Coefficient of expansion

Definition

CoE stands for Coefficient of Exapansion. What this essentially means is that when glass expereinces a change in temperature the bonds between the atoms within the glass change. This change is what is responsible for the stresses within glass and is why we need to anneal glass to control how it cools and eliminate the stresses.

Why do I need to know the CoE of my glass?

Glasses of differing CoE are incompatible, for example Bullseye glass has a CoE of 90 and Effetre has a CoE of 104. This means that if you make a glass bead using both of these in the same bead the bead will crack and break as the 2 different glasses expand and contract at different rates.

Are there exceptions to this?

It is possible to use different CoE glass in one bead but in very small quantities. For example frit is very often found with a CoE of 86-94 and it can be used on a glass bead made from glass with a CoE of 104. Under these circumstances the 5% rule should be applied. This means that the total glass of the lower CoE used should not exceed 5% of the total volume of the bead.