|Monthly Tech-Tip |
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
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 iron stone 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 (the iron percentage is so high that it fires black at cone 10R). 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 liquified (because of the reduction atmosphere in the firing), 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) was mixed as a slurry, dewatered to plastic form and then wedged in to the B-Mix (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 better. 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 stunning on a body like this.
Brilliantly glossy. The body is Plainsman Polar Ice porcelain. Firing is cone 6 oxidation. The reduction fired effect is particles (or agglomerates) from one of the raw metal oxides in the recipe (iron, cobalt, rutile; most likely the cobalt). If this glaze were ball milled the effect would be lost. Even though the glaze is so glassy, it is not running down off at the foot. The blue where it thickens on contours is because of the rutile, this can be removed for a truer Celadon effect (if it is not causing the specks).
On the right is a porcelain used in China, renowned for its whiteness and translucency. On the left is a body made from Grolleg kaolin, this is commonly used by potters. They were fired in reduction. The tiny iron specks that potters do not even notice are enemy number for the blue-white porcelain like this. Although they might be small the reduction atmosphere makes them blossom out in full glory to ruin the piece. These specks come as contaminants in the materials (especially the silica) and they are easily picked up during fabrication. For very white bodies like this, it is incredibly difficult to prevent the specks. For a perfectly white flawless result, the entire factory must be dedicated to this one body; they use wet processing, magnets, filter pressing, stainless steel equipment and impeccable procedures.
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.
Cone 10R firing of Plainsman Fire-Red (left), St. Rose Red 100 mesh (center) and St. Rose Red 10 mesh (right). The 10 mesh material produces a reduction speckle and deep red color that is very unique.
Courtesy of Susan Clarke
By Tony Hansen
The body is Plainsman M340S. Cone 6. Left to right: G1214Z 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.
|Materials||Iron Oxide Red|
Specking is a fault in fired ceramic ware where speckles of ion bearing contaminants are visible and mar the appearance of the surface.
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.