Annealing is the heat treatment that softens metal after it has been hardened by working. When a jeweler rolls, draws, bends or hammers gold, the metal gets stiffer and eventually cracks if work continues. Heating it to the right temperature and cooling it correctly restores its workability, so the jeweler can keep shaping it.
Work hardening
Metals are made of crystals called grains. When metal is bent or compressed, defects in the crystal structure (dislocations) multiply and tangle, which makes further deformation harder. The metal becomes stiffer, springier and less ductile. A jeweler feels this as a piece of wire that resists bending or sheet that springs back after hammering.
Work hardening has uses. A finished ear wire or a prong is often left in a hardened state so it holds its shape. During shaping, though, hardened metal needs regular annealing, or it can crack at the edges or split during drawing.
What annealing does
Heating a work-hardened metal above a certain temperature lets new, strain-free grains form, a process called recrystallization. The dislocations are cleared out and the metal becomes soft again. Held at temperature too long or heated too high, the grains grow large, and coarse grains can leave a rough "orange peel" surface when the metal is bent later. Good annealing uses enough heat for long enough, and no more.
Annealing temperatures
There is no single annealing temperature for gold. It depends on the alloy. Pure gold barely work hardens and needs little annealing. Karat golds contain silver, copper, zinc and sometimes nickel or palladium, and their behavior varies. As a broad guide, many karat gold alloys anneal somewhere in the range of about 600 to 750 degrees Celsius, but alloy suppliers publish exact figures for each product, and those should be followed.
Jewelers without a kiln judge temperature by color, working in dim light so the glow can be seen. Most yellow gold alloys are annealed at a dull red glow. Heating to bright orange risks overheating, grain growth and, on thin sections, melting. A kiln or annealing oven gives more control and is common for large batches or for sheet and wire.
Colors and alloys
Different karat golds react differently:
- Yellow gold in the 14K to 18K range usually anneals well at a dull cherry red and is often quenched once the glow fades.
- Rose and red golds have more copper. Some of these alloys harden again if left to cool slowly, so suppliers often advise quenching them promptly after annealing to keep them soft.
- Nickel white golds are harder and springier, harden quickly when worked, and need more frequent annealing. Some can crack if quenched while still glowing, so suppliers may advise letting them cool to black first.
- Palladium white golds tend to be softer and easier to work.
The yellow gold, rose gold and white gold pages cover the alloys themselves.
Quenching
Quenching means plunging the hot metal into water. With many yellow golds it is done once the red glow has just disappeared. Quenching speeds up the work, and for some alloys it helps keep the metal soft, because slow cooling through a certain range can let some karat golds harden again by an ordering reaction in the metal. Other alloys must cool slowly to avoid cracking. This is why the supplier's recommendation matters more than a general rule.
Quenching in pickle directly is a habit some jewelers avoid, since it can splash acid. Quenching in water first, then moving the piece to the pickle, is the usual practice.
Pickling
Heating copper-bearing gold in air forms surface oxides. A pickle, a mild acid solution, dissolves these oxides and leaves the surface clean. Traditional pickles used dilute sulfuric acid. Many workshops now use sodium bisulfate based products, often warmed in a small pot. The piece is lifted in and out with copper, brass or plastic tongs; steel tongs in the pickle can cause a thin copper plating on the work.
Pickling removes surface oxides but does not remove deeper firescale. Flux applied before heating limits oxide formation, and some jewelers coat pieces with a boric acid and alcohol mixture before annealing or soldering.
Annealing in the workshop sequence
A typical fabrication sequence alternates working and annealing:
- Roll or draw the metal until it stiffens.
- Anneal to a dull red, then quench as the alloy allows.
- Pickle, rinse and dry.
- Continue shaping, and repeat as needed.
- Leave the final shape hardened where springiness or strength is wanted.
Soldering also anneals the area it heats, so a soldered ring shank is softer near the join until it is worked again. The page on how jewelry is made puts annealing alongside casting and soldering, and the metallurgy glossary defines terms such as ductility and grain growth.
Stones and annealing
Most gemstones should be removed before a piece is annealed. Torch heat can crack, discolor or alter stones, especially those with inclusions, fractures or treatments such as fillings; see gemstone treatments. Pearls, opals and organic gems are particularly heat sensitive. Diamonds can survive moderate heat but can burn at high temperatures in air, and they need protection if left in place.
Frequently asked questions
Why does gold get hard when I bend it?
Bending multiplies defects in the metal's crystal structure, which makes it stiffer. This is called work hardening, and annealing reverses it.
What color should gold be when annealing?
Most yellow gold alloys are annealed at a dull red glow seen in dim light. Bright orange usually means it is too hot.
Should gold be quenched after annealing?
Many yellow and rose golds are quenched once the glow fades, but some nickel white golds can crack if quenched too hot. Follow the alloy supplier's instructions.
Does pure gold need annealing?
Pure 24K gold hardens very little when worked, so it rarely needs annealing. Karat alloys harden much faster.