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 and held there for the amount of time necessary to remove all the water (the "water smoking soak"). The degree to which the drying process has removed water determines not only the soak time needed at boiling point, but also the temperature at which the water smoking soak can be conducted. Contrary to what you might assume, for well dried ware this smoking is more effectively conducted beginning at 140C and proceeding gradually to 260C. However, for a typical studio potter, care and testing are needed, since ware is normally only air-dried when it goes into the firing and bodies are more plastic and fine grained.
Put almost-dry ware into a kiln. This happens!
An example of what can happen if ware is heated too fast during early stages of firing. This bowl was not quite dry on the base, it is Plainsman M370. Even though the firing proceeded to 220 degrees and soaked for an hour, it was not enough time for the water to escape before the second step in the firing schedule.
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