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Reduction Speckle


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.

Pictures

Cone 10R dolomite matte glaze with 5% manganese dioxide

By Tony Hansen

Dolomite bamboo matte glazed cone 10R mug

Courtesy of Susan Clarke

1970s cone 10 reduction stoneware bowl by Tony Hansen

This bowl was made by Tony Hansen in the middle to late 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.

Reduction iron speckle much heavier one one body than another

An example of how iron stone concretions contained within two clay bodies (a white and brown stoneware) blossom and produce speckle at cone 10 reduction.

Varying sizes of iron speckle in a high-iron reduction stoneware clay

Cone 10R firing of Plainsman FireRed (left), St. Rose Red 42 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.

Ravenscrag Saskatchewan clays fired at cone 10R (top) and at cone 10R with glaze (bottom): A1, A2, A3, 3B, 3C, 3D. The bottom row has also shows solubles salts (SOLU test).

Reduction speckle: a product of iron particles in the body

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.

A magnesia speckle matte at cone 6 oxidation is impossible, right? Wrong!

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.

Making my own home-made fired speckle for cone 6

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.

Making your own crucibles

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.

Cone 10R celadon at cone 6? Ravenscrag:Alberta GR6-N recipe.

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).

Laguna B-Mix on Steroids! I have wedged in 10% and 20% Plainsman P.E.S.

Both pieces have a transparent glaze, G1947U. The bar in the front is PES (Performance Enhancing Substance)! PES is made from 50:50 Plainsman A1 and St. Rose Red, it behaves like a red fireclay. BMix has some specks anyway, so why not concentrate them into some awesome aesthetics? The addition does not affect the working properties of BMix. Well, actually it does. Pieces dry better. Fired strength and maturity are minimally affected (porosity increases from about 1% to about 1.3%). With 20% addition the surface of the unglazed clay is almost metallic. Silky matte glazes are stunning on a body like this.

Want to make a cone 10R super translucent porcelain? Think again.

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.

In Bound Links

  • (Glossary) Reduction Firing

    A kiln atmosphere which is deficient in free oxyge...

  • (Troubles) Specking
    Specking is a fault in fired ceramic ware where sp...
  • (Materials) Iron Oxide Red - Fe2O3 - Synthetic Hematite

    Ferric Oxide, Red Iron Oxide, RIO, Iron(III) oxide, Fe2O3, Hematite


By Tony Hansen




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