What is COE?
Whenever you are dealing with glass, in almost any context you will encounter a value called COE. This is the short for Coefficient of Expansion.
It is a measurement of how much a batch of glass will expand when heated and shrink when cooled. If glass with different COE is mixed, they will expand differently when molten and mixed, and then shrink to different degrees when cooled. This will cause stress that will make the beads break during cooling or soon thereafter.
COE also indicate at what temperature a glass can be worked, Soda Lime Glass (COE104) melts at much lower temperature than Borosilicate (COE33).
This stress can also increase from simple temperature changes, such as warm sunlight (or a dishwasher) that makes the glass slightly expand, this can for example cause glasses on a shelf to break without warning.
Ancient glassworkers must have been aware of this. The simplest way to measure COE is to pull rods of glass from the batches one wants to test, and then heat them in an oven and watch if they expand in the same way.
It should also be noted that COE is not the only reason glass may break without explanation. Glass needs to be annealed (un-harden by being kept heated). This is a process where the glass is kept at a temperature where the molecules move, but the glass is not visibly flowing. This gives the glass time to build a stable structure on the molecular level, making it strong. For most soda lime glass the recommended annealing temperature is 516 degrees Celsius. If glass is not annealed, it may carry stress and break even when made from only one batch of glass.
