Why do conservators soak some ceramic objects in deionized water?

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Conservators soak ceramic objects in deionized water as a primary method for desalination. This process is necessary when soluble salts—such as nitrates, phosphates, or chlorides—have migrated into the ceramic's pore structure from the environment.1

These salts are problematic because they undergo crystallization as water evaporates. Because salt crystals occupy more volume than salt in solution, they exert significant internal pressure, which can lead to microfracturing, a powdery ceramic body, or the flaking of glazes.1

By soaking the object in deionized water, the salts are dissolved and drawn out of the ceramic body. Conservators regularly refresh the water and test it for salt content, continuing the process until the levels are deemed safe. For some objects, this desalination can also be achieved through water-based poulticing, such as using paper pulp.1