The term 'make rigid' can be defined as making something hard and stiff. This can be accomplished in a variety of ways, such as ossifying (6 letters), stiffening (7 letters), petrifying (7 letters), or starching (6 letters). Ossifying is the process of turning organic material into bone or a similar substance. Stiffening is the act of making something more rigid or firm. Petrifying is the process of turning organic material into stone. Lastly, starching is the process of treating fabric with a STARCH-based adhesive to make it stiffer.