Soldering, Annealing and Heat Treatment
What is examined
This domain tests heat control: choosing solder and flux, designing a close joint, bringing the work to temperature without melting it, sequencing multiple joints, annealing work-hardened metal, and pickling safely. It also tests the difference between soldering, brazing and welding.
Core rules
- In jewellery, 'hard soldering' with silver or gold solder is technically brazing because the filler melts above 450 °C. The parent metal must not melt.
- Solder grades flow at different temperatures: extra-easy, easy, medium and hard, typically across a range of roughly 620–800 °C depending on the product. Always use the manufacturer's flow chart. Cadmium-free solders flow higher than cadmium-bearing equivalents and are preferred for health and safety.
- Joint design: parts must fit closely, clean and fluxed. Capillary action draws solder into a narrow gap; a wide gap fills with solder but gives a weak joint, and an excessively tight joint can starve the capillary.
- Flux prevents oxidation and helps solder flow. Use the flux recommended for the metal and solder; remove residues after soldering.
- Heating: use the largest soft flame that suits the work, heat the whole assembly evenly, and bring the solder to flow rather than playing the flame directly on it. Direct heat can oxidise the solder and pit the parent metal.
- Annealing: heat silver to about 650–700 °C until dull red in dim light, hold briefly, then quench and pickle. Gold alloys anneal around 600–700 °C depending on composition; follow the alloy data. Overheating causes grain growth and firestain on silver.
- Pickle removes flux and oxide. Use warm dilute sodium bisulphate or approved pickle, plastic tongs, and rinse thoroughly. Do not put steel in the pickle.
Worked example
A pendant has three joints: a bail, a decorative plate and a setting. Solder in the order hard, medium, easy. Fit and clean all joints, flux them, and use hard solder for the bail first. Pickle and rinse. Add the plate with medium solder, keeping the flame away from the first joint as far as possible. Add the setting with easy solder last. The reasoning is thermal hierarchy: each later solder must flow below the previous joint's flow temperature, so the earlier joint is not disturbed.
Traps
Using too much solder; relying on solder to fill a loose joint; failing to clean or flux; overheating and melting the parent metal or a previous joint; quenching a complex assembly that may warp; leaving flux residue that causes corrosion; contaminating pickle with steel; ignoring cadmium fume hazards.