I’d like to first dispel a myth about Precious Metal Clay. Precious Metal Clay (PMC) is NOT some magical clay material that mysteriously turns into silver. In fact, it is quite the opposite. This material was developed in Japan in the early 1990s, by Dr. Morikawa of the Mitsubishi Company. Pottery has been an important art form throughout the history of the Japanese culture. In an effort to join the art of jewelry making with the art of ceramics, Dr. Morikawa, along with a team of scientists, developed Precious Metal Clay. It is silver and gold in a form that is very unique from the conventional hard sheet and wire that metal workers have used for centuries. During the manufacturing process of PMC, tiny particles of silver and gold are mixed with an organic binder and water. This is how the pliable, clay-like consistency is achieved, and where the name Precious Metal Clay comes from. The basic process for working with it is as follows. A piece is created with PMC, and is set aside to dry. After the piece is completely dry, it is fired in a kiln. During the firing process the binder burns away, and what remains is pure metal; fine silver, and high karat gold. After firing, the piece is cooled and can be finished in a variety of ways.
PMC is non-toxic, is an incredibly versatile material and can be worked using many techniques. For example, in its soft form, it is worked with the hands much like earthen clay. It can be molded, shaped, joined, blended, textured, pushed, pulled, stretched, twisted, rolled, coiled, flattened, pierced, and cut. When it is dry, and before it is fired, PMC can be sanded, carved, hand drilled and pierced. After it has been fired it is pure metal and can be worked using metalsmithing tools and techniques, such as soldering, hammering, texturing, filing, bending, etc.
PMC can also be combined with other materials such as precious and semi-precious stones, glass, polymer clay, and ceramic shards, to name a few. Some of these materials can be embedded into the PMC and fired, others are combined or added after firing. For example, there are certain stones that will withstand the lower firing temperatures of PMC. This allows a piece to be completed, stones and all, before it is fired. Stones that cannot withstand the firing temperatures are set after firing.
PMC is the perfect metal to accept the application of vitreous enamel. After its initial firing, PMC can be re-fired any number of times, as long as the highest allowable firing temperature is not exceeded. The maximum recommended temperature of 1650°F is well above those needed for enamels. PMC also fires clean, meaning that between applications of enamel there is no oxidation to remove from the exposed silver.
There are currently three formulations of silver PMC, and two of gold ,each with their unique characteristics. All of them will accept applications of enamel. Standard PMC is the original formulation, and is fired for two hours at 1650°F. PMC+ (PMC Plus) is the second generation formulation, and can be fired in a temperature range from 1470°F to 1650°F, and a time range of ten minutes to two hours. PMC3 is the third generation formulation and can be fired as low as 1110°F, and as high as 1650°F, also with a time range of ten minutes to two hours.
In addition to these three formulations, the silver PMC is available in several forms: lump, syringe, paste, and paper thin sheet. The gold comes two ways, in lump form and in a paste called Aura 22. The creamy consistency of Aura 22 makes it perfect for use in a painterly way. It can be applied with a brush or clay shaper directly onto fired silver PMC, or to enamels, or even to copper. It is also particularly suited to highly textured work, or pieces where the application of gold by alternative methods would be difficult.
Properties of PMC
There is a shrinkage factor with PMC. This is advantageous in that a piece is made larger than the desired finished size. As it shrinks during firing, the detail becomes more enhanced and beautiful! Standard PMC is 26% water and binder, which means that, as the binder burns away during firing the piece will shrink about 28%. PMC+ and PMC3 are 10% water and binder, and shrink approximately 12%. There is a slight variation in shrinkage, plus or minus 3%, depending upon the firing temperature and the length of firing time.
The tools for working with PMC are simple, inexpensive, can be found around the house, and are as varied as your imagination. Virtually anything that can be used to cut and texture clay can be also be used with PMC. Plastic and metal tools are best as the PMC can be easily cleaned from them. Metal tools, however, should not be in contact with the PMC for extended periods of time. For example, to cut a shape with a metal clay cutter is fine. But to lay a sheet of PMC over a piece of metal in order to shape it is not recommended. The PMC would be in contact with the metal while it was drying. This extended length of time could cause the silver in the PMC to have an adverse reaction with the metal it is laying on.
Since PMC needs to be fired to transform it from the soft, pliable stage to the hard metal stage, a kiln is essential. Programmable kilns are ideal. There are some made for PMC that can also be used for enameling. Enameling kilns and manual kilns that are equipped with a pyrometer can also be used. There are three main points to consider to ensure consistent, successful firings: temperature, time, and uniformity. The kiln must be able to hold a consistent firing temperature for the entire firing time. Under-fired PMC is weak and will break. The times and temperatures listed in the chart below are minimum requirements. If there is a question as to if a piece has been under-fired or not, simply re-fire it at the required temperature and time. Also, there should be no cold or hot spots within the kiln chamber. A cold spot could have the same effect as being under-fired. A hot spot could cause over-firing. This point is most critical for Standard PMC. It has only one firing time and temperature. The varied time and temperatures for PMC+ and PMC3 allow more leeway.
Standard 1650°F 1000°C 2 hours
PMC+ 1650°F 1000°C 10 minutes
1560°F 850°C 20 minutes
1470°F 800°C 30 minutes
PMC3 1290°F 700°C 10 minutes
1200°F 650°C 20 minutes
1110°F 600°C 30 minutes
PMC+ and PMC3 can also be torch fired.
Gold 1650°F 900°C 10 minutes
1560°F 850°C 30 minutes
1380°F 750°C 60 minutes
1290°F 700°C 90 minutes
Gold PMC can also be torch fired.
Aura 22 Gold
Apply to clean silver PMC or copper, let it dry, then fire in the kiln at 850°F (450°C) for 30 minutes.
Keum-boo method – Set the PMC piece onto a hot plate and heat to medium high. To test the temperature, touch a piece of wood to the surface of the hot plate. If it chars the hot plate is ready. Slightly reduce the heat and burnish the gold with a steel or agate tool.
Torch fire – Set the piece on a solder board. Gradually heat it to a glowing red color. Remove the flame; at this point, fusion has occurred.
After firing, PMC is hard metal, either silver or gold. The silver has a white, matte look to it as it comes out of the kiln, and the gold has a yellow, matte look. This is due not to any lingering residue, but to the way light is reflecting off of the metal. During the firing process the particles realign themselves in a somewhat random, jagged order, much like a mountain terrain. During the finishing process these particles are pressed down into a more flat, uniform existence, causing the light to reflect differently. As the PMC is rubbed with a wire brush a matte or satin finish emerges. To achieve a high polish PMC can be hand polished, burnished or tumbled. Once the PMC piece has been fired and finished, it is ready to be enameled.
Next article: Designing PMC Pieces for Enamel
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