Most ceramics glazes are simple to formulate and very easy to fire. But there are some glaze effects that have the reputation of being elusive and difficult to obtain. This was certainly the case early in the 20th century with copper reds and celadon blues. These glazes had a history of secrecy and myth surrounding them. One story even claimed that a Chinese emperor had slaves thrown into the kiln during the firing in order to produce the elusive copper red. However later potters found that by reducing the kiln early in the firing produced beautiful copper reds.

The most important factor in obtaining oil spot glazes is firing in an oxidation atmosphere. This cannot be overemphasized. Oxidation is essential because of the mechanism by which the oil spots are created. Simply stated, the red iron oxide molecule, Fe2O3, will let go of an oxygen atom at approximately 2250° F (1232° C). At this temperature, the red iron oxide molecule cannot maintain its complex structure and will release an oxygen atom to become the simpler FeO molecule, or black iron oxide. As the oxygen leaves the red iron oxide molecule it bubbles to the surface of the molten glaze, dragging a bit of iron with it. When it reaches the surface and leaves the glaze it deposits the “spot” of iron which creates the characteristic oil spot look. However, if the glaze is reduced early in the firing cycle, the red iron oxide will already have changed into black iron oxide, and there is no possibility of an oxygen molecule being released later to create the oil spot effect. So you can see that without an oxidation firing cycle there will be no oil spot effect.

The second most important factor is the glaze recipe. It must contain some red iron oxide and be sufficiently stiff to hold the oil spots that are created, but not too stiff to prevent the bubbling out of the oxygen. Feldspathic bases are perfectly suited to create oil spots. There are a variety of recipes that potters covet.