Annealing: why copper must be heated to be shaped.

Try bending a copper wire back and forth a few times: it becomes stiffer with each bend until it breaks. That's work hardening in practice. Every time you hammer, drive, or bend copper and brass, the metal's crystal lattice is squeezed together, it becomes harder and loses its ductility. If you continue without intervening, the workpiece cracks. Annealing is the metalsmith's answer: a controlled heating that softens the metal again, so work can continue.

During cold working internal stresses build up and the crystal grains are distorted. Through annealing the metal is heated evenly above its recrystallization temperature after which it cools slowly in still air. The result is a fine-grained stress-free crystal structure. According to the theory of heat treatment annealing is precisely the process that restores a cold-worked metal's formability by allowing the crystal structure to transform and internal stresses to disappear. This is typically done after heavy forging before further processing.

At the workshop, temperature is rarely read on an instrument but on the glow color. The workpiece is placed on a heat-resistant plate and heated evenly with the soft flame until it takes on a dull, dark red glow color — around 500 °C for copper. It must be heated evenly so the entire workpiece reaches the same temperature; otherwise some areas are softened more than others. Overheating should be avoided because the melting point is not far away and a melted corner is ruined material.

Annealing forms a layer of dark oxide (scale) on the surface. It is removed by pickling — a short soak in an acidic bath (diluted acid or a milder alternative such as citric acid) that dissolves oxide and any remaining flux residues. After pickling, the object is thoroughly rinsed in water. The pickling solution is caustic and requires gloves, eye protection and ventilation; read more in the article on acids and pickling.

When should you anneal?

• 01When the piece feels stiff and resists the hammer or pusher
• 02With deep driving: anneal at regular intervals before the metal cracks
• 03Before you bend an edge that has been hammered hard
• 04After hard soldering, where the heat has nonetheless changed the hardness locally
• 05When a wire or plate is to be drawn or rolled thinner

Pitfalls

• 01Uneven heating gives soft and hard zones in the same component.
• 02Too much heat can melt thin edges or burn zinc out of brass
• 03Missing staining leaves oxide giving poor solder joints
• 04Forgetting to anneal along the way is the most common cause of cracks in forged items

“The metal itself tells you when it should glow — you just have to learn to feel it.”