Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Regular versus Tempered Glass
Name: Cheyenne
Status: student
Grade: 6-8
Location: OH

Does regular glass have a stressed layer/weak layer like tempered glass?

Hi Cheyenne,

Regular glass does not have a stressed layer, but tempered glass is made in such a way that its outer surfaces are in compressive stress. Neither regular glass nor tempered glass have an intentionally-made "weak" layer.

Perhaps it would help to understand the way both types are made. First, you must remember that glass is very weak in tension, and very strong in compression. When you bend a piece of glass, the surface that is on the inside of the bend is in compression, but the surface on the outside of the bend is stretched and therefore in tension. Cracks start at the outside surface, and once the crack starts, the glass breaks.

Tempered glass is made by taking a very hot, nearly molten sheet of glass and cooling it very rapidly (usually by exposing it to a blast of cold air). This instantly cools and freezes the outer surfaces, while the inner part is still nearly molten. Then, as the glass sheet fully cools, the inner part shrinks, but since the outer surfaces were already cold and hard, the shrinking inner part causes the outer surfaces to be stressed in compression. So once totally cool, the inner part is "normal" glass, but the outer surfaces are highly stressed in compression. When you bend a piece of tempered glass, the outer surface that normally "sees" tension, is already in compression, and so resists the tensile stresses, and therefore resists cracking and breaking.

Normal glass is cooled very slowly, and therefore has no stresses. As a result, as soon as it is bent, tensile stresses occur and breakage results.

Bob Wilson

Click here to return to the Material Science Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory