Sterling silver will oxidise rapidly when exposed to the high temperatures used in annealing and soldering. Sterling silver is an alloy of silver containing 92.5% by mass of silver, and 7.5% by mass of copper. Oxygen in the atmosphere reacts with the copper in the alloy to initially form cuprous oxide (Cu2O), and then cupric oxide (CuO) as the process continues. This surface layer of oxide should be removed by pickling before work continues. At high temperatues, silver has the capacity to absorb oxygen. When the oxidation penetrates further than just the surface, and is visible as a patchy grey/pink bloom, it is known as firestain. This stain is often only noticed once the final polishing stage is reached. Firestain is a common problem, and can easily mar the otherwise perfect mirror-like surface of a highly polished piece of silver.
Avoiding firestain
In a commercial operation, it may be viable to solder in a protective atmosphere such as a vacuum furnace; however this isn’t an option for the amateur. Some tips for preventing excessive oxidation and firestain are:
- Avoid prolonged heating. The torch should be powerful enough to heat the metal to the required temperature in a reasonable time. If the process is too time consuming, all the protective flux will eventually burn off.
- Remove the flame as soon as the solder flows.
- Make sure that sufficient flux is applied; otherwise it may burn off before the solder flows.
- Areas not to be soldered can be covered with a boric acid mixture, Prip's flux, or Argotect. These will form a glassy protective layer across the metal.
- Use a different silver alloy: Argentium, Brilliante, and Reflections are all brand names for alternative silver alloys. These alloys are designed to provide similar working properties to sterling silver, but without the copper content.
- Use a reducing flame, i.e., one which has slightly more gas than air. The excess oxygen near the surface of the metal should therefore be burnt rather than oxidising the metal.
- Use a charcoal block as the heating surface, another way to produce a reducing heating environment.
Boric acid
This is a traditional method of preventing oxidation. A saturated solution of boric acid in methylated spirits is used. The resulting mixture should be similar in consistency to single cream, and will need to be given a shake occasionally to keep it mixed. The metal should be dipped into the solution (adhesion will be helped if you gently warm the metal first). As the mixture is highly flammable, only keep a small container near your hearth. Alternatively, a solution of boric acid and hot water can be used.
Argotect
This is a commercially produced alternative to boric acid, and is used in much the same way.
Prip's flux
Prip's flux is another popular solution that can be mixed in the home studio to help prevent firescale.
Ingredients
- 2 pints (1 litre) water
- 4oz (113g) boric acid
- 3oz (85 g) trisodium phosphate
- 3oz (85 g) borax
Boric acid
This is often sold as an insecticide to deal with infestations of cockroaches, silverfish, or ants. It is a skin irritant and the dust should not be breathed in. Repeated exposure will increase the body burden of this chemical.
Trisodium phosphate
Also known as TSP, this is sold as a heavy duty cleaner, degreaser, and stain remover. Pure TSP is a strong chemical and can burn unprotected skin and cause severe eye damage. For many years, TSP has been used as a component in household detergents, however, this use has declined in recent years due to awareness of the environmental damage that phosphates can cause. It is still possible to buy TSP, most commercially available products are supplied at about 12% concentration. Always wear suitable clothing and eye protection. Keep away from your skin by wearing gloves and only use in a well ventilated area.
Borax
Powdered borax is available as an alternative flux to the traditional borax cone.
Mixing
Mix and then bring to the boil to dissolve the ingredients into the water. More water may be added, but it is important to retain the 3:2:2 ratio of the dry ingredients.When the solution cools, some of the chemicals may crystallise as a sediment; shake the mixture before use.
Application
- Gently warm the metal with a torch flame.
- Spray on an even coat of Prip's flux. A standard spray bottle can be used, but the nozzle will clog if it's not cleaned after every use. The best type of sprayer to use is the pipe for mouth blowing that ceramists use to apply glaze.
- Repeat several times until an even coating is built up.
- Avoid applying the flux whilst the work is on the soldering block, as overspray will quickly build up on the hearth surface.
The metal can be quenched in Prip's flux rather then spraying. Again, several coats will be required.
Soldering with Prip's Flux
Prip's flux takes much longer to reach an active temperature, and with care can completely prevent firescale. Another flux intended for soldering should be placed in the joint area itself. The coating of Prip's flux can be maintained for multiple soldering operations provided that the metal is not cleaned in pickle, as this will remove the flux. For complicated soldering jobs, all the the components can be coated once with flux provided that the joins are cleaned at each stage.
