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Simple Lesson in Glassmaking
The common glasses we see in items such as bottles and window panes are
made mainly from three crystalline materials: crushed white sand;
crushed limestone; and sodium carbonate. Small amounts of other additives
are used to improve the glass quality, but these three are the principle
elements.
In a factory where items such as bottles are made, these three
ingredients are blended and heated for extended periods of time in order to
form the transparent material we call glass. The temperature involved is
in the neighborhood of 2500° F. and the residence time of glass in a
furnace may be as much as 24 hours. These conditions are necessary because
glass is a viscous material even at high temperatures. Thus it takes a
long time to rid the glass of small bubbles.
To visualize the conditions in a glass furnace, one can mix powdered
sugar in a glass breaker of corn syrup and then imagine how long it would
take to for all of the tiny bubbles to rise to the surface.
Since it would be impractical to try to duplicate the time and
temperature necessary to melt a commercial glass in a school laboratory, let us
try a simpler technique that will form an inorganic * glass on a much
reduced scale.
Silica (sand) based glasses generally have very high melting
temperatures, hence we will choose a different inorganic base for our
experiments. A common crystalline material that will melt to form a glass is borax
(sodium borate). The borax that can be purchased in a grocery store for
use as a laundry additive contains a lot of water, so it must be dried
thoroughly before being used in our tests. If anhydrous borax is
available, that is much easier to use for glass making. However, even dry
borax requires a temperature of about 1400° F. in order to melt to a
glass.
The best way to experiment at such a temperature is to use a small
quantity of material. This brings us to the old technique known as "e;the
borax bead test."e; A small quantity of borax can be melted on a wire loop
in a Bunsen flame or a propane torch and will produce a genuine sample
of an inorganic glass. In addition, if a tiny amount of a colorant oxide
is added to the borax, a colored glass can be formed.
The tools needed for this experiment are the following:
Propane torch and igniter
Inert metal wire with loop at end; handle
Sample of dry sodium borate crystals
Safety glasses and leather gloves
Put on safety glasses and gloves. Light the propane torch and adjust it
to a moderate flame. Insert the wire loop into the flame and wait until
it becomes red hot. Then remove the loop from the flame and touch it to
the borax to pick some up. Return the loop and adhering the borax to
the flame and hold it there until the borate melts. Continue, alternately
touching the flame and the borate with the loop until enough has
adhered to the loop to form a small droplet of borate glass. Allow the bead
and loop to cool (2 minutes).
On examining the bead, first by eye and then with a magnifying glass,
it will be seen that the crystalline sodium borate has been converted to
a sodium borate glass. This is the process that takes place in a
commercial glass furnace, but with different ingredients. Commercially, the
crystalline silica (sand), calcium carbonate, and sodium carbonate have
been converted to a soda-line-silicate glass. We have made do with a
single ingredient and have converted crystalline sodium borate to sodium
borate glass.
If it were possible to examine the raw materials and the finished glass
with the appropriate apparatus, it would be found that the glass
end-product has none of the crystalline aspects of the initial raw materials.
The high viscosities of glasses are what make them formable into window
panes and bottles. Most crystalline materials are very fluid when
melted and would be difficult to form into large transparent objects. Silica
is the magic component of most glasses since after melting it normally
forms a glass upon cooling.
One other technique can be used to make a glass sample if your school
has an art department with a ceramic kiln. such a kiln will reach a high
temperature for an extended time so that a somewhat larger bead of
glass can be produced. It will be necessary for you to be on very good
terms with the art teacher though, because the teacher will have to do most
of the work. If the art teacher will provide a small clay crucible, a
fill of dry glaze raw material ** (covered), and then place it in the
hottest region of a ceramic kiln, at the end of a firing it will be found
that the glaze batch in the crucible will have melted to a glass. It
will not be bubble-free, but it will be a fine durable glass. glazes are
indeed glasses and in fact the first historical glasses may have been
derived from glazes.
* glass can be named organic if, for example, the silica material comes
from the skeletons of small sea animals such as sponges, diatoms, etc.
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