The first precious metal clays were introduced in the 1990's - made from pure silver powder and organic binders (Mitsubishi Materials Corp. makes PMC = Precious Metal Clay and Aida Chemical Industries makes Art Clay). The silver clay behaved kind of like a ceramic clay, could be molded, sculpted, carved, extruded etc. The metal clay was then 'fired' in a kiln or by torch, to produce a pure metal item.
Fine silver (.999 pure silver) is a noble metal, meaning that it resists oxidation and corrosion. Many noble metals are also precious metal, gold, silver, platinum. Because it resists oxidation, fine silver can be fused by heat in an oxygen (normal atmosphere) environment such as in a kiln or open torch flame. When particles are heated, close to melting temperature, but not melting, they move close together and become sticky, adhereing to each other. This compaction and partial fusing is called 'sintering'. Sintering produces metals that are more porous and lighter than cast metal, also not as strong. Gold clay later became available, but required higher firing temperatures and was very costly.
Precious metals are costly, and in recent years have grown even more expensive. Many people worked on trying to make 'base metal' clays. Base metals, such as copper, bronze (copper and tin) and brass (copper and zinc) do oxidize and corrode. When heated in an oxygen atmosphere, base metal particles oxidize and do not stick together (sinter). Industrially, metal powders are sintered in special kilns filled with inert gases. This is virtually impossible to do in a home or small studio setting. The inventor of BRONZClay (tm) and COPPRClay (tm), Bill Struve, found a wonderful way to fire base metals in a home kiln. He used activated charcoal in a closed, stainless steel container to produce a low oxygen environment for sintering base metal clay. When heated to high temperatures, the charcoal tries to burn and uses any available oxygen in the area, thus preventing (and even reversing) oxidation of metal particles.
Step 1: Copper clay recipe
1/2 tsp. corn starch
1/4 tsp. powdered xanthan gum (do not breathe in the powder! It forms a VERY gummy slime when mixed with water)
glycerin - a few drops adds a little flexibility
3+ Tbsp atomized pure copper*
distilled water in a small spray bottle.
Wear your mask and carefully mix dry ingredients until you have a uniform mix. No clumps of cornstarch visible.
Spray surface with some distilled water and carefully stir. Add more spritzes of water and keep stirring until the mix starts to clump together. Mix well between spritzes until your mixture starts to feel clay-like. Lightly oil your hands with olive oil or natural hand balm and squeeze mixture. If it holds together then you can knead by hand. If mixture starts to crumble, work in a little more water. If it is very sticky, let it dry a bit and rework, or add a little more metal powder or cornstarch. Copper clay will have a reddish tine, shown in the second picture.
Set clay aside in a sealed container or plastic bag.
Let clay rest for a while and go make your trilobite mold in step two.
* Smallest metal particles are best, but it is hard to get finer than -325 mesh, about 44 microns. Really good if you can find 1-20 micron powder, but it is expensive reagent grade copper. Atomized metal is sold for use with plastic resins to 'cold cast' metal art objects. Check art suppliers, Douglas and Sturgess, or TAP plastics.