Additions of iron oxide are coloring, fluxing and crystallizing this base transparent

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% produces an amber glass (and the iron reveals 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 or Teadust), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln using the C10RPL firing schedule, the crystallization mechanism would be heavier if it were cooled more slowly (or less if cooled faster). The 7% one in this lineup is quite interesting, a minimal percentage of cobalt-free black stain could likely be added to create an inexpensive and potentially non-leaching jet-black glossy.

Related Pictures

Same iron red glaze on black stoneware and on porcelain

The same glaze on black stoneware and porcelain

The glaze is G3948A iron red fired at cone 6 using the C6DHSC schedule. The bodies are Plainsman Coffee Clay and Polar Ice (the insides are different glazes). They were in the same kiln. These mugs demonstrate how much reactive glazes can interact with the body beneath and how much that affects their fired properties, especially when they have high melt fluidity like this one. On the left the glaze is drawing color out of the body. The porcelain on the right has no color to give but it does have sodium - and it is supplying enough to act as a catalyst to the creation of the iron crystals.

A cone 10R high iron glaze has run off the vase during firing. And crystallized.

An high iron glaze has melted too much run

Iron, in the FeO form, is among the most powerful of fluxes in reduction firing. That fluxing action, dependent on the percentage of iron oxide in the recipe, produces two obvious consequences: Running (depending on the degree of reduction) and crystallization (depending on the speed of cooling and the chemistry of the glaze hosting the iron). This piece was slow-cooled during firing, resulting in total crystallization of the surface. The crystals are larger and densely packed at the neck. Their presence, as a thin surface layer, has completely matted it. And, because of the fluxing power of the FeO (present because of the reduction atmosphere in the kiln) enough glaze ran downward off the piece that it was left sitting in a pool of molten glass.

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, converting from Fe₂O₃ to FeO, 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.

Links

Glossary	Celadon Glaze A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.
Glossary	Mechanism Identifying the mechanism of a ceramic glaze recipe is the key to moving adjusting it, fixing it, reverse engineering it, even avoiding it!
Glossary	Metallic Glazes Non-functional ceramic glazes having very high percentages of metallic oxides/carbonates (manganese, copper, cobalt, chrome).
Glossary	Crystallization Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Glossary	Reduction Firing 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.
Glossary	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.
Materials	Iron Oxide Red Red iron oxide is the most common colorant used in ceramic bodies and glazes. As a powder, it is available in red, yellow, black and other colors.

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