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KILN CONNECTION: Focus on Color

February 1, 2011
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I am flying home as I write this column, having spent the last week working for a small company in the center of Mexico. It was a wonderful trip-great weather, good flights (by that I mean that they arrived on time and my luggage actually arrived with me), and a lovely location in San Miguel de Allende. On top of that, it was the first international trip that my new wife and I had taken together.

What made a good trip even better, though, was the opportunity to work with my client-a company located outside of San Miguel. It is an unusual company; the owner/manager is a degreed architect who gradually developed a number of ceramic decorative/structural designs. Over time, he decided to design ceramic products as functional art, and he started his own company.

His design genius can be seen in many high-end construction projects all over North America, and he has begun to market his products in the Far East and Europe as well. The company's products include floor and wall tile, architectural moldings, and outright art products. They all have one thing in common-they are so strikingly beautiful that it is almost a shock each time you see them. At every exhibition, visitors routinely stand transfixed when they see these products.

Important Considerations

From a manufacturing point of view, however, the products are difficult. Dozens of glazes, stains, engobes, multiple red and white bodies, and separate bisque and glaze firings make this a tough plant to run, particularly because the quality levels are so stringently set. Colors must be matched, of course, but the subtle tones around puddled glazes outlining relief areas have to be perfect.

The facility runs as a job shop and has literally a dozen kilns, ranging from continuous kilns (tunnel and roller) to a variety of periodic kilns. Because production lot sizes are varied, the kiln selected for firing a particular product can vary from one day to the next, and cycles for a single product can range from 3 to 17 hours, cold-to-cold.

Despite these variables, colors must match. Our studies of color vs. firing cycle data show that several key parameters must be maintained, regardless of kiln cycle, if colors are to be correct. The basic key factors affecting color include the following.

Atmosphere

If the O2 levels go below 3-4% at soaking temperatures, subtle unacceptable color changes can occur. I have not found much difference in color above the level of 4% O2. Some pieces can cause their own atmosphere reduction, however, by evolving CO+H2 in delayed oxidation. The reducing agents strip oxygen atoms away from oxides on the glaze, causing extreme color variations and changes in localized areas.

Temperature

When firing so many different kilns and demanding a consistent result, operators can only use the kiln thermocouples as a guideline, at best; the use of PCE monitors and/or firing rings is absolutely imperative. We all know that short cycles require higher temperatures to achieve the same level of body maturity as long cycles. In addition, thermocouples do vary and are not the accurate sensor that everyone would like to believe. Field temperature and moisture conditions, calibration, ageing of lead wire and thermocouples, etc. can result in significant errors of 1.5% or more.

Cooling Rates

The initial cooling rate affects crystal growth in the glaze, and can alter the glaze shine or degree of matte appearance. The final cooling rate can subtly change the glaze coefficient of expansion until the fictive temperature is reached. This can be an issue in glaze fit, when the body and glaze coefficient of expansions are a little too close for comfort.

Color Mastery

Mastery of these factors-along with continuous testing and improvement-is essential to match glaze colors. Especially when ceramic products such as these sell for $100-300 per square foot, they have to meet demanding standards. Knowing the impact of key variables is the best way to manage color.

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