All Glossary

Flashing

A visual effect that occurs in wood and salt firing of ceramic ware. Many potters value the effect and use special materials and firing methods to enhance it.

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Details

A fired visual effect on bare clay surfaces in fuel burning kilns. 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 suitability and the nature of the fuels and firing. In recent years there has been a focus on controlled reproduction of it.

In sodium vapor firing, the reaction with the clay surface forms a very thin sodium alumino-silicate glaze. Flashing happens where this reaction is slight enough that iron and other body colorants show through rather than being covered by a full glaze. “Flashing slips” for salt and soda firing are typically formulated to stay only partially active (thus non-vitreous). It is a balance. Too refractory and flashing is weak. Too fusible and one gets a glaze instead. Slip formulations having high alumina content react best with the kiln atmosphere. For that reason a high kaolin percentage is typical (for its reaction-available Al₂O₃). Some pure fine alumina powder can also be employed in slip recipes (using so-called ‘reactive’ finer particle sizes). The color development is understood to happen during cooling. Matters of study include understanding kaolins: Some are known to produce better flashing than others. And better understanding the sodium reaction and when it happens, iron state changes and crystal development are invaluable.

In wood firing, flashing is mainly a surface staining and very thin glaze-forming effect. Wood ash contains fluxes (like oxides of potassium, calcium, sodium). As wood burns the ash and these volatile alkalis enter the flame stream and react with hot clay surfaces. The reaction lightly attacks the clay surface, forming a microscopic alkali-rich glass layer producing oranges, reds, greys and subtle sheens. Where ash deposition becomes heavier, flashing can turn into actual ash glaze. A large part of flashing color comes from iron already present in the clay body or slip, the alkali reaction changing how that iron appears (according to kiln atmosphere). So flashing can be seen as an iron-response to alkali vapor. Pieces facing direct flame often flash more. Like salt/soda firing, slips improve flashing. Cooling also matters as it affects iron oxidation state and crystallization.

Related Information

Flashing from wood firing

The range of possible surfaces

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Made by Robert Self. This is Laguna White Stoneware body fired to cone 13 in a Manabigama wood fired kiln.

Test piece to demonstrate flashing

From a Manabigama wood fired kiln

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From Robert Self. This firing went past cone 13. The body is Laguna Speckstone.

Intense flashing at cone 10R, courtesy of nepheline syenite tiles

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This flashing method works in reduction or oxidation. It was done by bisque firing a thin nepheline syenite tile leaning against the piece. The tiles are made using a 5:95 mix of Veegum and Nepheline (or a 10:90 mix of raw bentonite and nepheline). Sodium vapours from the tile are deposited on the surface and affect it profoundly enough to influence the glaze firing. A similar effect can be had by spraying on a saltwater solution, it should work whether done prior to the bisque or the glaze firing (of course lots of testing would be required to perfect it).

Soda fired porcelain vessel

By Heather Lepp

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This is a small cup-sized object made from Plainsman P600 (simply composed of Tile #6 kaolin, nepheline syenite and quartz). It is valued as a product-of-the-process piece, consigned to the "kiln God" as unglazed. It exhibits carbon-trap, soda glaze deposition and flashing. The soda-vapour atmosphere of the kiln glazed one side of the vessel early enough in the firing to trap carbon under a crystal-clear glass. Often such glazes are crazed, but this one likely is not because the body contains 25% quartz, giving it a high thermal expansion. The other side of the piece exhibits tones of red, brown and yellow on the bare, vitreous porcelain surface - this is characteristic of "flashing".

Links

Glossary	Salt and Soda firing Salt firing is a process where unglazed ware is fired to high temperatures and salt is introduced to produce a vapor that glazes the ware.
Glossary	Wood Firing A method of firing pottery in a fuel burning kiln (using wood) rather than gas. Temperatures beyond cone 10 can be achieved.

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