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Encapsulated Stains

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.

A glaze incompatible with chrome-tin stains (but great with inclusion stains)

A glaze incompatible with chrome-tin stains (but great with inclusion stains)

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 sufficient CaO and has alot of MgO). Yet this same matte glaze intensifies the #6021 encapsulated stain at only 5% (using 20% or more encapsulated stain is to develop the color is not unusual).

Pushing the limits of an encapsulated stain

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

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.

When inclusion stains are fired beyond their range

When inclusion stains are fired beyond their range

They begin to decompose and do this. It happens first where the glaze is thicker. While a stain may be documented to go to 2300F for example, the proof is in the actual use of that stain in your base clear glaze.

Encapsulated stains are firing with a mass of bubbles and pinholes

Encapsulated stains are firing with a mass of bubbles and pinholes

This is happening on five different stains at 8% concentrations. The body: A fritted porcelain. Temperature: Cone 03. The glaze: 85% frit. The solution? Documentation for inclusion frits notes that adding 2-3% zircon can brighten the color. Although this does not seem intuitive, we added 2% anyway and refired another sample. You can see the dramatic difference on that tile below. The color is brighter because the micro-bubble clouds that were diffusing it are gone! Of course, it is apparent that the percentage of stain also needs to be increased to get more intense color. What happened to the bubbles? It could be that the particles of zircon that float, unmelted in the glaze melt, act as seed-points for bubble agglomeration and the bigger bubbles then break the surface and it heals behind them. But where do the bubbles come from? I do not know.

The amount of encapsulated stain needed for intense color

The amount of encapsulated stain needed for intense color

This is about 1/2 gallon of glaze with 150 grams of yellow stain ready to mix in. That stain retails for about $30 so its cost alone is about $60 for each gallon (in 2017).

Out Bound Links

  • (Materials) Cadmium Oxide - CdO
  • (Oxides) CdO - Cadmium Oxide
  • (Suppliers) NGY COLOUR
  • (Materials) Stain

    Ceramic Stains

In Bound Links

  • (Glossary) Colorant

    Although colorants are added to bodies, most peopl...

  • (Articles)

    Glaze and Body Pigments and Stains in the Ceramic Tile Industry

    A complete discussion of how ceramic pigments and stains are manufactured and used in the tile indus...

  • (Project) Stains

    We make no attempt to classify or compile stains a...

  • (Glossary) Glass-Ceramic Glazes

    Glass-ceramic frits are made by the controlled dev...

  • (Glossary) Stain

    Stains are man-made colored powders used in glazes...

  • (Glossary) Food Safe

    In recent years potters and small manufacturers ha...

By Tony Hansen

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