CoO (Cobalt Oxide)
Notes-Cobalt is the most powerful and stable colorant used in glass, glaze, enamel, and even paint. As little as 2 PPM can produce a recognizable tint, thus cobalt is often cut in a medium to make it easier to weigh and distribute in a mix.
-Various raw forms are available and all break down to cobaltous oxide (CoO), which is the stable form that combines with the glass melt to produce color. These include black stable cobalto-cobaltic oxide (cobaltosic oxide) Co3O4, which has a 93% conversion ratio and decomposes to liberate oxygen at 800C. Grey cobaltic oxide (Co2O3) is 90% CoO and mauve cobalt carbonate (CoCO3) has 63% effective stain content. Cobalt dioxide (CoO2) is not marketed for ceramics.
-Although cobalt has a high melting point by itself, is dissolves readily in most glazes and acts as a powerful flux, especially alkaline and boron types. This active nature causes it to diffuse, making it difficult to maintain a clean edge on painted decoration, especially overglaze. Decorated areas employing color in or on glaze can thus be locally more melted depending on the concentration of CoO or its particle size, this can result in increased tendency to blister or crawl in some glazes.
-It is very color dependable under both oxidizing and reducing furnace conditions, fast and slow firing. Cobalt is used in a wide array of decal inks, underglaze colors, glaze and body stains, and colored glazes. It is not volatile even at 1400C.
-Cobalt is a trace element in vegetables and an important vitamin (B12) in stock raising. Cobalt metal is used in steel and chrome alloys.
Cobalt in a transparent glaze: Color varies with thickness
If you add an opacifier to this the color will become pastel and lighter and vary less with thickness.
How do metal oxides compare in their degrees of melting?
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.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
Transform the yellow-white of cone 6 to blue-white of cone 10R
Adding a little blue stain to a medium temperature transparent glaze can give it a more pleasant tone. Some iron is present in all stoneware bodies (and even porcelains), so transparent glazes never fire pure white. At cone 10 reduction they generally exhibit a bluish color (left), whereas at cone 6 they tend toward straw yellow (right). Notice the glaze on the inside of the center mug, it has a 0.1% Mason 6336 blue stain addition; this transforms the appearance to look like a cone 10 glaze (actually, you might consider using a little less, perhaps 0.05%). Blue stain is a better choice than cobalt oxide, the latter will produce fired speckle.
How reduction firing can affect glaze color
An example of how the same dolomite cobalt blue glaze fires much darker in oxidation than reduction. But the surface character is the same. A different base glaze having the same colorant might fire much more similar. The percentage of colorant can also be a factor in how similar they will appear. The identity of the colorant is important, some are less prone to differences in kiln atmosphere. Color interactions are also a factor. The rule? There is none, it depends on the chemistry of the host glaze, which color and how much there is.
Bleeding underglaze. Why?
This cobalt underglaze is bleeding into the transparent glaze that covers it. This is happening either because the underglaze is too highly fluxed, the over glaze has too high of a melt fluidity or the firing is being soaked too long. Engobes used under the glaze (underglazes) need to be formulated for the specific temperature and colorant they will host, cobalt is known for this problem so it needs to be hosted in a less vitreous engobe medium. When medium-colorant compounds melt too much they bleed, if too little they do not bond to the body well enough. Vigilance is needed to made sure the formulation is right.
Pure cobalt carbonate and copper carbonate at 1850F
Cobalt carbonate (top) and copper carbonate (bottom). Left is the raw powder plastic-formed into a sample (with 2% veegum). Right: fired to 1850F. The CuCO3 is quickly densifying over the past 100 degrees and should begin to melt soon. It is long past the fuming stage.
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