|Monthly Tech-Tip |
Both pieces have a transparent glaze, G1947U. The Fire-Red (a blend of Plainsman A1/M2 and St. Rose Red native clays) was slurried up, dewatered to plastic form and then wedged into the B-Mix (the left piece has 10%, the other 20%, the bar in front shows the pure material). The A1 supplies most of the speckle, the St. Rose and M2 impart the color. This addition does not affect the working properties of BMix (it still throws very well). An added benefit is that pieces dry harder and with less cracking. Fired strength and maturity are minimally affected (porosity stays around 1%). With a 20% addition, the surface of the unglazed clay is almost metallic. Silky matte glazes, like G2571A, are also stunning on a body like this.
This is a high iron two-material stoneware fired in reduction, purely a test body I am working on. Normally such bodies are rendered unsuitable because of black coring associated with FeO. But 10% added Custer Feldspar solves that problem, combining with the iron to vitrify the body to excellent fired strength (it does have 5% porosity but this is mainly due to the cavities formerly occupied by the iron pyrite particles. The color and iron speckle is being contributed by both of the raw materials in the body. Saint Rose Red supplies most of the iron to produce the red coloration. A1 bentonitic clay adds plasticity and contributes the pyritic iron particles. The fired bars of this body show it at cone 10 reduction (top) and cone 10 oxidation down to 7 (downward from top). The glaze is GR10-CW, pure Ravenscrag Slip with 10% talc opacified with a little Zircopax and tin oxide. Adding just a little more feldspar produces a metallic firing surface!
B-Mix is a popular high-ball clay very plastic grey cone 10R stoneware in North America. The two mugs on the left have pure Ravenscrag Slip on the inside (the middle on the outside also), it fires almost transparent with a slightly silky surface. Pure Alberta Slip is employed on the outside of the left one and the inside of the right one. The outside of the right one is RavenTalc silky matte. In all cases the Ravenscrag and Alberta Slip are mixed half-and-half calcined and raw. B-Mix fires dark enough and with enough specks that a normal transparent glaze is not very interesting. But these Ravenscrag ones look much better (for use as a liner glaze).
Left two mugs are glazed with pure Ravenscrag Slip (roast:raw combo), far right one is RavenTalc silky matte (GR10-C). The speckled mugs have 10% of a Plainsman Fire-Red (a blend of a red fireclay, M2 and a heavily speckled ball clay). Ravenscrag Slip is an ideal base for cone 10R glazes, so many glazes can be made by adding pigments, opacifiers, variegators and matting agents.
We get this clay from St. Rose, Manitoba. Four tandem loads arrived this week. Just seeing the pile inspires me to make more pieces! It is a red fireclay and it is highly unusual. St. Rose Red has issues. They at first seem to be problems, but in combination they give it magic powers! It fires with very heavy iron speckling. The iron pigmentation is so high that it burns almost black at cone 10R. It has low plasticity. It shivers glazes: The vase on this picture lasted an hour after kiln exit, it spontaneously fractured because of the outward pressure from the under-compression glaze on the inside. But, by combining St. Rose Red with our more vitreous clays, which are highly plastic, we can make H440 and H443. A mix of only 45 St. Rose with 40 Ball clay and 15 feldspar produces a rustic metallic surface (like the cup shown). Such a body cannot be made from a low fire red clay (like RedArt), it would just warp and collapse in the kiln. It is the refractory character, heavy pigmentation, iron speckling and low plasticity of St. Rose that make metallic ware possible.
A sought-after visual effect that occurs in reduction fired stoneware. Particles of iron pyrite that occur naturally in the clay melt and blossom up through the glaze