Furnace atmospheres - ceramics and porcelain

October 2002 Analytical Instrumentation & Environmental Monitoring

Porcelain, and especially its more expensive derivative, bone china, is one of the more expensive types of ceramic material made for general use. Many households still have a set of best porcelain tableware that is used when guests come to dinner.

Manufacture of these products is a skilled process that requires careful attention at many stages of production to prevent high rejection rates. Bone china (porcelain with the addition of bone ash) should be almost transparent with an ultra-smooth surface. There are other domestic ceramic goods, such as bathroom or sanitary ware that also require careful attention during manufacturing, although not quite as critical as fine porcelain. However, the surface quality of a washbasin or bath is very important and greatly influences the selling price.

The manufacture process for porcelain has four main steps:

* Working: Special white clay (kaolin or china clay) is worked to shape by a potter or by an automatic machine.

* Pre-firing: The object is fired in a kiln under oxidising conditions from ambient to approximately 900°C. This dries the product and removes organic substances.

* Enamelling: After applying glaze the product is fired under oxidising, reducing, or neutral conditions between 1300 and 1450°C. This stage is critical and requires careful control of the oxidising or reducing conditions of the atmosphere and of the temperature profile.

* Decorating: The product is now decorated with special paints, coatings, or precious metals. After decorating, the object will receive a final firing under conditions determined by the coatings used.

* Kilns are of many types, but fall into two main categories: batch and tunnel kilns. In tunnel kilns, the products are moved through the length of the kiln on wagons. In batch kilns, the products are loaded and remain in place until the firing cycle is completed. There are many variations in design, degree of automation, and energy source (gas, oil, or electricity). Most glazing kilns are natural gas-fired, which allows fine control of the furnace atmosphere during this critical step.

Analysers allow the manufacturer to maintain the exact reducing atmosphere needed to impart the right finish to his porcelain or ceramics. Each product type will have its own processing characteristics and once the correct atmosphere for that product is determined, the analyser reading is used to reproduce and maintain those conditions. For such applications, OEN Enterprises recommends its Thermox WDG-IV or WDG-HPII as the analysers of choice - they would usually operate at the high temperature zone of the kiln.

For more information: Mike Andrews, OEN Enterprises, 011 792 2859, [email protected], www.oenenterprises.co.za

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