Digitalfire Ceramic Glossary

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A reduction fired glaze having about 10-12% iron oxide that fires to a highly glossy deep maroon to black. Tenmokus normally break to iron-red crystallized areas where thinner and thus work well to visually highlight incised decoration or abrupt contours. Tiny yellow iron silicate crystals adorn thick sections of the glaze when sufficient melting has occurred. Tenmokus behave differently on porcelains and stonewares.

Needless to say, the effect requires a that the degree of melting be exactly right. It looks the best when the glaze is melting and flowing as much as possible. Of course, if it is running too much it runs down off the ware onto the shelf and thins on vertical sections. The difference between these two extremes is quite small. For example, if the glaze flux is calcium carbonate, as little as a 1-2% change may be significant. Thus you need to be prepared to adjust the flux and iron content in the recipe to tune the melt and color in your circumstances. Of course, the rate of cooling also affects how much the crystals grow.

Tenmoku glazes typically are very red in the bucket because they contain so much iron oxide. This iron makes the slurry gel overtime. These can be really messy to use. The glazes also tend to have high feldspar and low clay content so they settle in the bucket, apply to the ware poorly and do not harden well on drying. Using glaze chemistry it is possible to produce a recipe that sources the iron from a low fire red clay so that little red iron oxide powder is needed. Another alternative is Alberta Slip. By itself it is almost Tenmoku, it just needs 1-2% added iron and possibly 5% calcium carbonate.

Iron-crystal glazes are closely related, however they contain more iron oxide, the rust colored crystallization covers the whole surface.


A cone 10 reduction tenmoku glaze with about 10% iron oxide

Fired on a porcelain in a gas kiln.

Tiny iron silicate crystals that often grow in tenmoku glazes when they are cooled sufficiently slowly.

The same Tenmoku on a buff stoneware and a Grolleg porcelain

This is Plainsman H550 and P700. The inside glaze is G1947U. They were fired in 10 reduction.

How iron particles in the body affect the look of a Tenmoku glaze

A buff stoneware (left) and brown speckled stoneware (right). Notice that the iron specks crystallize in a manner similar to the edges of contours.

Add 5% caclium carbonate to a tenmoku. What happens?

In the glaze on the left (90% Ravenscrag Slip and 10% iron oxide) the iron is saturating the melt crystallizing out during cooling. GR10-K1, on the right, is the same glaze but with 5% added calcium carbonate. This addition is enough to keep most of the iron in solution through cooling, so it contributes to the super-gloss deep tenmoku effect instead of precipitating out.

Ravenscrag Tenmoku vs. Alberta Slip Tenmoku on porcelain

Body is Plainsman P580. Far left: G2894 Ravenscrag Tenmoku with 10% whiting and 10% iron oxide added. Center: Pure Alberta Slip plus 5% whiting and 1% iron oxide. Right: Pure Alberta Slip plus 5% whiting and and 2% iron. The Alberta Slip versions are less messy to use because so much less iron is needed (iron also causes the slurry to gel). The Ravenscrag and higher iron Alberta Slip versions are running, they are too fluid. The rust colored crystals are not developing the way they did with these glazes on an iron stoneware (in the same firing).

Tenmokus made from Alberta Slip and Ravenscrag Slip

GR10-K1 Cone 10R Ravenscrag Tenmoku (right) compared to Tenmoku made from Alberta Slip (left, it is 91% Alberta Slip with 5% added calcium carbonate and 2% iron oxide). Left is Plainsman P700 porcelain, right is H570. Tenmokus are popular for the way they break to a crystalline light brown on the edges of contours.

Compare Ravenscrag and Alberta Slip tenmokus at cone 10R

GR10-K1 Ravenscrag tenmoku (left) compared to Alberta Slip tenmoku GA10-B (center) and pure Alberta Slip (right).

The multitude of things iron oxide can do in reduction

Iron oxide is an amazing glaze addition in reduction. It produces celadons at low percentages, then progresses to a clear amber glass by 5%, then to an opaque brown at 7%, a tenmoku by 9% and finally metallic crystalline with increasingly large crystals past 13%. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

Which tenmoku base is better: Alberta Slip or a clear glaze?

Right: Alberta slip is almost a Tenmoku glaze by itself at cone 10 reduction. To go all the way only 1-2% more iron is needed (plus a little extra flux for melt fluidity, perhaps 5% calcium carbonate). Compare that to crow-baring a clear glaze into a tenmoku (left): This is G1947U plus 11% red iron oxide. That produces a slurry that is miserable to work with (it stains everything it comes into contact with) and turns into a jelly on standing.

Cone 10R kiln variation shows in tenmoku glaze

The mug on the left was in a hotter part of the kiln (gas reduction), it's surface is brilliant glassy smooth and metallic. The one on the right is dull, pebbly, much less interesting. The temperature difference is about one cone. This is not enough to make much difference in the transparent glaze, but the tenmoku is sensitive, it needs to reach the full temperature.

FeO (iron oxide) is a very powerful flux

This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux in reduction firing and produces a glaze melt that is much more fluid. In oxidation, iron is refractory and does not melt well (this glaze would be completely stable on the ware in an oxidation firing at the same temperature, and much lighter in color).

Ravenscrag Tenmoku vs. Alberta Slip Tenmoku

All of these are on a cone 10 reduction fired iron stoneware (Plainsman H443). Far left: G2894 Ravenscrag Tenmoku with 10% whiting and 10% iron oxide added. Center: Pure Alberta Slip plus 5% whiting and 1% iron oxide. Right: Pure Alberta Slip plus 5% whiting and and 2% iron. The Alberta Slip versions are less messy to use because so much less iron is needed (iron also causes the slurry to gel). The Ravenscrag version is running, it is too fluid. Likely 5% calcium carbonate would be enough (and maybe less iron).

Cone 10R Tenmoku Ron Roy cup walks a delicate balance

Each potter using Tenmoku has their own preferences about how the glaze should look. Ron clearly likes the iron crystals to develop well on the edges of contours. He has learned how to walk a delicate firing and recipe balance to achieve this effect. If the percentage of iron is too high, or the glaze is applied too thin, reduction is too heavy or the cooling too slow there will be too muchy crystallization. If the iron is too low, cooling is too fast or the glaze it too thick it will be a solid black. Additionally, this effect depends on a glaze having a fluid melt (the iron is a strong flux), if the glaze is too thick it will run downward during the firing.

In Bound Links

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  • (Glossary) Kaki

    Kaki (persimmon) glazes are high fired reduction t...

  • (Recipes) GA10-B - Alberta Slip Tenmoku Cone 10R
    You can make a tenmoku from Alberta Slip by adding only 2% iron oxide and 5% calcium carbonate
    2003-12-12 - Tenmoku glazes normally contain 10%+ iron oxide, t...

By Tony Hansen

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