I have been fooling around with my special blend of home made bronze metal clay for awhile and have made some beautiful things but have gotten inconsistent results. Then I moved up to a Paragon Max 119 kiln so I could fire cone 6 pottery as well as glass and metal clay. I also started trying to make hollow forms in the bronze clay. I was having problems with under firing and over firing, so I needed to tweak my firing schedule. I found this article by Mardel Rein to be invaluable.
Here’s a pan of unfired bronze clay before kiln firing. I prefer to fire in these heavy, shallow stainless steel pans I get from my local Asian supermarket. I find that the more you use them, the less they flake.
The ring on the right is perfectly sintered and not overfired. The one on the left, from an earlier firing, is over fired.
Here are a couple of hollow beads. The one on the left has been repaired. The one on the right has not. I have found that you must put hollow forms through two firings. The form will come out of the first firing looking sintered, but will break if you drop it or hit it with a hammer. I save up beads that have made it through one firing and put them through the next firing with whatever else I have. I don’t plug the holes and I don’t construct screen cages to fire them in. I just cover them with carbon and whoop-de-do. You can’t use cork clay to make hollow beads from bronze clay, because you will never be able to get the cork fired out in an oxygen-free environment which is what you create when you fire in activated carbon. But if you can construct a hollow form with holes and get it through two firings, you should be able to bounce it on concrete without it breaking. In theory.
The piece on the left has been over fired. The pieces on the right went through a later firing and the tip of one broke off. Rather than try to reattach it, I just sawed the other tip off and will design something around the new shapes.
The piece on the left broke in the middle during an earlier firing and I repaired and refired it. The piece on the right is made up of broken sintered and unsintered pieces from earlier firings for a kind of mosaic pendant.
These pieces went through one firing schedule and sintered perfectly. What I learned from all my experiences is that when you have thicker pieces, the trick is not necessarily to fire hotter, but to ramp up to temperature more slowly. I started out firing to 1550 and holding for two hours. Then I tried two and one half. Then I tried three. Thin pieces were over firing, but hollow beads were breaking. Then I tried lowering the temperature to 1500. A little better but same problem. Then I read that a slow ramp worked best with bigger pieces. I tried ramping at 250 degrees F to 1000F, holding one hour, then ramping on full to 1500F and holding for three hours. That did the trick. All the single layer pieces are coming out fine. I take the hollow ones out and put them in the next kiln batch through the firing cycle a second time, and they have been fine so far. You have to experiment to find out what will work for you.