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Tenmoku

Tenmoku is a kind of high temperature reduction firing ceramic glaze. Glossy, very dark brown or maroon, fluxed by iron oxide to have high melt fluidity.

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Details

A reduction fired glaze, having about 10% iron oxide, that fires to a high-gloss deep maroon to black color. The amount of iron is tuned to be near the concentration that will precipitate as red or yellow crystals during cooling. The percentage added will depend on cooling rate of the firing and the degree to which the chemistry of the host glaze supports the crystal growth and can hold iron in solution. When the amount of iron is right, Tenmokus normally break to iron-red crystallized areas where thinner (on edges, incised decoration or at abrupt contours). The right balance of iron and cooling rate also enables a distribution of tiny yellow iron silicate crystals on flat areas). Tenmokus behave differently on porcelains and stonewares.

Tenmokus look the best when the glaze is melting and flowing as much as possible (making it a balancing act between appearance and damaging kiln shelves). The recipe makeup must be tightly controlled. For example, if the glaze flux is calcium carbonate, as little as a 1-2% change may be significant to the degree of melt. So be prepared to adjust the flux and iron content in the recipe to tune the melt and color in your circumstances. The rate of cooling determines how much the crystals grow.

The simplest way to make a tenmoku is to simply add 10% iron oxide to a good transparent cone 10R recipe (like G19147U). Understandably, tenmoku glazes typically are very red in the bucket. This iron makes the slurry gel over time. This makes the slurries messy and difficult to use. Tenmokus 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 and craze on firing. Using glaze chemistry it is possible to produce a recipe that sources some of the iron from a low fire red clay (having a benefit that less red iron oxide powder is needed). To reduce crazing is possible to swap KNaO (the high expansion oxides in feldspar) for other fluxes having much lower thermal expansion. An very good approach is using Alberta Slip. By itself it is almost Tenmoku at cone 10R, it just needs 1-2% added iron (for color) and ~5% calcium carbonate or dolomite (for melting).

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

Related Information

Tiny iron silicate crystals often grow in tenmoku glazes

This occurs as they cool in the kiln. The slower the cooling and the more iron oxide there is, the bigger and more plentiful these crystals will be.

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.

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 act as catalysts to crystallize in a manner similar to the edges of contours.

The multitude of things iron oxide can do in reduction

Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% is producing an amber glass (and the iron is showing its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

Add 5% calcium 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.

FeO (iron oxide) is a very powerful flux in reduction

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).

A way to prevent a tenmoku glaze from running onto your kiln shelves

Tenmoku reduction fired glazes can be so beautiful yet few people use them. One reason is the melt fluidity - runs stick pieces to the kiln shelf. While the melt fluidity is the key to the appearance it is also the curse. These glazes also pool on inside bottoms producing glaze compression issues. And they stretch thin over rims roughening them with any grit from the body or glaze materials. The running onto the shelf issue at least does have a simple solution: The GR10-A base as a catcher glaze on the outside bottoms and a liner on the inside (and even optionally wrapping over the rim). I use a dipping glaze version of it for the insides and a brushing glaze version for the bases (and up the side walls about 1cm). The tenmokus GR10-K1 (left) and GA10-B (right) can be applied thickly and it’s no problem, 5-10 mm of catcher glaze is all it takes to stop the running.

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.

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.

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