In the Studio: Mastering Mica Update Kate and Will Jacobson

Appears in the Jul/Aug 2018 issue of Pottery Making Illustrated.

Recent collaborations by people using mica with raku and other bare-clay techniques have produced some noteworthy results. While the first discovery of the use of mica in clay came from a natural curiosity, these new works are the result of shared methods and ideas.

Reflective or regular micas were addressed in the March/April 2013 issue of Pottery Making Illustrated. This new update is about the use of interference and selective types of mica. No longer are potters limited to a black crackle on a white or a colored background. Now with the use of these two types of mica, crackle patterns of all colors are available. These micas only show their true colors when placed on a dark or black background. That’s good news for raku potters, since we know how to trap carbon on our pots, creating dark or black areas. These micas work well with naked raku, horse hair, and saggar techniques, as well as the sodium-silicate crackle method.

Interference and Selective Mica

Interference micas are always a single color. They come as a white powder with very little, if any, hint of their color potential. You can easily color test them by rubbing a small amount on a black surface. That will show their approximate fired color. The selective micas will color shift. Some are red or blue when looked at from one direction and then shift to green or yellow when looked at from another direction. They come in all different kinds of color combinations.

When using these smooth micas in terra sigillata, it’s important to start with a very refined terra sigillata. Shake up your terra sigillata very well and let it rest overnight. Then use only the top 25% to mix with the micas. Start with 6–8 grams of mica per cup of terra sigillata. You can increase that amount up to 12 grams, depending upon the size of the mica particles. For the mesh size, 10–60 microns is a good range to go with—that is more or less the standard range for smooth mica powders.

The sparkle and glitter micas are much larger in size and don’t work that well in terra sigillata, but work well when mixed with sodium silicate and then applied to the surface of pots.

Mica and the Sodium Silicate Raku Technique

Sodium silicate can be applied to the exterior of pots, hardened quickly using a heat gun, then (because only the exterior is hardened) the piece can be expanded from the inside, and the surface of the piece will break into a crackle pattern. The idea of using mica in the sodium-silicate raku technique was given to us by David Hodapp. He attended a workshop we gave and we asked him to collaborate with us on using the new interference micas in naked raku. He subsequently tried combining the micas with sodium silicate, then applying that to the pots before drying the exteriors and expanding the pots to create a crackle pattern.

Based on his experience, he has this advice: “Mica in sodium silicate needs to be used quickly after mixing, so prepare small batches, and if you’re going to use it on several pots, cover it well between applications. It generally doesn’t store overnight. Start with approximately 15 grams of mica (regular or interference types) per cup of sodium silicate. For ¼ cup of sodium silicate, use 2½ teaspoons of mica. These pots can then be bisque fired to cone 017–015. Then, heat them to 1450°F (788°C) and reduce. Try reducing for 90 seconds and adjust accordingly. Remove the pots from the reduction chamber and let them cool.”

Mica Sources

We buy most of our micas from TKB Trading (https://tkbtrading.com). They offer an interference Basic 7 package and the interference Extra 6 collection to get a nice sample to start with. For the color shift micas, they have the Shift Collection.

We highly recommend the book, Naked Raku and Related Bare Clay Techniques, edited by Eduardo Lazo, and published by The American Ceramic Society (in which we contributed a chapter) as an excellent source of further information.

Kate and Will Jacobson have been collaborating artists for 40 years. They are widely recognized as early originators of naked raku. Their studio is on the Big Island of Hawaii and they will be teaching workshops in Italy and New Zealand in the Summer of 2018. To see more or learn more visit www.JacobsonArtStudio.com.