Iron red reduction clay suitable for functional ware? How?

This is a high iron two-material stoneware fired in reduction, purely a test body we are 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 are 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). Adding just a little more feldspar produces a metallic firing surface! The glaze is GR10-CW, pure Ravenscrag Slip with 10% talc opacified with a little Zircopax and tin oxide.

Related Pictures

Plainsman Fire-Red reduction fired vase

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 it is too refractory. The M2 supplies iron staining but also enough natural feldspar to mature the body enough to make it strong. 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.

Amazing iron-blossoms in a vitreous reduction stoneware body

Two vessels made from a vitreous metallic reduction stoneware

Fire-Red is an unusual material for several reasons. It has a high iron content yet is a fireclay. It is also non-plastic. Most important, it is not ground to 200 mesh like industrial materials. These bodies demonstrate it well, left: 42.5% Fire-Red, 42.5% ball clay and 15% Custer feldspar, right: 60% Fire-Red, 30% ball clay and 10% feldspar. The ball clay adds plasticity. The feldspar gives control of the degree of vitrification (the left one has 1.3% porosity at cone 10R, the right one 1.5%). This recipe vitrifies so it does not exhibit the deep red color that Fire-Red would give if there was no feldspar. Look closely at the surface: It is covered by thousands of tiny iron-eruptions, they occurred as the iron pyrite particles liquify as FeO (because of the reduction atmosphere in the firing), and these produce a metallic appearance. And, they will bleed through an over-glaze, if present, to give stunning speckled surfaces.

Laguna B-Mix on Steroids: Wedge in some Plainsman Fire-Red!

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 (a commercial porcelainous whiteware body made by Laguna Clay). The left piece has 10% added Fire-Red, the other 20%, the bar in front shows the pure material). The A1 clay supplies most of the speckle, the St. Rose Red and M2 impart the color. This addition does not affect the working properties of BMix (it is highly plastic). 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 fired to cone 10R with Ravenscrag Slip as an inner glaze.

This can fire almost black on reduction. However, as a pure material, it is prone to dunting as it is high in larger quartz particles.

Links

Troubles	Black Coring A common fault in reduction gas fired ceramic ware made from iron bearing clays. The interior cross section of the clay turns black.

Got a Question?

Buy me a coffee and we can talk