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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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