Coefficient of Expansion

COE, or Coefficient of Expansion, describes by how much a material will expand per degree of temperature increase. Different types of glass have different COE's.

Glass, like many materials, expands as it gets hotter and contracts as it cools. The Coefficient of Expansion for glass is a way of describing the degree of expansion. When two rods of glass are melted together, it is important that their COE is the same so that they are expanding and contracting at the same rate. If you attempt to melt two rods with different COE's together, they will contract at different rates as they cool, producing stress within the piece you have made which could lead to breakage.