How to Oxidize Precious Metal Clay (PMC) with Liver of Sulphur
Finishing Precious Metal Clay Pieces
There are several ways to finish Precious Metal Clay pieces to make them look polished and shiny, brushed and matte, or rough and antiqued. My favorite look is a polished, antiqued finish, and in this article, I will explain to you how I get that finish.
Liver of sulphur is a form of sulphur that is often used to oxidize metal. Copper oxidizes the fastest with liver of sulphur, but it is also used to oxidize sterling silver and fine silver. You cannot use it to oxidize gold.
Precious Metal Clay is made from 995 silver which means that it is made of fine silver or almost 100% silver. Sterling silver is 925 silver and the rest of the sterling is made from copper. That is why sterling silver will oxidize faster than fine silver. For people looking for jewelry that will not oxidize, there is a new silver available called argentium silver. This is made from silver and germanium, and does not tarnish or oxidize like sterling. Argentium silver is still new and rare but can be found in some jewelry.
Liver of sulphur is the easiest way to oxidize PMC, and it is readily available from jewelry suppliers, or anyone that sells Precious Metal Clay. Liver of sulphur comes in a solid or liquid form. If you are new at working with liver of sulphur, I recommend the liquid form. If you are comfortable with liver of sulphur, the solid form is better because it lasts almost indefinitely.
Recommended Source for Liver of Sulphur
Preparing Your PMC for Oxidation
Depending on if you prefer a matte or a polished finish, once you have fired your PMC, you need to either brush the dried binder crust off of your pieces with a wire brush, or you need to polish them with a polishing cloth. You can also clean and polish your pieces with a tumbler.
To tumble Precious Metal Clay pieces, you need a small single barrel tumbler with mixed stainless steel shot. Place your fired PMC pieces and the stainless steel shot into the tumbler, fill the tumbler with water leaving about an inch of air at the top of the barrel. Then add a mild liquid soap. I use dishwashing soap like Joy. You only need to add a small amount of soap. Then cap the barrel, and let it run.
Tumbling PMC will both clean and harden the silver. I usually let my tumbler run for a minimum of two hours with Precious Metal Clay pieces fresh from the kiln. You can let the tumbler run overnight if you wish, it will not hurt the PMC pieces.
How to Oxidize Using Liver of Sulphur
The next step is to oxidize your Precious Metal Clay.
Set out the following things on your work table:
- Paper towels
- A small bowl of clean room temperature water
- A small bowl filled with liquid room temperature liver of sulphur
- A plastic spoon or fork
- The PMC pieces that you want to oxidize
Here is the process:
Either wear gloves or be careful not to touch the liver of sulphur with your bare hands. Although Precious Metal Clay is non-toxic, that is not true of sulphur. I use the plastic silverware to handle my pieces.
Start with a single piece or a pair if they are going to be earrings. Place your pieces on the spoon or fork. I prefer a fork because I get a little more control of the process.
Dip the PMC pieces (on your fork) into the bowl of liver of sulphur. The longer the silver is in the liver of sulphur, the darker they will get. Pieces start out becoming a pale yellow, then in this order, darker yellow, orange, red, purple, blue, gray, black. The easiest way to slow down the process is to dip the PMC into the liver of sulphur for a short period of time, then dip it into the clear water to stop the oxidation.
Take a look at the pieces, then if you want to continue, repeat the process. Keep dipping the Precious Metal Clay until you achieve the color you want, then stop the oxidation by dipping in the clear water, and place on the paper towel to dry.
After Oxidizing the Precious Metal Clay
If you do not like your results, you can do two things:
- Repolish the piece.
- Reheat the piece in the kiln or with a torch.
Then you can start over with the liver of sulphur.
If you do like the results, then you are ready to protect the finish. I like to use an old jeweler's trick of applying a thin coat of beeswax to protect the finish. To use the beeswax, dissolve white beeswax in naptha. Both beeswax and naptha can be purchased from an old fashioned hardware store, or from Amazon. I am listing the Amazon links on this article.
Get a cheap child's paintbrush, dip it in the thinned beeswax. Paint the PMC piece with a thin coat of beeswax. Let the wax harden for about 1 minute, then wipe it off with a paper towel or rag. This thin coat of beeswax will protect your liver of sulphur oxidation for a very long time, even on a piece that is worn often.
Buy White Beeswax on Amazon
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