What is salt damp?
Salts can get into the fabric of historic buildings quite easily and frustratingly they really are quite mobile. They easily form solutions in moisture and can migrate through the substrate.
Where do the salts come from?
Both Nitrates and Chlorates can enter the fabric of the building from the ground. Depending on the use of the building in the past, it may have been exposed to salt-laden materials that have contaminated the walls over time. This is generally associated with animal waste/ manure and its decomposition, generating Nitrates. This is a problem experienced with barn conversions. Atmospheric pollution can cause salts from the creation of acid rain that reacts with masonry and leaves salt deposits. These are Sulphates. Sulphates are also common within chimneys and are originate from the burning of fossil fuels. This is often an issue with Victorian buildings in large cities due to the volumes of pollution created during the industrial revolution. It is also possible for salts to originate from the cement-based renders/ plasters applied to the walls (See 'Acid gases and their effect on stone'.
Efflorescence verses cryptoefforescence:
Salts in a humid or damp environment will be mobilised to the wall’s surface by the moisture (either moisture within the wall or moisture absorbed from the atmosphere). The moisture will evaporate and the salts will be left behind. This is called efflorescence.
In low humidity environments, or otherwise dry areas, the moisture within the wall evaporates more quickly than the salt-laden moisture arrives, resulting in salt deposits within the pores of the substrate. The salts crystallise and build up gradually. Eventually, the materials crumble under the stress of the growing crystals. The extent of damage is in part attributed to the size of the pore structures they accumulate within. The smaller the pores, the more likely the salts will break them open and cause decay to the substrate.
Salt damp has the appearance of dampness within the wall however these are often hygroscopic salts which attract moisture from the air. They can self mobilise in solution and travel surprising distances. The wall itself is dry but the salts on the surface are taking on moisture.
