Cover Glass Deep Dive: How to read a glass spec sheet

Suppliers love to throw numbers at you. Most of them do not mean what you think they mean. Here is what to actually look for.

CS (Compressive Stress)
Measured in MPa. This is the squeeze on the glass surface. Higher CS means better resistance to bending and drops. Typical range: 600–900 MPa for ion-exchanged glass. Anything below 500 MPa is basically unstrengthened. Anything above 1,000 MPa is suspicious unless the supplier shows independent test data.

DOL (Depth of Layer)
Measured in microns. This tells you how deep the compression goes from the surface. Thin glass (under 0.5mm) needs DOL under 50 microns. Thicker glass (0.7mm and up) can take DOL of 80–150 microns. The trap? Some suppliers give you CS and DOL but hide the CT.

CT (Central Tension)
This is the one nobody puts on the spec sheet. It is the tension in the middle of the glass that balances the surface compression. If CT gets too high (above 60 MPa for most aluminosilicate), the glass can explode spontaneously. Yes, spontaneous failure is real. It happens when a tiny scratch grows into a crack that the compressed surface cannot contain.

Weibull modulus (m)
The most honest number on any spec sheet. It tells you how consistent the glass is. A Weibull modulus of 5 means one in ten parts will fail at half the stated strength. A modulus of 15 means consistent performance. A modulus above 20 is excellent. If the supplier does not provide Weibull data, they either do not know their process or do not want you to know.

The shortcut
Ignore peak strength numbers. Ask for three things: CS, DOL, and Weibull modulus. If they cannot give you all three, find another supplier.