Monday, October 1, 2012

October 1st Hint of The Week


HINT FOR THE WEEK Getting ready for Raku. We are contemplating doing our first Raku firing around the middle of November. Last Wednesday Sarah made a presentation to help us get ready for the Raku firing. She will do another presentation a week or so before the firing, walking us through the steps and safety procedures for firing. Our discussion last week centered around clays and shapes for the raku, with a little info on glazes. Here’s what we discussed:

“A wide variety of clays can be used to make raku pottery. Be aware, though, that the clay used determines much of the character of the finished piece. As a general rule, earthenware clays should be avoided if you like dark, bold crackle effects. The closer a clay body gets to vitrification, the less effect carbon will have on it in reduction (earthenware clays are closer to vitrification at raku temperatures than stoneware clays). Iron content also resists carbonization, so earthenware clays should be avoided.

We are then left with the light-colored stoneware clays, and nearly all of these will work well. A stoneware body with just enough iron to give the fired piece a light tan color will have a pleasingly warm appearance. Some of the white-firing clays have an attractive ivory appearance…” many clay “suppliers also have bodies that are designated as “ovenware” clay. These clays, which contain less grog (easier on the hands), often make an ideal raku bodies too” (Ceramics Arts Daily).

Using clay bodies with somewhat more grog are better able to handle the stresses of thermal shock, but may be harder on the hands if you are throwing. According to the Archie Bray Catalog Sculpture High Fire (SH) clay has the highest grog content (12%) followed by McKenzie (MACS) with 9% grog. Talk to the Archie Bray folks in the Clay Business and see what recommendations they have for raku glazes.
Use well-wedged clay to avoid the air pockets that could cause the ware to break during the firing.
Ware must be bisque before firing.
Pick a shape that can easily be picked up with tongs. We are also reaching into the top of the kiln to “grab” the piece. So shapes should be made to be picked up from the top with tongs.

“No matter what type of glaze or decorative material you use, raku is inherently unsafe for use as domestic ware. The rapid firing, removal of the ware, and subsequent post-firing phase all contribute to fragility, porosity, and thin, easily flaked glaze surfaces. Not all materials used in raku glazes are toxic. In fact, most are not. Confusion arises when you realize that over the centuries some of the most prized teabowls by tea masters have been raku fired. Be safe and think of your raku ware as decorative and not functional” (Ceramics Arts Daily).

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