Hello. Thanks for the useful advice. Frost would to my opinion only be detrimental if they would hold the water and freeze. Since this is not the case (as you mentioned) I don't see a problem but I'll check it out my self next winter.
Kris
You're right. There fired at 1050°c. The temperature for Raku.
It means that the inside (not glazed) will absorb some water.
If they need to be used outside in winter, a thin indelible transparent coating would solve that problem.
Kris