The term 'Harden by Heat' can be solved by two crossword solutions, 'BAKE' (4 letters) and 'BKE' (3 letters). 'BAKE' is a cooking method that involves using dry heat, usually in an oven, to cook food. It is often used for cakes, breads, and other baked goods. 'BKE' is a type of heat treatment process that involves heating metal to a high temperature and then cooling it quickly. This process is used to harden the metal and make it more durable.