Advancer Oxidation in Wood Fired Kilns

 

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By Marshall Browne

Important information regarding the oxidation differences between Advancer and CN192 shelves in wood burning kilns.

Advancer forms a glass layer on the surface in an oxidizing atmosphere. The actual process is oxidation of the silicon carbide grain. We double fire Advancer in an oxidizing atmosphere to intentionally form a glass layer. The resulting glass surface is actually a protective layer (when intact) that prohibits further oxidation of the Advancer surface  beneath. Advancer forms more of a glass layer than CN192 because it has a  higher surface area of silicon carbide grain (i.e., fine grain sizing and virtually no porosity).

The combustion material (wood) is a likely source of alkalis (e.g., potassium) in the atmosphere of these kilns. The alkalis are likely fluxing the glass layer and dropping its viscosity so it drips more readily. When the glass viscosity drops, two things occur that compound the problem.

(1) Oxygen can penetrate the glass layer faster by diffusion thereby increasing the oxidation rate.
(2) Oxidation off gases produce bubbles in the glass which is the "frothing" or "foaming" observed. The bubbling is more pronounced in humid environments because a surface reaction with water can give off extra CO gas. The wood is also a likely source of water that can make a bad situation worse.

Once again, the main difference between the oxidation rate of Advancer and CN192 is surface area. CN192 will likely experience an increased oxidation rate when the surface oxide (glass layer) is infected by alkalis; however, the oxidation rate will be much slower than Advancer given the low surface area. Bubbling or "frothing" would also be less pronounced with CN192 given its low surface area. Generally speaking, Advancer has much better oxidation resistance (i.e., a protective bond) than CN192 but the ultimate life of Advancer in this environment will have to come from experience. The exact properties of the altered glass layer are difficult to predict and are greatly influenced by temperature and the type of alkalis that are created as a by-product of the combustion of wood.

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