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Desalination is the process of removing soluble salts from water. Sodium chloride (yes, table salt) is a naturally occurring soluble salt in soil in many parts of the world. There are also soluble salts such as nitrates, phosphates, and sulfates. In addition to causing pottery breakage, they can turn a ceramic vase into a pile of dust.
Many years pass before archeological pottery is discovered. In wet conditions, salts dissolve in the soil and seep into the tiles. Sunlight dries out soil and pottery, pulling moisture and salts to the surface, where they dry and crystallize.
Until the archaeologist digs the object out of the ground, the pottery is perfectly stable. Porous ceramics have a lot of air spaces between the grains. Water fills the pores of ceramic when it is wet. Salt will be dissolved in the water if the soil in which the pottery is buried is salty.
As ceramic dries, water and dissolved salts rise to the surface.
In pores near the surface, water evaporates and salt crystallizes. As crystals grow, they exert pressure on the pores, fracturing them.
Pottery is sometimes decorated with engobe, a refined clay that is slightly denser than pottery. The denser engobe makes it difficult for salt to migrate through it, and most of it crystallizes under it, pushing it out.
In the end, the ceramic loses surface detail and decoration and becomes brittle.
Have you ever seen objects like this?
Want to know more about this subject? We have a brand new course waiting for you here!
See details of the course with Profa. Ana Carolina Delgado Vieira on Conservation of Archaeological Materials
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Photos: Google - Penn Museum
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