Porous ceramics may occasionally contain water-soluble salts absorbed from various sources. In dry conditions, the salts are visible as loose white crystalling powder.
Salts can occur in low fired porous ceramics such as:
In some cases, salts were part of the original constituents of the clay body itself. In others, materials used for repair in the past, e.g. plaster, or inappropriate cleaning materials, may react and cause salts to form in the ceramic.
Repeated cycles of changing humidity will cause the salts within the pores of the ceramic to alter their state from solution (in damp conditions) to crystals (when dry). As the crystals tend to take up more space in the pores of the ceramic than when in solution, the pressure this exerts can disrupt the ceramic causing the glaze to flake, or the body to crumble.
Surface salts, visible as a white crystalline 'powder' can usually be gently brushed away with a soft brush, though they may reappear (and cause damage) if relative humidity fluctuates. Relative humidity measures the amount of moisture that is in the air relative to the maximum amount of moisture that the air could hold at that temperature.
Conservators use poulticing or immersion techniques for removing salts from badly affected pieces. An object with extensive salts or a flaking / crumbling surface should be referred to a ceramics conservator.