ago?" They started out at CCC melting some glass in a wood fired furnace that was built for firing ceramics, but decided that the gathering/heating port wasn't very well designed for forming glass, so they built another furnace. I believe Fred Herbst and Steve Gibbs were the principals for the project. They have melted SP87 and Spectrum nuggets in crucibles, and fired ceramics at the same time. It seems like it works fairly well, but takes some getting used to. Lewis Olson, Davide Salvadore, and Elio Quarisa did demonstrations on Thursday. They went fairly well for the most part, Lewis had worked out of the furnace before, and Davide's piece was pretty simple, Elio had some problems with heat, and soot preventing his bitwork from sticking. I was also told by Dane Jack that the temperature gradient isn't very sharp. I think Steve said they usually fire for about four days, and use two cords of wood over those four days.
Ben Solwitz
Registered User Join Date: May 2008  Location: Chardon, OH  Posts: 221
Re: Permission to quote
Sure go right ahead.
[Mike Firth] Can I have your permission to quote on my website your paragraph on the wood burning glass furnace? Will ask Franklin or other source for permission to use picture.

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