BRITTLENESS, MALLEABILITY, AND DUCTILITY OF SOLIDS
 

The atoms of elemental solids can “stack up” in various ways. This is evident in the shapes of the crystals we observe in many different solid substances. Salt, for example, has a characteristic cubical crystalline shape. The same is true of sugar. Ice crystals, however, can appear in a fantastic variety of shapes, but they always have six sides, axes, or facets. Some substances, such as iron, don’t seem to form crystals under normal circumstances. Some materials, such as glass, break away along smooth but curved boundaries. Some solids can be ground up into a fine powder, whereas others defy all attempts to pulverize them.
Crystalline solids are brittle. If a sample of such a material is subjected to a blow with enough force, it will crack or shatter. These types of solids cannot be stretched or squashed or bent out of shape very much without breaking. Glass is an example, although you may have noticed that glass has a little bit of “give.” You can observe the flexibility of glass if you watch the reflections from large window panes on a windy day. However, you cannot bend a straight glass rod into a donut shape.
Soft copper wire, in contrast to glass, is malleable (it can be pounded flat) and ductile (it can be stretched and bent). The same is true to some extent of iron. Gold is one of the most malleable known metals. It is expensive but can be pounded into sheets so thin that towers of buildings can be gold-plated without breaking the government budget. Aluminum is more ductile and malleable than glass, but not to the extent of soft copper or gold. Wood can be bent to a variable extent, depending on its water content, but can’t be pounded into thin sheets or stretched into wire.
The brittleness, ductility, and malleability of some solids depend on the temperature. Glass, copper, and gold can be made more malleable and ductile by heating. The professional glass blower takes advantage of this phenomenon, as does the coin minter and the wire manufacturer. A person who works with wood has no such luck. If you heat wood, it gets drier and less flexible. Ultimately, if you heat glass, copper, or gold enough, it will turn into a liquid. As wood is heated, it will remain solid; then at a certain temperature it will undergo combustion, a rapid form of oxidation. That is, it will catch on fire.