All Glossary

Reduction Speckle

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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Details

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.

Granular additives can be added to any clay body refractory enough to host them at the intended temperature (~0.2% addition). Granular ilmenite, manganese or rutile can be used (typically 20-40 mesh size).

Specking bodies are not practical for industry because of the difficulty in maintaining a consistent fired appearance. Even slight variations in the particle size, PSD (particle size distribution), concentration, mineralogy or chemistry of the specking agent body can produce noticeable differences in speckle size, color and melting and diffusing patterns. Natural speckle sources (e.g. iron stone concretions in the clay) are, by nature, inconsistent. In addition, the same reduction and temperature firing schedules time after time are essential. Packing density within the kiln, and even weather conditions, can affect the ability of the kiln to ventilate normally (to impose temperature and atmosphere). In addition, speckle bodies often contain soluble salts, which are iron-stained. While these are not obvious when a body is fired in oxidation, they will often create a glassy surface and darken fired color considerably.

Related Information

Reduction speckle: a product of iron particles in the body

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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.

The same clay fired in reduction and oxidation at cone 10

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This is a ball clay from Manitoba. The oxidation sample has plenty of small black specks, these are an iron pyrite mineral that does not melt at cone 10 oxidation. But in the reduction firing they are being converted (reduced) to their metallic state by the lack of oxygen in the kiln. The result is vigorous melting and blossoming. Notice the surface is also glossy, this is due in part to a thin layer of iron-stained soluble salts that are melting like a glaze.

Reduction iron speckle

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Some of the clays we mine have contain natural ironstone concretion impurities, they melt and blossom to produce speckle when fired in reduction in a gas kiln. Both of these 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.

Here is something potters can do that industry cannot do!

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These mugs were fired at cone 10R. The body is L4168G5, I mixed it myself using 50% Plainsman Saint Rose Red, 40% Plainsman A2, 10% Custer feldspar. The Saint Rose clay contributes the color, the A2 the speckle and plasticity and the feldspar matures the body enough to avoid black coring. The heavy iron specking is being sourced by these very unique clays, both were ground at 42 mesh only. The left glaze is GR10-CW Ravenscrag Talc matte with added Zircopax. The right one has that same glaze on the inside and G2571A bamboo matte on the outside. The unglazed body is a beautiful deep red. These are certainly not porcelain strength but the glazes fit, the mugs are durable enough and serviceable for normal use. This type of ware is the domain of potters only, no industry would be able to babysit the touchy process it takes to make these, they would not be able to find a reliable material supply, they would not be able to maintain enough consistency and would not be able to justify to customers the high body porosity required.

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

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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.

Amazing iron-blossoms in a vitreous reduction stoneware body

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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.

Dolomite bamboo matte glazed cone 10R mug

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Courtesy of Susan Clarke

Heavily speckled reduction fired porcelain Shino bowl by Glenn Lewis

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This effect was created by wedging 10 mesh ironstone concretions into the soft porcelain.

Cone 10R dolomite matte glaze with 5% manganese dioxide

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By Tony Hansen

1970s cone 10 reduction stoneware bowl by Tony Hansen

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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.

Making your own crucibles to make your own speckle

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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, dry and break into small pieces. I then melted them at cone 6 in a Zircopax crucible (I make these by mixing alumina or zircopax with 3-4% veegum and throwing them on the wheel). This material does not completely melt so it is easy to break the crucible away (it does not stick to the zircon). I then break the black up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. If specks fuse too much I can lower percentage of glaze (and vice versa). Of course, the particles are glass, jagged and sharp-edged so care is needed in handling them.

Making my own home-made fired speckle for cone 6

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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.

Emulating a speckled reduction fired stoneware in oxidation

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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.

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

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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.

Oxidation fired speckled glaze!

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The magnesia and calcia matte base glazes we have developed for electric firings have soft surfaces that rival the feel and utility of those achievable in reduction-fired dolomite mattes. By adding 0.2% granular manganese to M340 clay we can achieve this speckled aesthetic (outside of mug) using G2934 matte glaze.

Black coring with L4168G and L4168F

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These are iron reduction test bodies. The L4168G (left) is far stronger yet it has a higher percentage of Fe₂O₃ and much more black coring. How is that possible? Because it has 10% added feldspar. The black FeO iron is staining the feldspar black (bleeding out from each pyrite particle), helping it do its job of producing a glassy black color is not coming from the kiln atmosphere? Because the buff-burning bodies in the same kiln did not have any of this. On the right, the iron is restricted by its ability to vitrify the body by limited glass development, ending up destabilizing it instead (by increasing body thermal expansion).

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Ravenscrag Saskatchewan clays fired at cone 10R

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