I made these coasters using an epoxy resin. They were quite fun to mold.
I used UltraCast XT Art Resin. I picked this resin as it explicitly stated that it was heat resistant to 90°C - perfect for molding coasters. I was expecting the resin to have quite a chemical smell to it, but I hardly noticed it. I'd imagine other resins smell worse.
For the dye I used a powder dye (rather than a liquid dye) - just because there was a pack of mixed colours available at a good price.
The resin was quite easy to mix - the ultra cast is a straight up 50/50 mix and the advice is then to stir slowly which helps prevent air bubbles.
To make these I first molded a bunch of letters, with a mix of black and silver dye. Once these were dry, I placed the letters face down in the center of a coaster mold and poured on a thin layer colour layer.
I learnt a few things making these. Mostly - check the weather before starting - I made the coasters in the middle of a heat wave - whilst the letters I made dried overnight - the coasters took days and days to set.
I've got a few different ideas for my next batch...