In the right-hand pane, under Hadar’s Clay™ Metal Clay Powder – Copper and Bronze, you will find a link to download an Instruction Manual for Hadar’s Clay. This manual consolidates most of the information that has been posted here sporadically. The other, topic-specific files are still there, but have been updated. Below are some highlights from the manual and some new information.
Mixing the Clay
I have noticed how some of my students mix the clay. Sometimes a rolled layer from freshly mixed clay looks like this:
The white spots are grains of binder that have not comletely dissolved. If you see them, continue rolling the clay until they disappear.
Copper and bronze clay shrink about 10% while they are drying. That causes flat pieces to warp, even if you dry them in the air. Don’t leave flat pieces to dry unattended. You can safley dry them on a heating pan at 200-250°F (95-120°C). They will start to curve within seconds. Turn them over. They will soon start curving to the other side. Keep flipping them over until they stop. The whole process is very quick.
Alternativeley, once the top side has started to dry, tape it down to your drying surface.
If pieces have already warped, you can wet them so they soften and can be flattened again under heavy objects. However, this is a messy procees. If you leave them in the refrigerator for half an hour, they will regain some flexibility, and you may not need to rehydrate them.
Flat bronze clay pieces shrink about 25% after firing. Flat copper clay pieces shrink 15%.
Three-dimensional pieces shrink very little. Bronze shrinks a little more than copper, while both shrink less than low-shrinkage silver clay.
The diameter of the bronze circle above shrank from 20mm to 15mm, while the diameter of a copper piece shrank from 20mm to 17mm.
Pitting (as opposed to blistering) is common, especially with bronze clay, and often on the back of pieces. It happens with copper as well, although at a higher temperature. The best way to avoid it is to fire all pieces, even flat ones, positioned vertically in the carbon.
The heavy bronze piece was fired with its back on the carbon. Note the carbon granules that got stuck in the pits. It seems that the softness of the metal when it’s hot, combined with the weight of the piece, caused the carbon granules to penetrate the back side. If pieces are fired vertically, weight is not an issue.
Blistering refers to “bubbles” the sometimes show on the surface of the metal after it has been fired. It is caused by overheating, and will happen to bronze at a lower themperature than copper. If it happens consistency, you need to lower the hold temperature at the second phase of firing.
If the surface is not textured, these “blisters” can be eliminated by grinding them down with grinders or sanding bands, starting with 80 grit.
Silver clay is easy to repair. The fresh low-shrinkage clay applied to fired silver seems to stick pretty well. This is not the case with bronze and copper clay. Fresh clay doesn’t stick to fired metal, and if it does, once it’s dry it tends to separate and peel off.
Once you have finished applying the mending clay, don’t wait for it to dry. Stick it wet into the carbon. You don’t have to fire right away. The repair clay will stick just fine.
Out of curiosity, I cut copper and bronze clay circles and stuck them wet into the carbon, right before I started to fire. They came out just fine. Go figure!