POTTERY PRODUCTION PRACTICES: Mixing it Up

September 1, 2007
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The thorough blending and grinding of glaze and raw materials can yield consistent results in a finished glaze.    



Glaze mixing is a central component in many ceramic operations. The labor-intensive steps involved in forming, glazing and firing ceramic ware can often be negated by a single error in the glaze mixing process. Essentially, glazes are composed of alumina and silica combined with fluxes in varying ratios to form a vitreous glass surface when heated.

While numerous raw materials are finely ground at the mine or processing plant, some are not, and coarse materials can result in glaze imperfections. A thorough blending and grinding of raw materials can yield consistent results in the finished glaze. Based on a proven grinding method, a new glaze mixer* uses a ceramic mortar and pestle operated by an electric motor. When the revolving pestle comes into contact with the ceramic bowl, raw materials are mixed and reduced in particle size, which can have a dramatic positive effect on the fired glaze.

Small test batches (0.5-9 oz) can be placed into the bowl either wet or dry. The mixer is ideally suited for use in the potter's studio, allowing for the consistent blending of small test batches of glaze or the grinding of raw materials.

* Available from Nidec-Shimpo America Corp., Itasca, Ill.

Figure 1. Natural bone ash before (left) and after grinding in the mixer for 1 hour.

An Even Mix

A direct relationship in ceramic materials exists between particle size and the material's ability to melt when subjected to heatwork within the kiln. Large particles, having less surface area, do not melt as readily as smaller particles, which have greater surface area. Raw materials like Cornwall stone, borax, gerstley borate and natural bone ash clump together in storage and do not easily merge into a glaze mix (see Figure 1).

Often, large particles do not blend uniformly, causing defects in the fired glaze. This effect is also noticeable with metallic coloring oxides, which can have relatively coarse particles in comparison to their finer carbonate forms. The action of the new mixer grinds and blends glaze materials to provide uniform color and raw material distribution.

The glaze mixer's dimensions are 8.5 x 13.75 x 24.5 in., and the mixing bowl is 6.5 in. wide. The mortar mixing speed is 7 rpm, while the pestle's speed is 115 rpm. These speeds provide for the controlled formulation of ceramic raw materials.

Figure 2. Glaze samples on the left of each photo were not mixed with the new mixer. Defects include a) black iron oxide specking, b) chrome oxide specking and c) nepheline syenite specking. Glaze samples on the right were mixed with the new mixer and show no glaze defects.

Put to the Test

The new mixer was recently tested with three glaze samples (see Figure 2). All of the glazes were applied to vertical test tiles and fired to c/06/ox (1828°F). The mixed glazes reduced the particle size of black iron oxide, chrome oxide and nepheline syenite in the three glaze samples.

Figure 2b.

The tiles on the right side of each image represent thoroughly mixed glazes that were placed in the new glaze mixer, which provided an even dispersion of metallic coloring oxides and raw materials.

Figure 2c.

A studio potter's ability to produce reliable results is an important factor in any successful ceramic endeavor. It is often said that the only thing consistent about ceramics is its inconsistency. While many raw materials are processed for direct use by potters (in terms of particle size), some of the most widely used materials surprisingly still need a finer degree of processing. The glaze mixer is a useful tool in eliminating one source of inconsistency by efficiently grinding and blending ceramic raw materials.

For additional information regarding the glaze mixer, contact Nidec-Shimpo America Corp., 1701 Glenlake Ave., Itasca, IL 60143; (800) 237-7079 or (630) 924-7138; fax (630) 924-0340; e-mail info@shimpoceramics.com; or visit www.shimpoceramics.com.

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