The process of failure due to salt-promoted efflorescence in pottery by digital and scanning electron microscopy and by analysis of the effects of NaCl and CaCl₂ on the phase equilibrium of salt water systems. These studies indicated that the growth and connection of the micro-cracks in the particles of samples under the stress of salt crystallization was the destructive cause of pottery efflorescence. Furthermore, there were significant changes in quality, morphology and flexural strength of pottery due to salt crystallization, depending on environmental relative humidity. The most devastating effect was due to crystallization of sodium chloride. Crystallization of sodium chloride could be slowed down by the addition of CaCl₂ , which therefore can play a role in preventing the destruction of pottery due to efflorescence.