Copper reds, tenmoku and oil spot glazes

Potteries and porcelains in these glazes can be stunning, and deliciously vulgar. At least, I think so. I really like them.

Copper reds:
Three glazes are produced with oxides in thick glaze mixtures. What else they have in common is that the kiln should be dedicated only to that glaze for that firing. This is because firing is unique to the effect: reducing the kiln early in the firing makes beautiful copper reds, and having only copper oxide glazes in the kiln at that time will have a deeper red than mixed firing because the copper atoms will "fertilize" the other pots.
Whatever the recipe, all copper glazes have feldspar, silica, copper carbonate, tin oxide, and iron oxide.


Tenmoku:
Tenmoku is a dark glaze with subtle spotting that has disipated slightly. It contains feldspar , limestone , and iron oxide. The name tenmoku comes from 天目 (Tien Mok, Tian Mu) which means 'heaven eye'. The Japanese use it a lot in teabowls. The darkness of the glaze depends on the cooling of the kiln.They can be very different - long firing and an iron-rich clay, as well as a firing-down of the kiln, will keep the glaze molten longer, and let the iron crystalize finely in the surface of the glaze.


Oil spot:
Oil spot glazes are stiff glazes containing red iron oxide and magnesium oxide. The glaze is applied thickly and fired in an oxidation atmosphere. Oxidation is essential to create the oil spots.The red iron oxide molecule will release an oxygen atom at around 2250° F (1232° C) and become black iron oxide. The oxygen heads to the surface of the glaze, pulling a trace of iron along. The oxygen leaves, the iron remains on the surface, creating the oil spot effect. Oil-spot glaze contains a high amount of feldspar, often over 50%. There is also magnesium oxide (dolomite or talc), and from 6.0 to 8.5% red iron oxide.
The thicker the glaze mixture is applied to the pot, the larger the resulting spots. If the glaze is to be applied really heavily, more feldspar is needed to stiffen it, so that it doesn't run too much and fuse the pot to the kiln shelf. But that is usually not a problem. If the firing is very hot, the glaze may run enough to pull the oil spots into the hare's fur effect so prized by Sung dynasty potters.

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Oxidation firing: If there is more than enough oxygen present to burn the fuel completely, especially during the last stage of temperature increase, then the firing is an oxidation firing. The metals in the glaze mixtures will convert to fully oxidized, the colors will be more uniform. This is best done with an electric kiln, to avoid the fuel drinking up the oxygen. And by having fresh air coming into the kiln.