Salts of the Earth

Check out Gary Holt’s Chemistry, Application, and Firing of water-soluble metal salts, but don’t do anything until you read the Practical and Safety Concerns. Then, you can look at the Recipes, Color Chart, and Test Tiles.

Latex resist was painted on the lip and underside of this porcelain vessel and 10% potassium dichromate was painted on the entire bowl. The latex was then removed and the following WSMS solutions were dotted and brushed on: 15% cobalt chloride, 50% cobalt chloride, 25% iron chloride, 50% nickel chloride and an "all gray" solution (10 grams each of potassium permangantate, cobalt chloride, molybdic acid and iron chloride in 100cc water).

Beautiful, soft, muted-color brushstrokes and washes of water-soluble metal salts decorate Gary Holt’s translucent porcelain bowls and plates. The simplicity and quiet presence of his works belie the years that Holt spent experimenting and perfecting his technique. Using water-soluble metals salts (WSMS) demands excellent technical skills and careful attention to details.

Water-soluble metal salts are often compared to watercolors in application and decoration. They produce a variety of interesting effects on ceramic works, such as halos of color, fumed or smoky halos, solid shapes with soft, diffused edges or solid shapes with crisp sharp edges. They can be used to color terra sigillata and will not dull or matt the surface as oxides will.

 Holt has been testing and experimenting with metal salts for more than twenty years, while running a successful pottery studio in Berkeley, California. With little research literature available on WSMS, he has had to develop his own techniques through trial and error.

Chemistry

Water-soluble metal salts are simple solutions that are composed of nitrate, chloride and sulfate forms of metals, which dissolve in water. They are simpler solutions in comparison to glazes, which are usually composed of fluxes, alumina and silica, as well as oxides, carbonates or stains, and which may contain metal elements. Metal carbonates and oxides are the most commonly used form of metals in glaze, but more than 20 water-soluble metal salts may also be used.

Water soluble metal salts are extremely toxic and should always be used following the utmost safety precautions. Carefully read and adhere to the guidelines in this post whenever using these salts.

This article was originally published in Ceramics Monthly.
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Application

After latex resist was applied to the lip and underside of the bowl, a 2% gold chloride solution was painted around the entire bowl and a 50% tin chloride solution was painted in broad vertical strokes. The latex was removed and a 15% cobalt chloride solution was painted in dots and stripes on the inside and outside of the vessel.

Holt prefers to use Southern Ice porcelain, formulated by Australian ceramist Les Blakebrough. The plasticity of the clay compares to Limoges porcelain clay. It does not have bone ash as part of its body. Holt likes Southern Ice for its translucency and whiteness, and he has noted that the color of the clay affects the brightness or clarity of the metal salts. The darker the color of the clay body, the more muted the colors will be. As an alternative to porcelain, Holt suggests a white clay body, stoneware covered with white slip or plain stoneware.

Often the color may “sink” into the clay body, which may or may not be desirable. Holt applies an opaque glaze to the inside of a pot if he does not want the color to migrate to the other side of the wall. Also, to keep his metal salts on the surface of the wall longer, he uses a nonreactive thickener. The thickener has the added effect of intensifying the colors.

Firing

Latex resist was painted on the lip and underside of the bowl. A 15% cobalt chloride solution, a 50% cobalt chloride solution and a 50% tungsten solution (with a small amount of sodium hydroxide to help dissolve the salts) were applied with an eye dropper onto the surface of the bowl. Phosphoric acid was added with an eye dropper to create halos by "removing" the central area of a previously painted color.

Holt states that the clay vessel or form must be bisque fired between applications of metal salts. This technique is called “setting” the color. All water-soluble metal salt colors are temperature sensitive. The colors will change depending on the firing temperature.

Practical and Safety Concerns

It is absolutely essential to observe safety and health precautions when using these materials. Holt refers to the Merck Index whenever he uses an unfamiliar material. As always, the potential hazards depend on the concentration of the chemicals used and the safety practices of the ceramist. Holt believes everyone can use WSMS with the required attention and care. A Materials Safety Data Sheet (MSDS) should accompany each product when purchased. If the supplier does not provide an MSDS, buyers should ask for one. These information sheets will provide the precautions for storing, using and disposing of the products.

Latex resist was used to mask two rectangular areas before applying salts. On the left, a 50% cobalt chloride solution was painted on. Then phosphoric acid was added to create dots. On the right, a 2% gold chloride solution was painted on. A 50% tin chloride solution was dotted on with a brush. A second bisque firing was done to set the colors. Then a 30% vanadyl sulfate solutionwas painted on the left and 15% iron chloride was painted on the right. Separate solutions of 15% cobalt chloride, 50% cobalt chloride and 50% nickel were dotted on with an eye dropper.

Water-soluble metal salts should be stored in containers separate from regular glaze mixtures. The containers should be well labeled to avoid any accidental mixing of the chemicals. In addition, acids and bases should be kept in separate containers.

A NIOSH-approved respirator should be worn when measuring and working with water-soluble metal salts so the chemicals are not inhaled or ingested. Eye goggles should also be worn, especially when using acids. For hand protection, Holt wears two sets of gloves-a latex glove over a nitrile glove-because skin can easily absorb these chemicals.

Besides the health and safety concerns, local laws and regulations regarding the proper and safe disposal of the chemicals should be checked. When mixing the chemicals, Holt mixes only small amounts so that the disposal of the remaining solution is kept at a minimum.

Though the health and safety concerns are numerous and may appear overwhelming, they are necessary precautions to a rewarding and exciting facet of ceramics decoration that has been explored by few ceramists. Holt continues to experiment and add to his extensive body of knowledge on the subject and generously shares this knowledge through seminars and workshops. As evidenced by the fruits of Holt’s experimentation, water-soluble metal salts present many possibilities for new forms of expression in ceramics.

Color Chart for Water-Soluble Metal Salts

Key to Test Tiles	Background: Molybdic Acid	Background: Ammonium Chromate
Background: Copper Chloride	Background: Sodium Chromate 30%	Background: Vanadyl Sulfate