After going over the test results of the experimental pieces I did at the last saggar fire, I learned a few things. Some are probably obvious to those more experienced than I.
For any color at all besides black, some kind of metal salts/oxides are required. I probably should have realized this, but I was hoping perhaps some of the trace minerals in certain organics would be strong enough on their own, trapped under the aluminum foil. They weren’t. Their main function seems to be to influence the colors/shades of any major metals already present. Ferric chloride, copper carbonate, and copper sulfate are all good starting points.
Pieces of broken off pinecones give a nice dark black. If you want good blacks, pinecones and needles are a good bet. It also doesn’t take much, start conservative.
Brushing on ferric chloride works just as well as spraying. And it’s a LOT safer, in my opinion. Airborne ferric chloride is just bad news. You still want to wear plastic gloves and be very careful. It’ll also ruin your brush, so use a cheap one, foam works well. I want to experiment with building up varied layers of diluted FeCl in the future.
Tinfoil saggars are the way to go, in my opinion. Comparing with other work from clay saggars, and pit firings where some was wrapped in foil the same way and some not, the colors just seem to come out more intense. You can also achieve good results with much less raw material since you don’t have to fill the space between the saggar and the pot. That’s just my personal taste. Some people like more delicate, muted coloration and that’s cool too. And despite reading in several places warning about not firing past the melting point of aluminum (1100F), I’ve never seen or heard of a problem with it. The aluminum foil doesn’t melt, it simply burns away. I think the regular weight stuff works fine, but experiment with some heavy duty and see what you like.
Terra Sig takes the color just as well as burnished or unburnished clay. I thought that maybe since it is a tightly packed surface of smaller platelets it would be less permeable to the smoke and fumes, but it didn’t seem to be a problem. Your clay, however can make a big difference. Earthenware bisqued to cone 06 is much less permeable than a mid/high fire stoneware bisqued to cone 06. The colors are still very nice, but are lighter and more delicate. Again, lots of room to experiment.
More detailed instruction is available in Alternative Kilns and Firing Techniques by Watkins and Wandless. It’s chock full of ideas, many of which can be mixed and matched between the different techniques. This one will really get your imagination flowing.
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