Rev. 2002-12-15, 2004-01-06, 2005-02-10, -12-19, 2007-02-10,
2008-01-15, 2009-05-04, 2010-12-08, -12-20, 2011-01-23
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|Why is glass transparent?|
Out of Glass
A Long, long article on the Glass Flowers at Harvard and their problems
with considerable discussion of glass chemistry at an easy level
Calculation and Development of Glass Properties
A large website of glass property calculations
Glass is sand
or other sources of Silicon Dioxide, like flint, with other ingredients added to
make it melt easier. The result is an amorphous solid - non-crystalline -
that melts over a range of temperatures. The fewer ingredients added, the higher
the temperature it melts at and the less it expands and contracts.
Almost pure sand would result in quartz, which melts at over 3000F (1650C), next down is
borosilicate glass (Pyrex), lower down from there are soda/lime
glasses, which melt for working at 2000-2100F (1090-1150C). Glazes for
clay and enamels for glass and metal are forms of glass and can melt as low
as 1150F Sodium Silicate (Water Glass) dissolves in water. Handling glass batch
chemicals requires safety
practices including cleanup, keeping chemicals safely, and using a
An alternative to melting batch is to remelt broken
glass, referred to as cullet. Often, some cullet is added to a batch melt
because it assists in melting the batch and saves money. An all-cullet
melt requires buying cullet in bulk which is done from firms mostly in the
mid-Atlantic states where pressed and blown glass objects produce a reliable
glass scrap. For a long time the primary supplier of cullet from this source was
Gabbert Cullet. Recently certain sources, especially on the west coast are
melting specifically for sale as cullet (no objects made before), some of which comes from
China. Cullet melt can be done with broken window or bottle glass
collected locally, but this glass sets quickly and is more awkward to work on
the pipe. Cullet as a product costs more than batch, especially
considering shipping, but savings occur in energy use since cullet does not have
to be taken to as high a temperature to melt and fine and the corrosive effects
of some batch ingredients are absent. Thus, a furnace does not have to be
built to as high a standard and fuel piping and maximum usage are reduced.
Cullet also permits a moderate design for electric melt since corrosive gases
can effect elements when melting batch and the higher temps are nearer the
failure limit of elements. 2005-12-19
Melting Point - Glass is an amorphous solid
which means that it is without a crystalline structure.
Crystalline structures have a specific melting point, amorphous do not.
Different formulas of glass have different ranges of softening/melting and, in fact, by manipulating the chemical contents one can make a glass that goes from molten to stiff over a narrow temperature range, which is desirable for mechanically making bottles, or over a wide range, which is good for manually blowing glass while making lots of little changes. In glass working the former is called short and the later long glass.
Soda-Lime glass formulated for glassblowing from a furnace will start to sag and sink into other pieces of itself at about 1300F and the pieces will melt flat with each other in a kiln at about 1500F, but it will be hard to drag (a design method) in the middle of the range. The glass will be workable on the pipe but resistive at about 1700F and be like thick honey in gathering from the pot at about 2000F. A glass blower can adjust the viscosity from thick to thin for gathering purposes by setting the furnace temperature from 2000 to 2100 or 2150 when it is getting close to trying to gather room temperature olive oil.
Lead crystal glass is very long, staying soft enough to work while cooling. When I took a class at Corning, the tech assistant was given the chance to work with Steuben crystal in the shop, he reported that is was soft for so long that he had trouble getting used to it.
Pyrex, a borosilicate glass, needs to be much hotter to flow easily. Torch workers (lampworking) work it with oxy-propane and can take it and out of the flame to control its stiffness. Almost no one hand works boro from a furnace where it requires protective gear and is so near the limits of the materials that the furnace does not last very long. 2010-12-20
Further down this page is a formula for
making a glass that is widely sold for art glass blowing and
which melts to an easy to work, very clear glass. Ten ingredients
resulting in 12 testable chemicals are listed, but the first four
ingredients make up 94.4% of the product (by weight.) Some of the
other ingredients are added to make other ingredients behave,
ease melting, or to balance color. Below the formula are
instructions for melting the batch. This link [BROKEN] NOTES
ON GLASS (John M. Rusin) has an excellent discussion of
various aspects of glass.
|At right is the basic soda lime recipe used in American Glass Practice for discussion and further development. Except for the sand, each of the other ingredients can be changed and others added with varying effects on the glass. The melted glass totals 1954 lbs., a shrinkage of 311 lbs., most of which loss as gas in the reaction to make glass, such as carbonate giving off CO2.||Sand
In choosing the sand, the maker of the glass looks for sand
pits that can provide a blindingly white sand. That is because of
the very last chemical in the right column below: Fe2O3. The ordinary sand
you find on the beach or sand box is a shade of brown and it is
so because of the rusted iron in it. Iron oxide in glass gives it
a green color. In fact, the easiest color to get in glass (and
the hardest to get rid of) is green. Glass melted in the early
colonial days is predominantly green (shaded to blue as a second
choice) because there was iron in the sand. Window glass, viewed
edge on, is green for the same reason (and because the chemicals
to reduce the color are costly.) In fact, one of the most popular
glass objects, the green glass Coca Cola bottle, is that color
because it is easy to make. (Not because a green slag volcanic
glass is added to it as gullible travelers to the southwest are
Soda ash or potash are added to lower the melting point of
the glass mix, while limestone brings durability and the feldspar is a flux to make the melting
process go forward more easily. In the history of glass, various
ingredients have substituted for the soda ash. The first source
for alkali ash was burned plants rich in the right chemical.
Another source is chemicals remaining from boiling (or
evaporating) sea water.
One way to make a (small) batch of glass would be to use white sand from a hardware store of the kind that is used in ashtrays in hotels, substitute whiting (calcium carbonate) used in plaster formulas for the limestone, and ... for soda ash buy it from a ceramics supply firm (which can also do calcium carbonate.)
Here are some old recipes, from Glass Gaffers of New Jersey, pp 101-102
Note that discussing glass from batch is complicated by the
fact that parts of ingredients will cook off. Note above that the
difference between the ingredient formula and glass component in
the carbonates is CO2 which passes off as a gas.
|What makes glass? One of the problems/factors in making glass is the difference between what goes in and what is in the result. Looking at the list of ingredients, it can be noted that many have CO3 as part of the formula. However glass is made of oxides, so during the process of cooking of glass CO2 is given off. Other compounds evolve other gases, in particular, fluorine in making opals. What this means is that the formula for making a glass batch is not the same as the chemical analysis of the glass that is used for determining coefficient of expansion. Traditionally, batch formulas are given starting with 100 pounds of sand - not 100 pounds of mix or percentages. To determine the final analysis, the pounds of ingredients are reduced to pounds of effective material. There is a fixed percentage for each ingredient which can be determined from the chemical formula and molecular weights, but is also available from sources such as Henry Halem's Glass Notes, along with a further discussion of the process. 2006-06-29|
The purpose in presenting the following material is to suggest what is involved in making glass from scratch and melting such a formula. Some details of the formula have been omitted, but other formulas will be added below from other sources. Most of the added ingredients are fluxes to make the glass melt at lower temps and clarifiers to remove color provided accidentally by small imperfections in the purity of ingredients (like iron in sand.) Please note that melting on a wrong schedule can cause fluxes to melt out early or ingredients to boil off.
SPRUCE PINE BATCH CO. HIGHWAY 19E - P. O. BOX 159
We mix glass batch for the studio glass artist. All batch is
agglomerated (pelletized) for easier, safer storage and use. All
materials are selected and tested for use by studio artists- not
large factories. Our agglomerated batch is prepared by weighing
and mixing the materials which compose the formula of the glass,
then processing in a rotating disk pelletizer which forms larger
granules, balls and pellets. The primary purpose of agglomeration
is the prevention of separation (unmixing) of the materials
during shipment and storage. However, it also has the advantages
of increasing safety and convenience by reducing the dusting of
the materials, most of which are finer than 200 mesh. In
addition, some researchers have found agglomeration results in
energy savings and reduced furnace wear when compared with loose
batch. We will custom mix your formula if it does not contain
lead, price depending on material and quantity.
English and Turner coefficients
Melting Spruce Pine Batch
[end of SPB copyright material]
Heating a batch on the wrong schedule - using the wrong temperatures and times at various temperatures can cause the glass to behave differently because the flux may melt first and actually run off the pile of batch before contributing to the melting of other materials.
There was a nice little example here of a batch being developed, but the guy who posted it to a discussion group decided after giving permission to use it to take away his permission, even though his name was given in the material.
This is a formula e-mailed for my use, this is a simple batch formula, with no added complications.
27# 2oz SILICA 325 [mesh]
"Janet" wrote on rec.crafts.glass
Lead oxide makes a glass "long" - workable
over a wide temperature range. When I took a class at
Corning, the tech assistant was given the chance to work with
Steuben crystal in the shop, he reported that is was soft
for so long that he had trouble getting used to it. Most
glassware is made from "short" glass that goes from
molten to stiff over a narrow temperature range because it is
blown into a metal mold, the neck being burned off with a small
torch flame (shown by a small bead on the lip at the last place
to melt free.) The desire is to get the piece blown,
out of the mold and standing on its own for annealing.
From: "Jeff" <>
From: "Jeff" <>
From: "TWB" <>
Glazes - Glazes are glass-like
mixes that are applied to pottery/ceramics to provide decoration and seal the
surface so that vessels will hold liquids without leaking or holding
contaminants. Below are two clear glazes from the site
http://digitalfire.com/4sight/material/index.html along with their summary
version of the ingredients in float window glass. The formulas and numbers
to the right of the percent (%) sign are the content of each compound in the
ingredient which are listed highest percent first.
Contact Mike Firth