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A special class of stains that enable bright and difficult-to-achieve colors in ceramic glazes. They are a recent development in glaze technology. Many companies that hesitated to use these stains in the past now use them in their biggest selling products.
These stains can strongly pigment a glaze in amounts as low as 5% (if everything is right, see below) but up to 20% or more is needed for some colors and glazes. Stain companies showcase these stains (on their sample boards) using a little opacifier (e.g. 3%) in the glaze (this likely intensifies the color).
Encapsulated stains are not, as the name suggests (and some misunderstand), a zircon capsule around an otherwise unstable compound. Rather, they are manufactured by sintering to form a crystalline matrix (in a process called encapsulation). After sintering they are ground, filter pressed and dried. In cadmium encapsulated stains, for example, the matrix between the zircon and cadmium is stable to 1220C and the selenide/selenium is released during combination. The stain is further rendered safer-to-use by washing with water or weak acid to remove any soluble uncombined compounds (e.g. cadmium or soluble impurities). This washing process does produce toxic byproducts that can only be tolerated in certain countries (e.g. India, China).
Manufacturers have specific recommendations for each stain that must be followed closely. For example, cadmium stains normally work best when glazes are slow cooled and many must be used in glazes with a qualifying chemistry (e.g. glazes for cadmium stains must have no zinc (affects color), no titanium (crystallizes and makes the color fuzzy, a little TiO2 in the clay is tolerable) and have low alumina. Obviously, these stains must not be ball milled, glazes must be milled prior to adding the stain. Suppliers may hesitate to publish some specifics that might give away trade secrets of their products, even relating to the chemistry requirements in host glazes.
Regardless of manufacturers claims, detailed testing is needed to establish a firing curve and a compatible host glaze to get the best possible color. Encapsulated stains will sometimes cause bubbling in specific glazes, even if the firing temperature is under the stability guideline.
The crystalline compound created must not be fired above its recommended temperature or the normally toxic compound will dissolve into the glaze (normally without volatilization unless the maximum temperature is greatly exceeded) and the color will be lost and the glaze rendered toxic. Cadmium content and cadmium release are separate issues. The manufacturing process of these stains is designed to create a stable coloring zircon-compound from a parent metal that would otherwise be unstable (e.g. leachable toxicity). Metal release tests must be done to monitor metal release.
Other kinds of encapsulated stains besides cadmium are: Zircon Pr Yellow, Zircon Vanadium turquoise, Zircon Iron Coral, Zircon Vanadium Orange, Zircon Pr/Vanadium Green. The color depends on the sintering temperature.
That middle sample is supposed to be red!
Left: a cone 6 matte glaze (G2934 with no colorant). Middle: 5% Mason 6006 chrome-tin red stain added. Right: 5% Mason 6021 encapsulated red stain added. Why is there absolutely no color in the center glaze? This host recipe does not have the needed chemistry to develop the #6006 chrome-tin color (it lacks enough CaO). Yet this same matte glaze intensifies the #6021 at only 5% (sometimes 20% or more encapsulated stain is needed to develop the color).
Pushing the limits of an encapsulated stain
Mason 6021 encapsulated stain in Plainsman M370 transparent fired at cone 6 on M370 (with second layer of two other colors). Notice the bubbling. This stain is rated to 2300F, it appears that even cone 6 is pushing its limits.
Transparent inner glaze over an encapsulated stained engobe
Encapsulated stains can reach their limits in a glaze host at cone six and begin to dissolve and decompose. That is an obvious problem on a food surface. But in a less fluid underglaze they can survive longer. The bright orange color on the left was likely done this way. The transparent over glaze is isolating it from any contact with food or drink. However people are more wary of the risk of glazes leaching heavy metals and having bright colours on food surfaces may not send the right message.
Out Bound Links
Cadmium Oxide - CdO
Red Stains - ZIRCONIUM ENCAPSULATED CADMIUM
CdO - Cadmium Oxide
In Bound Links
By Tony Hansen