Architecture : Preservation : Art: October 2010

I know when we think of threats to historic structure salt is generally not very high on the list, but it should be. All masonry and stone structures are susceptible to salt damage, for the sake of simplicity I will focus on brick. Bricks are porous and dissolved salts can enter them via capillary suction. The salt then crystallizes inside of the brick. Salt crystals expand and contract in the presence of water, over time this can damage the masonry’s structural integrity on a microscopic level.

Salt crystals can form on the interior or exterior of a masonry structure. Salt crystallization on the exterior of a structure is called efflorescence; this form of salt crystallization can cause pitting on the structures surface. Salt crystallization on the interior of a structure is called subflorescence; this form of salt crystallization can cause weaken and destroy the materials structural integrity. Efflorescence is externally visible which makes it easier to diagnose and treat than subflorescence. The presence of subflorescence is not externally conspicuous, making it a silent killer, the ninja of salt damage.

The susceptibility of different bricks to salt damage depends on the size and distribution of the bricks pores; large pores tend to allow salts to flow freely into and out of the brick, however small pores tend to trap salts within the brick.

One method of treating subflorescence is local desalination, and it is achieved by saturating a wall with water and applying a poultice over it. A poultice is a removable layer of material applied to a structures surface to absorb the dissolved salts from the wall. A poultice can be composed of paper fibers, cotton fibers or clay.

Here are some images of an experiment SCAD historic preservation students preformed to compare the effectiveness of paper and clay poultices in removing salts from Savannah gray brick.