The vast majority of glass produced in the world is soda-lime glass and it is certainly possible to make glass in the pottery kiln from soda ash, limestone, and silica. Since some students in a class might be making glass while others are working on metals, pottery, or lime, it would be desirable to use the same kiln program for all four kiln projects so that they might all be fired together. The kiln conditions required for making soda-lime glass, however, are at odds with those for smelting metals. For one thing, soda ash and limestone give off bubbles of carbon dioxide when fired and if the glass is to be free of bubbles it must be left at its highest temperature for an hour or more. In this time, the charcoal in a smelting crucible might burn away completely, spoiling the metal. For another thing, glass shrinks as it cools and consequently must be cooled slowly through its annealing point, considerably lengthening the time for a firing. We can address both problems by making a borosilicate glass instead of a soda-lime glass.

In a soda-lime glass the soda serves as a flux, lowering the melting point of silica. The lime serves to make the resulting glass insoluble in water. Soda serves as a flux in a borosilicate glass, as well, but boron oxide rather than lime (calcium oxide) serves to render the glass insoluble in water. Both sodium oxide (soda) and boron oxide are available in a naturally-occurring mineral, borax, available wherever laundry detergents are sold. Just as calcium sulfate or sodium carbonate occur in anhydrous and hydrous forms, borax occurs as the anhydrous Na2O2 B2O3 and as the decahydrate, Na2O2 B2O310 H2O. Since water will be driven off by the heat of the kiln, either form will do for making glass but the weight needed will depend on which one is being used. To eliminate guesswork, you should dry your borax for an hour in an oven at 130C (266F) to convert the decahydrate to anhydrous borax. Anhydrous borax may be stored indefinitely if it is kept in a moisture-proof container. In addition to borax we need silica, which might be provided in the form of ordinary sand, but it should be ground to a fine powder. Ground silica, known among potters as flint, can be had inexpensively from ceramic supplies. With borax and silica at hand, we are ready to formulate our glass.

Figure 13-2. Freeing the Glass from the Crucible

There is no "correct" formula for borosilicate glass. The more borax is used, the lower will be the melting point of the resulting glass. A very nice, colorless glass can be produced using 1 part anhydrous borax to 2 parts silica. Let us begin, then, by weighing by difference 15 g of borax and 30 g of silica into a plastic bag. Optionally, copper oxide may be added to provide a blue-green color; 1 g of Cu2O will render the glass completely opaque. Thoroughly mix your glass ingredients by kneading the bag and turning it end for end.

Figure 13-3. Removing the Investment

If you completed the mold of Figure 10-3(R) you have only to pack your mixed glass ingredients into the mold. If you have no mold, you must at least coat the walls of your crucible with investment, as shown in Figure 10-2(L), to prevent the glass from sticking to the walls of your crucible. Fill your mold or crucible with the mixed glass ingredients, cover your crucible with its lid, and fire it to cone 05. When it returns from the kiln, remove the lid and use a knife or screwdriver to pry the investment from the crucible. Chip away at the investment to free your glass from its cocoon, as shown in Figure 13-2. A knife can be used to remove much of the investment, but a Dremel tool with a cutting disk is useful for removing it from the surface of the glass. Provide yourself with a bowl of water and use it to wash your glass object twice a minute, or so. The water will help to soften the investment and it will keep the glass cool. Since glass is much harder than investment, it is easy to remove the investment without fear of damaging the glass. The finished glass casting is shown in Figure 13-3(R).

What lesson have you learned from old Theophilus? I have variously referred to you in the past as though you were my son or daughter, my brother or sister. But you and I are not animal, vegetable or mineral. We have no gender, no nationality, no ethnicity, or to be more precise, we are free to surround ourselves with identifications of our own choosing. My effort in this book is not to instruct you as if you were a recalcitrant child in need of smartening up. Rather, I am here to re-mind you, to bring together into mental solution thoughts which may have de-vitrified over the centuries. From this point I shall no longer treat you as a child, but as a comrade in flux.

ImportantQuality Assurance

Record in your notebook the weights of your glass materials and a description of your procedure. Your glass should be, well, glassy. Either it is glass or it is not. Photograph your glass and include it in your notebook as a testament to your ability to keep in the heat and withstand it.