Old Glass Miscellany
Rev. 2002-10-24; 2003-01-08, 2004-05-22, -07-08, 2007-04-20
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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.
- Viewers
- 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
- Employment
- 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.
http://www.rchme.gov.uk/thesaurus/mon_types/F/69089.htm
FRITTING FURNACES N A furnace for the calcining and roasting of a mixture of
sand and fluxes as a preparatory stage in glass-making.
http://digitalfire.com/material/m-01p1x1.htm
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.
http://machaut.uchicago.edu/cgi-bin/WEBSTER.sh?WORD=Fritting
Frit"ting (?), n. [See Frit to expose to heat.] The formation of frit or
slag by heat with but incipient fusion.
http://machaut.uchicago.edu/cgi-bin/WEBSTER.page.sh?PAGE=597
Frit
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.
http://13.1911encyclopedia.org/L/LE/LEAF.htm
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.
Contact Mike Firth