Ceramic Thermal Events

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.

By Tony Hansen

See Also

  • 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.

  • 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.

  • Melt Fluidity

    Glazes become fluid when they melt, they are molten. The fluidity (or viscosity) of this melt needs to be considered, especially when troubleshooting problems. While two different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more fluid th...

  • Decomposition

    Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view).

    "Elements" are one kind of atom which cannot be broken down any further (except b...

  • 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

  • Limestone, Calcium Carbonate

    Also called GCC (Ground Calcium Carbonate), limestone is a very common sedimentary rock. Calcite and Aragonite minerals are the pure crystalline forms of CaCO3 (limestone contains them), but limestone...

  • Water Smoking

    Refers to the period in firing where the last of the mechanical water in body and glaze are being released. Firing can normally proceed quickly after this water has been ejected (up to 750C/hour is common in industry). Typically firings are taken to the boiling point (or slightly beyond) and held th...

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