This problem is common in many terra cotta materials but can also surface in others.
Ceramic soluble salts.
In high humidity salts become soluble and in low humidity they crystallize.
How bad can efflorescence of soluble salts of fired ceramic surfaces be.
Barium carbonate can be used to precipitate the salts inside the clay matrix so they do not come to the surface on drying.
The water dissolves the salt and draws it out of the ceramic.
Soluble salt compounds may be introduced into the ceramic pores from ground water agricultural run off seawater or other sources.
Soluble salts respond to changes in humidity both high and low.
The most common method of removal is by soaking the ceramic in deionized water for extended periods of time.
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When soluble salts are present within the ceramic structure at a percentage considered threatening the conservator must remove them.
This terra cotta clay vitrifies here at 1957f cone 03.
If the ceramic is subjected to cycles of drying and wetting the salt compounds will crystallize and mobilize alternatively producing severe pressures on the ceramic body from within the pores ultimately.
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