STAINS

Stains are oxides that produce color. In ceramics the term is generally reserved to refer to commercially prepared colorants. The oxide or oxides, often combined with an opacifier, have been blended, then fired together (frited), and finally cooled and ground into a fine powder (usually ball milled and very fine so they can often be used in an airbrush without it clogging). They are produced by several companies for their intended use in industrial production situations that demand great color variety and extreme consistency, from which we ceramists benefit. For our practical purpose, some of the published terminology is misleading. For instance, a maximum quoted firing temperature does not mean it can’t be fired successfully to Cone 10 but that the color shade may change, perhaps for the better artistically. Likewise, glaze versus body stain designations really refers to the original intended industrial production use and some variation in color shade may occur if otherwise used.

The characteristics and advantages of commercial stains over the often less expensive oxides are:

ADDITIONAL COMMENTS, USES AND SUGGESTIONS

TO MAXIMIZE SUCCESS WITH COMMERCIAL STAINS, REFER TO THE MANUFACTURERS SPECS

1. Stains are refractory and need to be fluxed by the medium such as the glaze or slip or by direct flux addition (2-8%). Generally, the addition of additional flux is recommended even if the stain is added to a glaze or slip. If known, use more of the same flux that is in the base. Otherwise, use a frit or even low fire clear glaze if it is not a low fire glaze that the stain is being added to.

2. Gas (reduction) firing is more detrimental to successful color development than is temperature. This is especially true with pinks, yellows and purples.

3. More stain is needed to achieve a given color intensity in a slip than in a glaze because the glaze is transparent and thus more of the stain is seen than only what is on the surface of the slip.

4. The speed of a firing and the cooling cycle can effect the color

5. The addition of a "pinch" of tin oxide will brighten many colors

6. More pastel shades can be achieved by adding tin, zircopax or Mason extender (6700 for all but browns and pinks, 600l Alpine Rose for darker chrome-tin pink stains)

7. Start testing with 2% up to as high as 25% stain additions. Initially a 2%, 5% and 10% additions should provide sufficient range for a final determination

8. Black, being total color saturation, requires at least a 10% addition, usually higher to avoid grey

9. Do not try to judge the fired color intensity by the intensity of the mix before firing especially with the lighter yellows and pinks which invariably require more stain than one might think

10. Zinc oxide influences the color in a glaze more than any other element. Generally, zincless glazes should contain no magnesium oxide. Some stain colors containing zinc are to be used in a zincless glaze (The zinc in the color is in a combined form and will not harm the color, but free zinc oxide in the glaze can destroy the color).

11. Chrome-tin stains are adversely affected by the presence of magnesium, zinc, phosphorous and antimony

12. Calcium oxide in the most common form of calcium carbonate (whiting) should be between 12% and 15% for best color development in a glaze. Adding the molecular equivalent of calcium oxide in the form of wollastonite often gives better color uniformity. The increased silica from the wollastonite must then be subtracted from the glaze formula.

13. Even a very small presence of magnesium (even from talc) will cause a shift in cobalt stains towards a more violet shade in glazes

14. If applying over an unfired glaze or to add multiple coats, cover the glaze with a gum solution (or Karo syrup solution) to prevent disturbing the glaze

15. If applying a slip to a glazed surface for refiring, mix with alcohol instead of water to prevent running

16. Stains can be mixed or combined to create additional colors

17. Generally stains can not be successfully applied to a surface as iron oxide might be to emphasize texture without at least adding a flux to make them "stick" as noted above. Additionally, it is usually difficult to apply enough straight stain to get the intensity without adding it to a slip clay or gum solution so as to be able to apply more actual stain

SLIP, ENGOBE, UNDERGLAZE BASE

Perhaps the most common use of stains is in making colored decorating slips. Since slips (or underglazes, or engobes, all being basically colored clay) do not melt in the firing and the individual constituents thus chemically interact, many of the compatibility precautions noted above do not apply.

The base could be a slip made from the clay being used if the clay is not dark (any grog should be sieved out to prevent a rough surface). This assures compatibity in shrinkage rates. A premixed dry clay body could be purchased (we sell our Cone 10 Glacia porcelain in dry form for this purpose). A prepared low fire white underglaze or engobe could also be used up to Cone 6 although that would be more expensive. For a low fire base, a simple 50/50 mix of ball clay and talc would work. A very simple Cone 10 base would be equal parts of kaolin, flint, ball clay and feldspar. Additional flux as noted in 1 above should be added to all of these except the 50/50 mix. Following is a bit more complex but more dependable alternative, especially for Cone 10.

SLIP/ENGOBE (UNDERGLAZE) BASE