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A Quick and Dirty Compatibility Test for Float Glass
Recently a friend gave me some lovely cobalt blue wine bottles. It took me a lot of time to work up the nerve to smash one, but I did. It is fairly stiff to work with, and takes a lot of heat to melt, rather like borosilicate. I made some initial beads from it and a simple shell sculpture (see the attached picture). However, I realized that I wanted to add more decoration. It would be simple enough to use my other glass rods to decorate a bead made with the wine bottle. However, they would likely crack. As they cool, different types of glass expand at different rates, and each glass has a compatibility range known as its "Coefficient of Expansion." Moretti and Vetrofond glasses have an approximate COE of 104; Bullseye glasses have a COE of 90, Spectrum 96 glasses have a COE of ... you guessed it ... 96, and so on and so forth. Most float glass has a COE in the eighties. What that means is that if I mix the wine bottle glass with another glass that has a different COE, the glass will crack spontaneously as it cools. Without the sort of equipment that you would find in the Corning Laboratories, however, how can you tell whether it is safe enough to use teensy amounts of another glass as decoration? Enter two friends, "Tinroof" and "Gwacie," who suggested the following procedure: 1) Melt together equal parts of the wine bottle glass with another glass whose COE you know. 2) Pull a stringer (a thin rod of glass, anywhere from 3mm down to a hair's width). Let it cool. 3) As it cools, watch it. It will arc like a pulled bow, as one glass expands more than the other. The glass on the concave side has the lower COE. The more pronounced the curve, the larger the incompatibility. 4) If there is only a small amount of curve, you can -- carefully and with judgment -- mix the two glasses, or even build a bridge from one COE to the other by slowly adding layers in which each layer has a bit more of the glass below it and a bit less of the glass above. 5) Be prepared for the bead to crack after annealing anyway, just on general principles. Although it is imprecise, the stringer method can give you a good approximation of what glass is safe to use as a decorative element -- usually no more than 10% of the total glass in the bead. |
Seashell made with Wine Bottle Float Glass.
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May, 2012
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