The generic term pickle is used to refer to a dilute acid solution which will remove oxides produced by annealing and soldering as well as glassy flux residue.
Safety pickle
Many suppliers market an acid salt called sodium bisulphate as safety pickle. This is not corrosive if kept dry, making it a safer alternative to traditional acids. However, it becomes corrosive when wet so avoid damp storage conditions.
Usage
To mix, add approximately 50g of powder to 1 litre of hot (50°C) tap water in a container capable of resisting acids. Stir with a wooden spatula. The pickle should be heating to about 50°C, and this temperature maintained for rapid removal of surface oxides. However, if this is not possible, the pickling process will still work at a slower rate. A slow cooker can be used for the container, these are ideal as they have an integrated heating element and temperature control. Always use plastic, brass or coppers tweezers to remove items from the pickle solution. If steel tweezers are used, the pickle will act as an electrolyte and cause any copper dissolved in it to be plated over the surface of your work. Make sure that any fragments of binding wire are removed for the same reason. Try not to leave your work in the pickle overnight. Doing so may result in the solder joins becoming pitted as the pickle attacks the nickel in the solder.
Health & Safety
- As with any container used for chemicals, clearly and permanently label your pickle pot so that it can never be used for food preparation.
- Always add acid to water, not vice versa. Failure to do so may result in an exothermic reaction and the concentrated acid could spit onto you.
- Do not quench hot work in the pickle, the solution could spit and release vapour.
- Boiling should be avoided as this will release vapour. A maximum working temperature is around 80°C.
- Keep the container covered, avoid breathing the fumes, and use in a well ventilated area.
- Avoid skin/eye contact - if this does occur, rinse immediately with plenty of water.
- If you accidentally ingest any pickle, drink a plenty of water and rinse your mouth out. Do not induce vomiting.
- When handling, wear protective mask, gloves, goggles and non-absorbent clothing.
- Refer to the manufacturer's safety data sheet.
Disposal
Over time, the pickle solution will become saturated with dissolved metals. This will be evident as the solution will gradually take on a deep blue/green colour. Although the pickle will still work, eventually you will want to replace the solution. Add a large amount of sodium bicarbonate to the cold pickle. Once the fizzing has subsided, it may be flushed down the drain with plenty of tap water. If this is not possible in your location, then the water may be left to evaporate; you can contact your local council to dispose of the remaining waste.
Alternatives
- Sulphuric acid - This is a traditional pickling agent, and should be mixed 1 part acid to 9 parts water. Older textbooks may refer to it as Oil of Vitriol. In concentrated form, this is far more corrosive than safety pickle. Consult the manufacturer’s safety data sheet for advice.
- Alum - This can be bought from good chemists. Many Indian and Middle Eastern shops sell this as a cooking ingredient. Dyeing suppliers also sell this as a mordant used in the vegetable dyeing of fabric.
- Citric acid - This can be bought from a variety of sources such as good chemists, Asian food shops; or suppliers for baby sterilisers, winemaking, or homemade soap / bath salts. As this is an organic substance, mould may grow on the surface after a week or two.
- Vinegar and salt - This is the slowest acting pickle. This may not be an issue for the amateur, and the ingredients are readily available.
