GlassCast 10 and 50 both have the same heat resistance, which is is about 65 degrees C.

65C is about as high as any epoxy can be temperature resistant to, without some kind a special heat treatment (called a post-cure) and even after a post-cure, it's for a clear casting resin to exceed this temperature by much.

This does mean that you need to consider what you're putting on a GlassCast 10 or 50 surface. A typical hot-drink is unlikely to mark the surface but a large, freshly boiled black coffee, hotter than you could drink, could cause a light ring if you left it on the resin surface.

We have many, many customer making drinks coasters out of GlassCast 10 and GlassCast 50 and it's incredibly rare to hear of any issues or marking. That said, you should be aware of the limitations of these sort of epoxy systems, even those marketed as 'heat resistant'; we've tested them all and never found a 'heat resistant' epoxy to be any different in how it's affected by heat, no difference at all.

I hope this helps, Matt