|Monthly Tech-Tip |
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.
This is the lump form of Plainsman St. Rose Red clay, as received from the quarry. The bright red color is natural iron oxide. That iron cannot be washed out, it is part of the clay crystal structure. The pigmentation is heavy enough that even when this material is a minor part of a body recipe, that body will still fire to a dark color. The color and high cost of St. Rose Red means that it is always heavily diluted with other clays, many of which are highly plastic. Because St. Rose is non-plastic it is a perfect complement to these. It is also refractory, it melts at a much higher temperature than typical clays. It is mined near St. Rose, Manitoba.
Top to bottom: Cone 10 reduction, cone 10, 9, 8, 7 oxidation. Fire-Red is a mix of St. Rose Red fireclay, M2 Montana medium fire red clay with some dark burning A1 ball clay. This blend is less refractory than pure St. Rose Red and much more plastic, thus more suitable as a body addition. The color shift between cone 9 and 10 oxidation occurs because of the fluxing action of M2.
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.
Fire-Red is a 50:50 mix of St. Rose Red and M2 with 10% A1 bentonitic clay. The St. Rose is a red fireclay, not useful on its own in reduction firing because of black coring. The M2 supplies iron staining but also enough natural feldspar vitrify the body. The A1 contributes iron pyrite speckle and plasticity. In heavy reduction, with a little more feldspar added, this body can fire metallic. The glaze G2571A bamboo.
We are currently getting about 20 loads of M2 from our Montana quarry. This clay is what makes it possible for us to make clay bodies like M390 and M350. This super-long trailer has a conveyor belt on the bottom, driven by a powerful hydraulic motor - it conveys the material out of the back. The tub empties clean and enables dumping the clay at a controlled pace. These are much safer than the tandem dumps we have used in the past.
Saint Rose Red