182 Chapter 7 Conditioning Processes Conditioning materials changes their internal properties. For example, if a piece of steel is magnetized, the magnetiz- ing force makes the molecules of the steel line up in one direction. Heat-treating also causes changes within a material. When steel is heated red-hot and quickly cooled in water, it becomes harder. This process is called hardening (Figure 7.26). If the steel is heated again, not quite as hot, and again cooled quickly, the steel becomes less brittle. This process is called tempering. If the steel is heated red-hot and allowed to cool very slowly, the steel becomes softer. This process is called annealing. When a piece of metal is hammered, it becomes harder in a process known as mechanical conditioning. The metal's crystal structure changes, getting longer and thinner. When plaster and water are mixed, heat is given off because a chemical reaction takes place. The plaster hardens because of this chemical conditioning. When clay is fired in a kiln, it becomes harder and stronger. In each example, the change takes place within the mate- rial itself. There are many other examples of conditioning: exposing photographic film with light developing an image on photographic paper baking a cake freezing or boiling water melting ice putting metal in liquid nitrogen to turn the metal into a superconductor using radiation on a tumor making butter from cream sending electricity through the filament wire of a light bulb Figure 7.27 Hardening, tem- pering, and annealing all use heat to change the properties of metal. Interpreting How many times Each of these processes involves some change to the struc- has tempered metal been heated? ture of the material. Hardening Metal becomes harder. Tempering Metal becomes less brittle.