•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
A fired visual effect on bare clay surfaces in fuel burning kilns (especially wood). Clay surfaces that have been flashed have been subjected to a thermal history of variations in flame, ash, kiln atmosphere and even imposed vapors (like salt and soda). Historical ceramics often had flashing simply as a consequence of material quality and the nature of the fuels used in firing. In recent years there has been a focus on the reproduction of the look, various methods seek to reproduce the process, others only the final product. But the modern kilns and materials used by most are highly consistent in comparison so there is a need to understand the chemistry and process mechanisms that produce the effects to better reproduce them. Notwithstanding that, most practitioners just employ recipes they have found or received and fire as instructed. The degree to which flashing appears in the ware is attributed to the mystique of the process.
But some basic technical things are known about the process. It is known that slips of high alumina content react and respond visually (flash) to the atmosphere containing ash and soda/salt (which are high in SiO2 and KNaO). For that reason, formulations high in kaolin are typical (no common material has a higher percentage of Al2O3). But kaolin is so much more refractory than any clay body and its drying shrinkage is much lower, that means a slip made purely of it will flake off during both drying and firing. But it can be combined with other ingredients that make it more similar in drying and firing to the body (e.g. feldspar, stoneware clays, bentonite). Various kaolins are known to produce better flashing than others, this could be because of differences in the natural soluble salts present, especially those containing iron. Pure fine alumina powder can also be employed in the same way (but in a much smaller proportion in the recipe since it has zero plasticity and is even more refractory than kaolin). Alumina and even silica of the finest particle sizes are most likely to react with fumes in the kiln atmosphere.
Example of flashing on ware from a Manabigama wood fired kiln
From Robert Self. This firing went past cone 13. The body is Laguna Speckstone.
Example of flashing from wood firing
Made by Robert Self. This is Laguna White Stoneware body fired to cone 13 in a Manabigama wood fired kiln.
This piece is thrown from calcined kaolin
Calcined kaolin has zero plasticity. 25% bentonite had to be added to make it plastic enough to make this piece. Why bother? Because this will flash heavily in reduction firing.
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Salt, soda firing
Salt firing is a process where unglazed ware is fired to high temperatures and salt fumes are introduced into the kiln chamber (normally by a spray in the burner ports). The sodium in the salt forms a vapour cloud in the kiln. That sodium, along with the silica and alumina in the clay, combine to fo...
A firing technique used by necessity in many countries and by choice in others. In a properly designed kiln wood is capable of delivering high temperatures so it is possible to make stoneware and porcelain. The kiln chamber in a wood kiln subjects the ware to alot of ash and smoke and this profoundl...
By Tony Hansen