Whilst these defects may occur due to air bubbles or contamination in the glaze, they can also occur due to firing issues, as follows:
The ware may not be fired sufficiently during the bisque firing, or during the glaze firing before the glaze melts, for organic and sulfur compounds to burn out.To my mind, it is better to complete this in the bisque firing, as a lower risk strategy. Firing should be slow enough both to allow heat to spread through the clay, and to give enough time for matter to burn out. This is best achieved with a slow firing or a hold at about 650°C, rather than bisque firing quickly to a high temperature so that the outside may end up being overfired and the inside underfired, a bit like a burger on a barbecue!
There may be insufficient oxygen, or excessive carbon monoxide or dioxide, in the kiln for organic and sulfur compounds to burn out in the bisque firing. This may occur if there is insufficient air flow through the kiln at temperatures up to about 650°C. Airflow should be ensured through the use of a kiln vent, or firing with the bungs out, to this temperature.
Given enough air supply, air flow round the ware can be improved by good loading of the kiln. This includes using half shelves to make the kiln space more open, and mounting work on tile or plate setters, or on stilts, where appropriate.
Particles in the kiln may land on pots, causing defects. This may be bat wash, clay or glaze falling off a shelf or prop, or pinging off a kiln wall or the roof.<HR>
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