Bronze Clay and Copper Clays
One big note on these clays: That black stuff that often appears on these clays after firing is nothing more than oxidation: a chemical process that occurs when base metal clays are exposed to oxygen. To clean off the oxidation you can either use a rotating bristle brush 
or drop it (after quenching it in water) in a pickle solution. If you don’t know what pickle is, About.com has a nice little article on it. A safe alternate to the commercially available pickles is to mix 1/4 cup of citric acid per a quart and 1/2 of distilled water. Sugarcraft.com (look under hard candy ingredients) sells a one pound jar of citric acid crystals for about $6.00. You can also find it in health food stores or chemical supply houses (ask for reagent citric acid). If you heat it in a crock, potpourri pot or stove top in a Pyrex baking dish (or other stove safe glass or pottery container) - ventilate. You can also use undiluted, reconstituted lemon juice. Heating it will make it work faster. As I’ve never used the lemon juice, I’m not sure of the concentration – experiment. Don’t forget to neutralize the pickle (whichever type you use) by boiling it in baking soda (small handful) and water (two cups) for a little while. Baking soda is a base and will neutralize acid. If you don’t neutralize the pickle – the acid will continue to work and will eventually dissolve your work. Not such a great idea! Rinse off the baking soda!
Timelines
2004 – Hadar Jacobson experiments with the first bronze and copper clays.
mid – 2008: The first version of Bronze clay was developed by Bill Strueve of Metal Adventures in .
2009: Metal Clay Adventures brings out Copprclay.
2009: Prometheus clay arrives.
September 2009: Aida Chemicals copper clay arrives (Art Clay)
December 2009: Hadar Jacobson launches her powdered versions of Bronze , Copper and Stainless Steel Clays.
Early 2010: Hadar Jacobson presented White Bronze Clay
June 2010: Hadar Jacobson brings out Pearl Gray Steel Clay
July 2010: Bill Strueve of Metal Clay Adventures releases Fast Fire BRONZclay.
Want to make your own clay? Yeah, really, you can make it at home! Here’s instructions from Del Feast at the Metal Clay Academy’s site.
Metal Clay Adventure’s Clay
Bronze clay hit the scene at the 2008 PMC conference. BRONZclay is 11% tin and the other 89% is copper, water and a “non-toxic” binder (as we don’t know what it is, it’s hard to tell). BRONZclay is available at Rio Grande, Metal Clay Supply, Cool Tools, Whole Lotta Whimsey, New Mexico Clay, among others. Celie Fago has a great story to tell about her first encounters with BRONZclay here.
Recently, Metal Adventures came out with a quick fire version (which they are calling “fast-fire”) of BRONZclay. The big difference here is, obviously, firing time. Fast fire BRONZclay can fire in as little as two hours compared to the 4-9 hours required by regular BRONZclay. That’s a substantial savings in time but, it must still be fired in carbon and in a stainless steel container. 
It’s also available at Rio Grande Jewelry.
There is also a Copprclay by Metal Clay Adventures and also available at Rio Grande, Metal Clay Supply, Whole Lotta Whimsey, New Mexico Clay, Cool Tools . This clay must be fired in a coconut shell-based carbon ONLY!
Hadar’s Clay
Hadar Jacobson has developed a Bronze Clay, Cooper, White Bronze, Steel and Pearl Gray Steel in powdered form available at her Textures store.
Prometheus Clays
I’ve been having trouble finding out much about the company that produces Prometheus
clays. All that I can find is that they are from Greece. Odak, a Greek company carries their clay and has information on it’s use. You can purchase Prometheus Bronze and Copper Clays at PMC Connection.
Check out the great information on using BRONZclay from Cool Tools, written by Mardel Rein. Rio Grande has put out a PDF on working with their Bronz and Coppr clay. It’s a great source of information. On the bottom of Rio’s regular BRONZclay order page there is a short video. Art Jewelry Magazine also has much knowledge to share on its blog for February 2009. The Metal Clay Academy is a great source of information on all things metal clay.
Bronzclay videos to watch are from: Cool Tools and Tonya Davidson’s video from Whole Lotta Whimsey. Hadar Jacobson has a video on how to process her clay. There are many more out there now – just GOOGLE “bronze clay videos” – it will keep you busy for a while!