Salts in Masonry

Salts including sulphates, chlorides, hydrates and nitrates can cause accelerated damage and loss to porous stone. Naturally occurring sulphates include gypsum, mirabilite and magnesium sulphate; while the most frequently found chloride is sodium chloride (common salt).

Thus, one of the main risks to historic stonework is the presence of dissolved salts in the stone.

Salts present in masonry can be due to the use of dredged sands/ aggregates, but certain stones (e.g., carboniferous sandstone) can naturally suffer with it more, which are geographically localised. Outside influences can induce salt efflorescence, such as seawater in coastal areas, or salts present in cleaners and detergents. Salts may deposit as a film of a few microns thickness to a ‘crust’ of several millimeters.

The safe threshold for sulphates is in most cases no more than 0.1% by weight, though where sulphates are in the form of gypsum, a higher concentration is considered safe. For chlorides, a concentration not exceeding 0.03% is considered ideal, while for nitrates, 0.05% is often cited as the safe upper limit.

A key weapon in the armoury of stone conservators’ ongoing battle with salts is the poultice. It consists of the application of water in the form of a clay or paper pack. They work by there being a higher concentration of salt solution within the stonework rather than within the poultice, the poultice then draws the excess salt ions out and into the poultice by diffusion in the process of drying.

When these materials are applied to the stonework, the task of the water is to dissolve and remove salts/other contaminants in the stone that tend to cause damage.

Clays, talc, chalk, lime mortar, paper pulp, cotton fibre and cellulose powder are among the most common natural materials used in the poulticing of stonework for desalination purposes. Commonly used clays with good water absorption and adhesion properties include sepiolite, attapugite, bentonite and kaolin. Sand is sometimes included in poultices too.

There is a requirement not to use too much water, as this can sometimes have the unintended effect of causing dissolved salts to migrate more deeply into the stone. Restorative Techniques can advise on the ideal desalination poultices for stonework.

Poultices are usually applied by hand to areas of finely carved historic stonework, but cruder machine-based methods of application by spraying are often used on larger areas of rough stonework in order to increase efficiency.

Desalination via poulticing tends to only be effective to within a depth of a few centimetres from the surface. As this is the area most exposed to the elements/ wear and tear, reducing salt levels in this area is nonetheless a valuable aim for conservators dealing with masonry containing dissolved salts.

Because higher levels of salt, in deeper layers of thick stone walls can be reached by poulticing, salts are often mobilised and drawn towards the outer layers following desalination treatment. It may be necessary to repeat poulticing at regular intervals of once every few years in order to keep the surface salt levels of valuable stone detailing and masonry as low as possible. Our desalination poultice and all of our other poultices can be found here.

Electrochemical desalination is an alternative or auxiliary method that involves the use of electrodes to draw out salts by the principle of electroosmosis into either a poultice or a mortar that has been applied to the stonework. Because the use of electrodes can lead to unpredictable and extreme changes in pH in the area where they are used, electrochemical desalination methods can cause localised damage and are a higher-risk method than poulticing, though they can be more powerfully effective at desalination than poulticing alone.

Like poultice-based desalination, electrochemical desalination does not penetrate very deeply into the stone but can be effective close to the surface.

If salts cannot be safely or adequately removed from the stone substrate, it may be possible to treat them in situ by binding them with other chemicals to make them insoluble in water. Insolubilisation treatments are sometimes based on barium compounds.

Indoor stonework can be protected to a degree by controlling the temperature extremes and relative humidity to which it is subject. Moisture cycles affect the behaviour of salts; and the more pronounced the extremes, the more damage is likely to be caused, so limiting these extremes is a valid conservation goal.

For external historic stonework, this would only be possible by first taking the extreme measure of enclosing the entire structure within another building. This has been done with some historic ruins such as those of Hamar Cathedral, Norway, now conserved within a glass outer structure.

However, it may be possible to significantly reduce the exposure of external historic stonework to the rigours of direct rainfall by constructing an extended modern roof covering that project some distance outward from the tops of the exterior walls. If this additional roof covering is made of modern transparent materials, the detrimental visual impact upon the view of the exterior walls from afar will be minimal.

The ideal humidity level to which the substrate should be exposed will depend upon the range of salt contaminants found in it. Particular humidity levels have been found to be optimal for containing the detrimental effects of particular salts.