Tuesday, February 23, 2010

Copper Clays - Firing Tests

For a while now I have been wanting to try the new copper clay and finally broke down and got some. There are two different kinds, each with different firing requirements. Not knowing much about either, I decided to try both out and experiment with the firing options.

The two types are COPPRClay and ArtClay Copper. I bought the smallest packages of I could of each, made pieces out of each and tried both firing options with each. Details for each clay, including their firing requirements can be found at the links provided above. Here is what I found:

COPPRClay came in a 30 gram package. It has the longer and more complicated firing schedule and should be fired surrounded by activated carbon (the full firing schedule is available at the link above). It was darker in color than the Art Clay Copper and the manufacturer recommends storage in the refrigerator. Even after drying, the clay was bendable and felt more like PMC in it's leather hard state.

ArtClay Copper seemed to dry out less quickly. It came in a larger 50 gram package. It is not recommended that ArtClay Copper be stored in the refrigerator. Both can be dried on a warming plate, just like PMC. When dry, ArtClay Copper is lighter in color than the COPPRClay and feels more like PMC in its greenware state. The manufacturer recommends storing ArtClay Copper in a vacuum sealed bag (like the ones you can buy for food) and then placed in another bag with a moist paper towel. I don't have a vacuum sealer and I wasn't going to buy one just for this so I put the clay in a ziplock bag, pushing out as much air as I could, and then putting that bag inside another bag with a wet paper towel. I haven't used it again, so I don't know how well it retained it's moisture. That will be a subject for my next blog post.

I tried the COPPRClay firing schedule first, but used pieces of both kinds of clay to see how well they fared comparatively. That schedule requires first firing at a slow ramp on a kiln shelf to burn off the binder and then another slow ramp firing with the pieces embedded in the carbon (to create an oxygen-deprived environment and prevent black fire scale). Because of the slow ramps and the cooling in between, this process took me a good part of the day (I mean it took my kiln - I only had to program it and wait). It made my kiln quite messy and darkened the shiny stainless steel firing pan. I don't know if this will happen each time I fire or if it was only because it was the first time using the carbon.

After digging the pieces out of the carbon, the ArtClay Copper was shiny copper. The COPPRClay looked more like terracotta at this point. Once I brushed it with a brass brush, it took on more of a coppery sheen. The COPPRClay shrunk more than the ArtClay Copper and seemed bendier and less strong than the ArtClay Copper piece.

I next tried the firing instructions from the ArtClay Copper, with pieces of each type. The firing time isn't nearly as long - a hold time of 30 minutes. The directions suggested placing the pieces into a pre-heated kiln and also taking the pieces out while still red hot and quenching them. The instructions said this would remove the black coating that forms on the outside of the clay (the whole reason the COPPRClay is to be fired in carbon is to avoid the black coating). It does remove the black coating - at least mostly - and a few minutes in warm pickle removes the rest. The ArtClay copper did quite well with this firing, but the COPPRClay didn't survive. The piece was pretty much all firescale and it crumbled during the quenching process.

Below are photos of the three pieces that survived. All that has been done to them at this point is brass brushing (other than pickling the one marked ArtClay Copper b). The two pieces on the left are the pieces fired with the first (COPPRClay) method and the one piece on the right was fired with the second (ArtClay Copper) method.

The results and my opinion? Well, certainly the firing schedule for ArtClay Copper is much simpler, and it has been advertised that it can be torch fired (which I haven't tried), but the black fire scale coating that came off during quenching was quite thick and left a considerably thinner piece of copper. On the other hand, the ArtClay Copper came out of the first process just beautifully colored with no brushing needed (although brushing shined it nicely). It is stronger and less pliable than the same thickness of COPPRClay fired the same way, and substantially thicker than the ArtClay Copper fired with the ArtClay method.

So at first glance, I like the ArtClay Copper better, except that I prefer the way it comes out with the COPPRClay firing method. I haven't worked with the pieces after firing much, except to brass brush them and I will be anxious to see how actual jewelry pieces will come out using it.

To be continued...


  1. WOW-- you're a mad scientist!! I love it!!

  2. Lisa!!!! I know they're just "test subjects", but I love the way they look! Especially the first Art Clay one---it looks like shiny penny!! I can only imagine the beautiful pieces of jewelry you'll come up with using this clay!


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