How do I make cast resin more hard?

I have done a lot of casting with epoxy resin, but sometimes if the ratio is just slightly off (with more hardener-to-resin-ratio) my castings become bendable. I recently cast a ring and the resin was bendy, too. I don't want it like this. Does anyone have tips on how to make the resin harder on the ring, even though it's been cast? Would exposing it to high heat or high cold work? Is there some sort of catalyst that I can paint on top? Tips, please!