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Temperatures

Many ceramic problems relate to a lack of understanding about what is happening at each stage of a firing, there are just so many materials that are doing so many things. This part of the database will help solve that problem. In a material-centric ceramic information universe it quickly becomes evident that each material has its own way to decomposing and melting. Many materials (especially ground minerals) have multiple decomposition events where they change crystal structure (accompanied by volume and state changes), release gases (e.g. CO2, H2O), soften and melt. This area of the knowledge base brings together all of the events in the thermal decomposition that have been defined for individual materials or minerals (however there are obviously interactions, see paragraph below). The result is a master temperature line that can be examined for any specific range to see what is happening there and specific temperature events that are linked to other parts of the database that relate to them.

One key thing to remember about studying the thermal history of how a material decomposes, alters and melts is this: In glazes and clay bodies materials interact, often they do not evolve in the same way when they are part of a mixture of other materials that is being heated. For example, barium carbonate decomposes at 1450C by itself, but in a glaze it readily dissolves in the glass melt. The story is the same with calcium and magnesium carbonate. Kaolin by itself has a very high melting temperature, but dissolves readily into active melts at low temperatures. When low melting materials are part of a glaze recipe, for example, they act as catalysts that accelerate the reactions of other materials. Also, if these catalysts create a glass phase that actively dissolves materials that normally go through complex phase and crystal changes during heatup, none of these changes ever get a change to happen because the particles have dissolved. In addition, another level of complexity arises: the product of a mix of many material will often have its own complex thermal history that exists only as that mix. For example, certain crystal species only grow where the chemistry is just right, no material may have that chemistry, but a mix can.

Related Information

Links

Articles Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Articles A Low Cost Tester of Glaze Melt Fluidity
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Articles Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware
Glossary Decomposition
In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization
Glossary Melt Fluidity
Ceramic glazes melt and flow according to their chemistry and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
Glossary Water Smoking
In ceramics, this is the period in the kiln firing where the final mechanical water is being removed. The temperature at which this can be done is higher than you might think.
Minerals Limestone, Calcium Carbonate

By Tony Hansen


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