This is an excerpt from Technotes from Bullseye Glass Co. written by Daniel W. Schwoerer.
"A misunderstanding that the compatibility or “fit” of two glasses is solely a function of their expansion properties has lead to an overemphasis on “expansion” and the numerical value of the coefficient of expansion (COE) of glass. Studio artists continually ask us for the COE of a glass, hoping to predict whether it will “fit” other fusing glasses or their own furnace glass. We say the same thing each time: Matching COEs is simply not an accurate measure of compatibility.
The viscosity characteristics of a glass are equally important as its expansion characteristics. Together, these two properties determine whether one glass will fit another. But it will be useful to first discuss each individually as it pertains to this subject.
Expansion affects compatibility throughout the full temperature range (from the annealing point to room temperature). This is because by nature most materials—whether solid or liquid—expand upon heating and contract upon cooling. It is commonly assumed that if they expand and contract similarly, they will “fit” or be compatible once fused together. But this is not necessarily true. In fact, we have tested some glasses with the same COE and found them to be incompatible.
Why a Measured COE Alone Does Not Insure Compatibility
As stated above, the fitting of two different glasses is a function of both viscosity (resistance to flow) and expansion. Whereas expansion affects the compatibility predominantly in the lower temperature range—below the strain point—the viscosity properties affect compatibility predominantly in the annealing range, from the annealing point to the strain point. Differences in viscosity between two glasses will cause compatibility problems. If one glass is stiffer than the other, they will strain each other as they cool through the annealing range."
So those times when you are scratching your head, wondering why something came out of the kiln broken, think about this.
An amazing resource is also Lisa Vogt Art Adventure Blog.
Lisa Vogt Art Adventure Blog: Crack the Case –Your Guide to What Went Wrong Inside Your Kiln