Cobalt Oxide

Cobalt Oxide

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CoO is a metallic coloring oxide that produces blue in glazes at all temperatures (unless in very high percentages where it will be black). Black Cobalt Oxide is a key source of CoO used in glazes, glass, and enamels. Cobalt is the most powerful ceramic colorant and it is stable in most systems, it appears in many recipes at 1% or lower. Like copper, it melts very actively in oxidation. If it is mixed into a fluid frit base in high enough a percentage, it will completely crystallize during cooling. Cobalt is also useful as a body and slip stain (see the oxide CoO oxide for more information). However, cobalt materials are very expensive, this severely limits its practical use in many things.

Understanding exactly what cobalt oxide powder is and how it decomposes is complicated. Suppliers stock a product normally referred to as 71% Cobalt (or similar), this refers to the amount of Co metal. This product of commerce is theoretically Co3O4 (although possibly somewhere between CoO and Co2O4). Pure CoO, however, would be 78.6% cobalt metal. The difference is thus the extra oxygen in the Co3O4 that is liberated during firing. For this reason the chemistry defined here has a loss on ignition. Additionally, raw cobalt oxide powder does not decompose to actual CoO during glaze melting unless the kiln is fired in reduction. These complications will simply necessitate a small adjustment in the percentage to adjust color when needed (e.g. when switching brands).

Cobalt(II) oxide is a product of Co2O3 cobalt oxide decomposing at 900 °C. It occurs in ores with nickel, arsenic, sulfur, and manganese in deposits in Canada, Morocco, and southern Africa. During the roasting process toxic by-products of arsenic and sulfur are produced. The associated ores may contaminate the Co3O4 to some extent (i.e. with Na2CO3). CoO can also be made by heating the carbonate. Some people have tried roasting the ore themselves in a kiln, however as noted this can be hazardous, not to mention that if fired too high the ore can melt and eat through the container walls.

This material can be found in technical and ceramic grades. Commercial ceramic grades of cobalt oxide will often produce glaze specking if not thoroughly sieved or ball milled (depending on whether the particles are a product of agglomeration or are simply unground). Also, there is some inconsistency in commercial products, different batches or materials from different suppliers can vary in the amount of specking. Cobalt carbonate tends to disperse better in glazes to give even blue coloration because it is not as powerful and can produce some glaze blistering problems (as already notes). Consider a cobalt blue stain for the most consistent and reliable results.

The theoretical carbonate form has 63% CoO while this has 93%. So if you want to switch from the oxide to the carbonate in a recipe, multiply by 93 and divide by 63. To switch from carbonate to oxide multiply by 63 and divide by 93. But remember that this will be approximate (for the reasons discussed above), you will likely need to fine tune the amount based on fired results. In addition, the quality of the color may be different.