Hi Linda, I have never tried to make them hard afterwards, to do that quickly would likely require a bit of candy making, and the use of a syrup at the hard crack stage, although that is a guess on my part. Grampa was a candymaker, but long retired when I was born. That said, my marzipans do become hard over time, which happens faster when they are thin.The more water you used in binding the dough the longer it will take. I have had them dry relatively stiff when left uncovered overnight (under a pop-up screen). The flat leaves that I make dry very quickly as a rule. Yes, I do believe that refrigeration would possibly help, but exposure to air and possibly a fan, is likely a better route.