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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
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An effect created by firing a clay containing high iron mineral particles (e.g. ironstone concretions). The iron becomes a flux in reduction and the particles melt and blossom and can even run down vertical surfaces. Plainsman Clays in Alberta, Canada is particularly adept at making this type of body because they have raw clays that contain concretions and their grinding process can leave them large enough to blossom.
This bowl was made by Tony Hansen in the middle 1970s. The body was H41G (now H441G), it had large 20 mesh ironstone concretions that produced very large iron blotches in reduction firing. Luke Lindoe loved to use these clays to show off the power of the cone 10 reduction firing process that he was promoting in the 1960s and 70s.
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.
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.
I control the recipe and temperature I use to make it and now I need to control the particle size. I have already smashed it up (using a special flat hammer we have) and am now sizing it. That involves getting what I can through the screen and then going at the larger sized particles with a hammer again. I use three screen sizes in the procedure so that I can control the distribution of sizes in the fired product (to more closely match reduction fired ware). This can be a dusty procedure and those particles are angular and sharp and high in heavy metal, so it would be better to do this outside in a breeze or with a ventilator and mask inside.
I mixed a cone 6 porcelain body and a cone 6 clear glaze 50:50 and added 10% Mason 6666 black stain. The material was plastic enough to slurry, dewater and wedge like a clay, so I dried a slab and broke it up into small pieces. I then melted them at cone 6 in a zircopax crucible (I make these by mixing alumina or zircopax with veegum and throwing them on the wheel). Because this black material does not completely melt it is easy to break the crucible away from it. As you can see no zircon sticks to the black. I then break this up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. As it turned out, this mix produced specks that fused too much, so a lower percentage of glaze is needed. I can thus fine tune the recipe and particle size to theoretically duplicate the appearance of reduction speckle.
I am getting closer to reduction speckle in oxidation. I make my own speckle by mixing the body and a glossy glaze 50:50 and adding 10% black stain. Then I slurry it, dry it, fire it in a crucible I make from alumina, crush it by hand and screen it. I am using G2934 cone 6 magnesia matte as the glaze on this mug on the left. To it I added 0.5% minus 20 mesh speck. Right is a cone 10R dolomite matte mug. Next I am going to screen out the smallest specks, switch to a matte glaze when making the specks (they are too shiny here), switch to dark brown stain. Later we will see if the specks need to bleed a little more. I am now pretty well certain I am going to be able to duplicate very well the reduction look in my oxidation kiln. I will publish the exactly recipe and technique as soon as I have it.
In reduction firing, where insufficient oxygen is present to oxidize the iron, natural iron pyrite particles in the clay convert to their metallic form and melt. The nature of the decorative speckled effect depends on the size of the particles, the distribution of sizes, their abundance, the color of the clay and the degree to which they melt. The characteristics of the glaze on the ware (e.g. degree of matteness, color, thickness of application, the way it interacts with the iron) also have a big effect on the appearance.
An example of how the amount of ironstone concretion impurities, that blossom to produce speckle, can be drastically different in two clay bodies (a buff and brown stoneware). Both are made from locally mined clays ground to 42 mesh. The one on the left is H550 and the one on the right is a mix containing Plainsman Fire-Red and A2 clays. The glaze is GR10-C, an MgO matte recipe based on Ravenscrag Slip.
Courtesy of Susan Clarke
By Tony Hansen
The body is Plainsman M340S. Cone 6. Left to right: G1214Z1 calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon (this one produces the white "Kohler Toilet Bowl" appearance we are seeking to better). G2934Y silky magnesia matte base with 10% zircon.
Ravenscrag Saskatchewan clays fired at cone 10R
Iron Oxide Red
Red iron oxide is the most common colorant used in ceramic bodies and glazes. As a powder, it is available in red, yellow, black and other colors.
Specking, or speckling, can be both a fault or feature in fired ceramic ware - caused or produced by metal-bearing contaminants to metallic additives
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.
|By Tony Hansen|
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