The shortened firing schedules of Quick-fire copper and bronze clay may result in some fire scale, especially if pieces are not taken out of the kiln as soon as the firing is over. The reason is that by the end of the firing there is not enough carbon to protect them from oxidizing. This oxidizing is external and does not compromise the strength of the pieces, but sometimes it can be hard to remove by just buffing or sanding.
One way to avoid this is to take the pieces out of the kiln when they are hot and cool them in water. By “hot” I mean that you can absolutely wait for the kiln to cool down to about 900°F before you do that.
But if fire scale does occur, there are a few easy ways to remove it. Personally I don’t like to use pickle, but if you want to, a very mild and effective one is citric acid. (Citric acid is actually used to make pickles and exists in many food products and almost every soft drink, as well as naturally in the juice of citrus fruits.) You can get it at Mediterranean food stores. It’s also available at wine-making and beer-making supply stores, as well as at some supermarkets. It’s very inexpensive.
Here are two alternative methods that I prefer:
1. If a piece is small enough, you can heat it with a torch to red hot and immediately quench in water. The fire scale will disappear.
If you choose to do this, there are two things to keep in mind:
a. It is not recommended to do this with pieces that were fired by the “hot firing” method (in the air). It is also not recommended to repeat it more than twice or so since the copper is weakened by air firing and will eventually disintegrate.
b. After dipping in water, the piece must be completely dried on a warmer before you handle it any further. If you try to buff or sand a wet piece, you will not see metallic color and you may think that it is not properly sintered. It is not enough to dry it with a paper towel; the metal is porous and holds a lot of liquid inside, even if it looks dry on the outside.
2. Put the pieces back in the carbon. Heat the kiln to 1000°F and hold for 15-30 minutes. You can take them out hot or cold. The fire scale will be gone. This time there will be very little ash and there is no need to discard the carbon.
This piece was taken out of the carbon hot after re-firing for 15 minutes at 1000°F and cooled in water. The fire scale disappeared completely from the copper part.
I used a coarse mini-fiber wheel to clean up the steel part. It was quite easy.
With the spinner bead, I waited for the carbon to cool down and did not cool the piece in water.
Again, the fire scale was gone.