How many times have you gotten glass, in your studio, mixed up and thrown it away because you were unsure of the COE? There is a simple test that can help you save that glass. The test will require a torch (nothing fancy, a plumbers propane torch will work), some small pieces of glass to test (as small as ¼”x ½” will work), or glass rods or strips of glass. A couple of glass rods to use as punty handles (tweezers will also work).
Concept: The stringer pull - compatibility test consists of two small pieces of glass of equal mass, size and color. It is not always necessary to use the same color of glass for the stringer pull, it just makes the pull easier since some colors are softer than other colors and melt faster. One piece of glass is your control (the COE that you are trying to match i.e. 32, 90, 96 104). The other piece of glass is your test COE piece. I always start with the same COE as my control piece so that I have a stringer to compare to all of the rest of my test pieces.
The Test: You can tack fuse your two pieces together in a kiln and then torch work glass punties onto opposite ends for heating and pulling or you can attach each piece to a glass punty and then fuse them together in the flame of a torch. Tweezers can also be used if preferred. Glass rods can also be used or strips of glass. If you are using either of these, for the test, overlap one rod or stripof glass, onto the other about ½” in the heat of the torch.
It is important to heat the two pieces evenly on all sides while keeping them parallel to each other and not twisting or otherwise blending the glass together. If the glass gets twisted or blended together the stringer test will not work. When heated thoroughly, pull into a thin stringer at least 12” long. Cut the stringer loose from the pulls. The resulting stringer will stay straight if the two pieces of glass are compatible and will curve if they are not compatible.
Glass expands when heated and contracts when cooled. The higher the COE the more the glass expands and contracts. The concave side (inward curve) of your pulled stringer is the higher COE as it has contracted more.
Note: If you are testing glass that has a dichroic coating the coating must be removed. You can etch, scrape, or sand the dichroic off of the glass. If not removed, the dichroic coating will make the pull more difficult and irregular in consistency.
Sounds tricky and you really wish you could see the process being done? Poof, your wish has been granted.