Brittle Glass

When I was a kid I heard this question:

Boil water within two glasses, one thin and thick. Which one will shatter first?

And the answer: The thick one.

Am I remembering the question and answer correctly? Why does the thick one shatter first?

Oops, the two glasses should be one thin and one thick.

I believe it’s because the thick glass can’t heat through it’s… “thickness” fast enough leading to larger temperature (and expansion) differences between the inside/outside and being brittle the thick glass cracks since it can’t bend much.

Not to say that you can’t crack thin glass with hot water though - I’ve done that; especially when only part of the glass gets hot fast while the rest stays cool.

Right! Thanks for jogging my memory. But I wonder if this is really true, since I tend to disbelieve things I heard as a child…

Does the diameter of the glass make a difference?

If you decrease the diameter of the thin glass enough, you can reach a point where the ratio of outer to inner radius is the same as the original thick glass.

If the breaking occurs due to thermal expansion of the inside of the glass vs. the outside, the smaller diameter thin glass should crack as quickly as the original sized thick glass, correct?

In other words: How can I get drinking glasses that won’t crack?

The standard method is to choose a material that has a low coefficient of thermal expansion. That’s why Pyrex can be used with hot food - it’s no stronger than ordinary glass, it just doesn’t expand when heated.

There’s a much smaller temperature gradiant within the thin glass, as heat moves more quickly though it.

Pure silica glass - often called quartz glass - has such a low thermal expansion coefficient that it is often used to make apparatus to measure the expansion in other materials. You could take such a glass from a hot oven and pour in ice water and still it would not crack. Unfortunatly it would be very expensive!

Borosilicate glass - e.g. Pyrex - would be almost as good in most circumstances.

Good to know. I always thought Pyrex was some type of plastic, like plexiglass.

How do so-called tempered glasses hold up to temperature? I see that designation applied to drinking glasses all the time.

Tempered Glass.