How cold should it be outside for a hot coffee mug to break?

Question

So I like to go outside for a morning coffee with a cigarette. In winter here it's usually between -5C to -25C, and sometimes it gets down to -30C and colder. Assuming that my coffee is about 75-80C, what are the chances that the coffee mug will shatter in the cold?

My mug is made from some kind of organic glass with a seam in the middle like it's two halves glued together.

I don't know if this question is serious enough for this site, but, basically, I wanted to know which precautions should I take to avoid breaking the mug?

Answer 1

It sounds as if your mug is made from Pyrex or some similar low expansion glass. Experiment suggests that if I pour boiling water at 100C into a similar mug in my kitchen (around 15C first thing in the morning) the abrupt 85C temperature change doesn't shatter it. Given this, the 45C difference between room temperature and -25C isn't likely to cause problems, especially since the cooling to -25C will be slow.

You might want to avoid putting the mug down on a very cold surface (which would cause more rapid cooling) but I doubt if even this would have much effect.

Answer 2

I don't think the temperature of the mug outer surface will change much between being inside and outside. This is because there is not much convective heat transfer at the mug surface. If you were dunking your mug into a liquid at -25C, that would be different but in your case, I don't think the temperature gradient across the mug surface will change much and cause additional stress.