In the world of glass fusing,
two glasses are "compatible" if they can be fused together and have no
undue stresses in the finished piece that will lead to fracturing.
Glass, like most materials, expands when heated and contracts when cooled.
The viscosity (resistance to flow) characteristics of a glass are equally
as important. Together, these two properties determine whether one glass
will "fit" another.
If two glasses that do not expand or contract
similarly are fused together, unwanted stresses will develop in the glass.
The greater the difference in contraction or expansion of the fused
glasses, the greater these stresses become. If the stresses are too
great, the two pieces will break apart upon cooling.
Expansion affects the compatibility predominantly in the lower temperature range—below the strain point.
Viscosity properties affect compatibility predominantly in the middle temperature range—from the strain to the annealing 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.
For glasses of different viscosities to be compatible
(which is frequently the case) their expansions must be different. What happens in actuality is a process of compensating errors. Two different glasses will be compatible if the strain set up by the mismatch in viscosity is cancelled out by the strain introduced by the mismatch in expansion (once cooled to room temperature and assuming, of course that proper annealing has occurred). For instance, if the viscosity differences result in tension between the two glasses and the expansion differences result in an equal amount of compression between the two glasses, the two stresses cancel each other out. This is the critical phenomenon that results in
"compatibility" of two glasses with different expansion/viscosity properties.
Glass suppliers such as Bullseye and Urobros
spend a great deal of effort developing "recipes" of different colors of
glass which meet the stringent criteria of being "compatible". To
further insure the actual product performs as required, small samples are
tested in every batch. As you would expect, the research and testing for the
myriad of glass colors available increases the cost of production of
compatible glass dramatically over a sheet of common stained glass or
ceramic clay.
