Friday, 24 May 2013

Pickling and Quenching

Wear thick rubber gloves when you prepare a pickle that contains acid and always add the acid to water not the other way around. Pickling is the word used to describe chemically cleaning a piece of metal after it has been heated. Quenching describes the cooling of metal after heating. Both processes can be carried out in a variety of ways. 

 

Pickling

While they are heated, brass, copper, nickel silver, silver, and golds up to and including 18 carat will oxidize to different degrees. The oxidization appears on the surface of the metal as a grayish-black film, which is removed chemically in an acid solution. Oxides can also be removed with wet and dry papers, fine files, and so on, but this also removes minute amounts of metal, and chemical methods are, therefore, preferred.

Dirty or greasy metal must be annealed and then pickled in an acid solution to clean it before soldering can take place. After soldering, use pickle to remove any residual flux because if it is not removed immediately, the flux becomes crusty and is awkward to remove by hand.

Quenching

Metal can be quenched either in water or in a pickling solution. You should quench in water if you are using either alum or a hot pickle solution.

After annealing or soldering, allow the metal to cool slightly before quenching. If you do not, you may cause stresses within the structure of the metal that will distort it, and the pickling solution will spit violently and give off fumes.

Leave copper and silver for a few seconds, then use a pair of brass tweezers to place the hot metal in the pickling solution. Remove any binding wire that was used to hold pieces together for soldering. If you place binding wire or steel or iron tweezers in a pickling solution containing sulfuric acid, you will contaminate the solution and everything that is subsequently placed in it.


Safety Procedures
Most pickling solutions contain acid, and they are, therefore, potentially dangerous. When you are mixing a pickle, take a few obvious precautions:

• Make sure that you have easy access to running water in case of spills.

• Always store bottles containing acid where they cannot accidentally be knocked over and, ideally, in a lockable cupboard. Make sure that all bottles are clearly labeled.

• Wear rubber gloves, safety glasses, and, preferably, a heavy-duty apron or coverall.

• Work in a well-ventilated room or even outdoors because acids give off pungent fumes. 

• Always add acid to water; never add water to pure acid.

Before quenching gold, check the technical specifications that will have been supplied with the metal when you obtained it from the dealer. Some golds do not require quenching at all, and others must be quenched below specific temperatures.

Preparing and using pickling solutions

If you spill sulfuric acid, even when it is diluted, onto clothing, a hole will appear after the next washing. There is nothing you can do about this, which is why you should wear a thick apron or, at the very least, old clothes. If you get any sulfuric acid on your skin, rinse it under clear running water; it will leave your skin feeling sore and inflamed. Nitric acid solution splashes on your skin will cause black stains and make your skin feel itchy. Again, rinse the affected area under clear running water.

A suitable solution for pickling silver, gold, or copper can be made from I part sulfuric acid to 10 parts water. For pickling gold, use a solution of 1 part nitric acid to 8 parts water. Keep this separate and use it only for gold, because if you clean gold in a solution that has been used mainly for silver, a silver deposit may be left on the surface of the gold.

Sulfuric-acid solution can be used hot or cold. Mix it in an ovenproof glass bowl or pitcher with a lid to prevent evaporation. If you are using a warm pickle, place the bowl containing the solution in a second bowl of warm water, which is then heated. Do not place sulfuric acid in a pan or bowl that is subjected to direct heat.

It is possible to obtain special hot pickle tanks made of reinforced plastic. The outer casing containing the water has a thermostatically controlled heater, while the acid solution is held in an inner section.

Alum pickle

Alum is most often used as an astringent or paper mordant, but it can be used to clean and pickle metal. It acts more slowly than an acid solution, but is safer for use in the home workshop. Make a solution of 1½ ounces in 1 pint of warm water. Mix it either in a fireproof bowl or a pitcher with a cover so that it can be kept warm over a gentle heat or in a ceramic slow cooker.

Quench heated work in cold water, then place it in the warm alum and leave it until clean.

Commercial pickling solutions

If you leave binding wire on a piece of silver that is being pickled, the metal will turn red when the iron in the wire reacts with the acid. Anything subsequently pickled in the solution will be contaminated.
Your tool and equipment supplier will stock a ready-made commercial pickling solution. Read the manufacturer's instructions carefully before use.

"Spent" pickle

After it has been used several times, pickle becomes "spent" that is, inactive. This will be obvious as the color turns deeper and deeper blue and the pickle takes longer and longer to work.

When you dispose of spent pickle, turn on the cold faucet and let it run for a while before slowly pouring away the pickle. Leave the water running while you do this and for a short time afterward. Never pour away fresh acid in this way.

Writer - Jinks McGrath 

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