A little bit o' history
Dec. 3rd, 2015 08:49 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
offcntrSaggar. An enclosed vessel used in firing to contain a specialized atmosphere and the pots within it. A saggar can be as small as a covered jar or large as a brick box, chinked with clay and topped with kiln shelves.
Saggars were originally used in Chinese pottery to protect delicate celadon glazes from the ravages of wood ash. Entire chambers of the kiln would be stacked with ceramic boxes, each forming the lid of the one beneath it. Common peasant ware would be stacked around them, unprotected from the ash deposits and drips we so cherish now days.
In Europe, where they used sulfurous coal as a kiln fuel, saggar firing developed into the Muffle kiln where the entire stacking chamber was sealed off, isolated from the corrosive flame.
In modern times, with clean gas or electric firing kilns, we've reversed the purpose. Saggars contain dirty firing atmospheres within our clean kilns. Pots are nested in sawdust or charcoal, draped with seaweed or salt-soaked straw. Bits of copper wire or steel wool are strewn on top. Marvelous, atmospheric effects can be achieved at low temperatures.
In high-fire, saggars are often used to salt or soda fire in a kiln not normally devoted to that purpose. You have to be careful how much you use, though. A friend of mine in my Craft Center days tried a saggar full of salted peanut shells, and the resultant foamy mass of salt-and-ash glaze rose up and engulfed his poor little pot.
Salt fire, soda fire. At sufficiently high temperatures, sodium-bearing materials like salt, soda ash or baking soda will decompose into sodium vapor, a flux that will combine with silica on the surface of the pots to form a glaze. Clay color, amount of soda deposition, variations in kiln atmosphere and flame direction all will influence the color and texture of the glaze in unpredictable ways. Salt is the more traditional material, but urban kilns where a chlorine smell might be offensive favor a soda ash/sodium bicarbonate mix. (There are also glaze variations from the materials that I don't plan to get into.)
Because the soda is in the kiln atmosphere, it also deposits on brick, shelves and posts, with the result that a kiln once used for soda is always and ever after a soda kiln. Soda is corrosive to soft brick, so hard brick is generally preferred for the hot face on soda kilns.
Wadding. Left to its own devices, soda vapor would weld pots to the kiln shelf. To prevent this, ware is stilted on small wads of friable (crumbly), soda resistant material. The mix may include clay, alumina, fine grog, calcium carbonate, sawdust, flour or other material. Wads are usually applied wet while loading the kiln, though some potters dry them and glue them onto the pot beforehand. After firing, they're crumbled away.
Saggars were originally used in Chinese pottery to protect delicate celadon glazes from the ravages of wood ash. Entire chambers of the kiln would be stacked with ceramic boxes, each forming the lid of the one beneath it. Common peasant ware would be stacked around them, unprotected from the ash deposits and drips we so cherish now days.
In Europe, where they used sulfurous coal as a kiln fuel, saggar firing developed into the Muffle kiln where the entire stacking chamber was sealed off, isolated from the corrosive flame.
In modern times, with clean gas or electric firing kilns, we've reversed the purpose. Saggars contain dirty firing atmospheres within our clean kilns. Pots are nested in sawdust or charcoal, draped with seaweed or salt-soaked straw. Bits of copper wire or steel wool are strewn on top. Marvelous, atmospheric effects can be achieved at low temperatures.
In high-fire, saggars are often used to salt or soda fire in a kiln not normally devoted to that purpose. You have to be careful how much you use, though. A friend of mine in my Craft Center days tried a saggar full of salted peanut shells, and the resultant foamy mass of salt-and-ash glaze rose up and engulfed his poor little pot.
Salt fire, soda fire. At sufficiently high temperatures, sodium-bearing materials like salt, soda ash or baking soda will decompose into sodium vapor, a flux that will combine with silica on the surface of the pots to form a glaze. Clay color, amount of soda deposition, variations in kiln atmosphere and flame direction all will influence the color and texture of the glaze in unpredictable ways. Salt is the more traditional material, but urban kilns where a chlorine smell might be offensive favor a soda ash/sodium bicarbonate mix. (There are also glaze variations from the materials that I don't plan to get into.)
Because the soda is in the kiln atmosphere, it also deposits on brick, shelves and posts, with the result that a kiln once used for soda is always and ever after a soda kiln. Soda is corrosive to soft brick, so hard brick is generally preferred for the hot face on soda kilns.
Wadding. Left to its own devices, soda vapor would weld pots to the kiln shelf. To prevent this, ware is stilted on small wads of friable (crumbly), soda resistant material. The mix may include clay, alumina, fine grog, calcium carbonate, sawdust, flour or other material. Wads are usually applied wet while loading the kiln, though some potters dry them and glue them onto the pot beforehand. After firing, they're crumbled away.