Moisture movement and salt migration/ crystallisation cycles can have a severely detrimental impact on the content and fabric of a building. 

Moisture profiling

• Moisture movement is influenced by the external, interwall and internal environmental conditions (in particular relative humidity (RH) and vapour pressure (VP)).

• Moisture profiling can be undertaken using a capacitance meter and carbide meter.

• The capacitance meter is non-invasive and will give percentage moisture readings at 4 cm depth beneath the surface when calibrated for the plaster type found on the walls. Measurements of moisture concentrations can, therefore, be taken on all areas of wall surface, allowing extrapolation of moisture gradients and profiles, which can indicate sources of any moisture and its significance. Used in conjunction with a resistance or protimeter, capacitance meter readings can also give an indication of salt distributions on the surface of the walls, which can be correlated with moisture profiles and areas of deterioration.

• Carbide meter readings can give moisture concentrations at much greater depth in the wall than with a capacitance meter, and may reveal significant moisture not detected by the capacitance meter. However, carbide meter readings require drilling, preferably with a 5mm drill bit, to depths of up to 15 cm into the wall. As it is destructive, it should only be employed in areas of later repair.

• The carbide readings can help calibrate or standardise the capacitance meter measurements, since they give absolute values of percentage moisture by weight of plaster/mortar. The carbide meter can be used to measure the hygroscopicity (hygroscopic moisture content) of the sampled plaster caused by any hygroscopic salts present. This may help determine whether any moisture found is “free” moisture or that absorbed from the atmosphere by hygroscopic and deliquescent salts, when humidity levels allow.
   

Salt analysis

• Salt types may be related to a number of factors such as materials used both in the original construction of the building and or internal features such as wall paintings.  Salts have specific equilibrium relative humidity (RH) values. Fluctuating RH may cause crystallisation cycles of the salts, which can cause great damage to the substrate and interior features such as wall paintings, decorative surfaces.

• For the purpose of analysis, salts may be directly removed from pockets of efflorescence or extracted from plaster substrates. It may be necessary to take samples of salts from the surface and at selected depths across the thickness of the wall (especially if there is evidence of cryptofluoresence) to establish cross-sectional distribution of salts. This distribution will be dependent on fluctuations in vapour pressure/Relative Humidity within and at the surface of the wall, which in turn is conditioned predominantly by the air humidity/vapour pressure within the room.

• In order to identify salt types conclusively, information regarding their chemical composition (whether they are double salts or admixtures) is essential, since the equilibrium RH/VP of a multi-component salt solution is different from that of the individual salts that make up the solution. In addition, the degrees of hydration of the salt crystals can give information on the average RH/VP within a room because these too have different equilibrium RH/VP.

• Standard anion analysis can be undertaken at Hirst Conservation’s laboratories, to establish whether concentrations of salts are significant. Should salt levels be significant, sophisticated techniques such as SEM, FTIR analysis and ion chromatography may be recommended to identify salt types and characteristics conclusively. The results of this analysis can be correlated with the generally hygrometric parameters within the Church building environment and the hydrodynamics within the walls, as well as indicating the possible source of salts and their significance regarding mechanisms of deterioration of the plaster and paint.
   

Salt profiling

In addition to salt analysis described above, non-intrusive profiling can be achieved by qualitative identification and their relative concentrations determined using Merckoquant test strips for nitrates, sulphates and chlorides.