|Monthly Tech-Tip |
Alternate Names: Cobalt(II) oxide, Cobaltosic Oxide, CoO, cobaltous oxide
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.
Cobalt oxide particles can agglomerate. Glazes that contain them must be sieved to break these up. Glazes that get contaminated can look like this.
The insides are GA6-A Alberta Slip cone 6 base. Outsides are Ravenscrag Floating Blue GR6-M. The firing was soaked at cone 6, dropped 100F, soaked again for half and hour then cooled at 108F/hr until 1400F. The speckles on the porcelain blue glaze are due to agglomerated cobalt oxide (done by mixing cobalt with a little bentonite, drying and pulverizing it into approx 20 mesh size and then adding that to the glaze slurry).
The magic of this recipe is the 5% extra frit, that makes the melt more fluid and brilliant and gives the glaze more transparency where it is thinner on edges and contours. The extra iron in the Plainsman P380 (right) intensifies the green glaze color (vs. Polar Ice on the left). The specks are cobalt oxide agglomerates that were made by slurrying cobalt oxide and bentonite, then crushing it to sizes large enough to make the specks.
Using stonewares it is easy to get pretty sloppy in the studio because a particle of iron or cobalt in a glaze or body is no big deal. But on a ice white, translucent, transparent-glazed piece it is a really big deal. These specks are particles of cobalt that were trapped in my 80 mesh glaze screen from previous use. I use a soft brush to coax the glaze through the screen faster, but even that was enough to dislodge some of the cobalt particles. The lesson: I need a dedicated glaze screen for use with this transparent glaze, it gets used for nothing else.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
These have to be seen to be believed, it is the deepest, richest blue we have ever produced. This is Plainsman M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt (not raw) Umber instead of the normal 10% Zircopax) was applied inside and partway down the outsides (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue. Firing schedule: Cone 6 drop-and-soak.
The High Human Cost of Cobalt Mining
Cobalt II,III Oxide at Wikipedia
More hazards info
More Toxicology Information
Cobalt II oxide at Wikipedia
Metal oxide powders are used in ceramics to produce color. But a life time is not enough to study the complexities of their use and potential in glazes, engobes, bodies and enamels.
Cobalt Oxide and Carbonate
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Metallic based materials that impart fired color to glazes and bodies.
|Oxides||CoO - Cobalt Oxide|
|Frit Softening Point||1935C|
|Density (Specific Gravity)||6.07|