Basic metallurgy for jewelers: how metal behaves

Understanding how metal behaves explains why it cracks, why it needs annealing or why a piece changes size when cast. These are the key ideas.

Kinds of metal #

• Precious metal (metal precioso): gold, silver, platinum and palladium, among others. They are scarce and resist corrosion.
• Base metal (metal base): copper, brass, bronze, steel, aluminum… They are cheaper and some of them tarnish.
• Alloy (aleación): a mix of metals to change hardness, color or melting point. See jewelry alloys.

Purity #

• Fineness (ley): the proportion of pure metal, in thousandths. Sterling 925 means 925 parts silver in every 1000. See 925 silver.
• Karat (quilataje): the purity of gold in 24 parts. 18-karat gold is 18/24 gold, that is, 750 thousandths. It is not the same as the carat of gems: see carat and karat.
• Purity (pureza): the general term for pure-metal content.

Mechanical properties #

• Hardness (dureza): resistance to scratching or deformation. It is not the same as the Mohs hardness of gems.
• Malleability (maleabilidad): the ability to be flattened into sheet without breaking. Gold and silver are very malleable.
• Ductility (ductilidad): the ability to be drawn into wire. It is the basis of wire drawing.

Heat #

• Melting point (punto de fusión): pure gold melts at about 1064 °C, pure silver at about 962 °C, copper at about 1085 °C and platinum at about 1768 °C.
• Melting temperature of an alloy: not a single point but a range between the start (solidus) and the end (liquidus) of melting.
• Shrinkage (contracción): metal shrinks as it cools, which is allowed for when casting.

Changes when you work metal #

• Cold working (trabajo en frío): hammering, rolling, bending or drawing at room temperature. It hardens the metal and makes it brittle. Also called work hardening.
• Internal stress (tensión del metal): stress built up by working it; it can warp or crack the piece later.
• Annealing (recocido): heating the metal to remove hardness and stress and recover malleability. See annealing.
• Hot working (trabajo en caliente): forging and other processes at high temperature, where the metal is softer.
• Oxidation (oxidación): a reaction with oxygen; on copper and sterling silver it forms dark oxides. See pickling.

The practical rule #

Work the metal until you feel it push back, anneal it and carry on. Ignoring this is the most common cause of cracks.

See also: Basic metalworking operations.