If you're making homemade marshmallow or crispy rice squares, or if you just want to harden regular commercial marshmallows, remember that marshmallows become hard through exposure to air and cool temperatures. Storing them in a tightly closed container is the best way to keep them soft and fresh. Provide the opposite environment, and they become hard.
Place the marshmallows or rice treats on a plate or tray, and set them in the refrigerator or a cool, dry location. Do not cover them with a lid or plastic wrap, which will keep moisture in.
Chill the marshmallows for at least 1 hour to allow them to harden.
Remove marshmallows from the refrigerator, and cut crispy rice treats or homemade marshmallows into squares. If they are still too soft, return them and allow them to continue to harden. This method causes the marshmallows to set up faster without drying them out.
Spread commercial marshmallows on a baking sheet. Turn the oven on to its lowest setting, and preheat. Turn it off, and allow it to cool for 10 minutes.
Place the marshmallows in the oven for 20 to 30 minutes. The warm air will dry them out without melting them.
Remove the marshmallows from the oven and test them. Return them to the oven if they are still soft. This method works well if you are hardening commercial marshmallows for a craft project for kids. The marshmallows become firm and dry, and they are not appealing to eat.