Pottery/Safely Coloring Clay Bodies
I am new to pottery but am interested in proceeding past the beginner class. I would like to try and pre-colour my clay before throwing. I am somewhat concerned about the additives and what is "safe" to handle freely without problems (mason stains, oxides, under glazes etc.) What are the best material/methods to colour my clay that I don't need to worry about handling it. Is the concern more with just breathing in the dry contents than after they are mixed in liquid or clay ?
Thank you for your recent question. Coloring clay is as simple wedging colorants into the clay body. However, the colorants used to color the clay is not that simple. I will try to explain the methods of coloring clay as well as the safest raw materials to use. Remember though, all raw materials in their powdered form are toxic to some degree, primarily by inhalation. Others are more toxic and even hazardous when inhaled or ingested into the body by mouth or open wounds. Care should be taken when using any raw materials and all should be handled as if they are as hazardous as lead or barium.
Before I give you information on the types of raw materials to use to color your clay body, let me explain the process. Gather the clay you wish to color. Separate it into 1 pound balls. This will make it easier to add the raw material colorants. For each 1 pound ball, punch a deep impression into the ball. You will pour the raw material colorant into the impression. Carefully squeeze the ball together to close the impression without leaving behind any air pockets. The best way to do this is to begin squeezing the clay from the bottom of the ball to the top where the hole is.
Now, all you need to do is to begin wedging the clay. If you want to put two or more balls together to wedge them, then this is fine, however, you must make sure that all of the raw material colorant is completely homogenized into the clay body. You will know this when the clay has changed the color of the raw material colorant and there are no streaks in the clay body. This can take up to 30 minutes to wedge.
As for colorants, it is possible to add raw material colorants in their oxide forms to the clay body, however, because oxides forms are often unstable, you may not get a good clean uniform coating of the color throughout the clay body.
The best materials to use are stains. Stains are oxides that have been mixed to create a uniform and true color, fired and then pulverized into the powdered form you purchase. Safety in using stains is far more reliable than using oxides. Yet, stains contain the same raw material colorants, plus silica and alumina in the make up process so, each stain should be considered toxic in its own right.
I have included information on stains from a web sheet from Clay Planet about ceramic mason stains. Because there are literally hundreds of different stains available on the market, it would be impossible to tell you whether it contains any toxic materials.
You mentioned the term "handle freely without problems" in referring to safety when using. Never consider any clay or glaze raw material as safe. All have at least one toxic feature...dust consumption that can damage your lungs.
INCLUDED INFORMATION ON STAINS
Stains are in general terms, combinations of oxides that produce color. In ceramics the term is usually reserved to refer to commercially prepared colorants. The oxide or oxides, often combined with an opacifier, have been blended, then fired together (fritted), 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 cant 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:
1. COLOR: Stains provide more color variations than is practical for the average person to develop. In addition, since stains are fired, the color of the powder closely approximates the fired color in a glaze, engobe, etc. This also permits easier mixing of stains to create additional colors.
2. CONSISTENCY: Color variations from batch to batch are minimal and results are predictable.
3. USE: Stains are versatile and easier to use than straight oxides. Most are formulated to remain stable at high fire and are appropriate for mixing with a variety of mediums to produce colored glazes, slips and engobes (both are underglazes), china paints, enamels, silk screen colors, decals, colored clays or direct brush or air brush application. They may be applied to greenware, bisque or even glazed pieces if refired.
4. SAFETY: Stains are technically insoluble in water since they are fritted so risks in handling the powder and wet glazes is diminished (proper masks and gloves are recommended however).
ADDITIONAL COMMENTS, USES AND SUGGESTIONS
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 also 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. Normally 10-15% but that could make an expensive slip depending on the stain added.
4. The speed of a firing and the cooling cycle can effect the color. Test with different clays and at different firing speeds if possible.
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
TO MAXIMIZE SUCCESS WITH COMMERCIAL STAINS, REFER TO THE MANUFACTURERS SPECS
I hope this information assists you in your use of colorants for coloring clay bodies. If you have further questions, please feel free to contact me at any time. I am always at your service. Much luck to you.
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat