Novel approaches to the control of salt damage

Conservation scientists at the Institute of Archaeology have a longstanding interest in how the growth of salt crystals damages ancient buildings, wall paintings and objects in museums. They are now participating in a European research consortium that is investigating novel methods of reducing salt damage. The Institute's role in the project is to assess how environmental conditions affect the process of salt damage and to examine how salt crystallization might be inhibited.

Novel approaches to the control of salt damage Alison Sawdy & Clifford Price Conservation scientists at the Institute of Archaeology have a longstanding interest in how the growth of salt crystals damages ancient buildings, wall paintings and objects in museums.They are now participating in a European research consortium that is investigating novel methods of reducing salt damage.The Insti tute's role in the project is to assess how environmental condi tions affect the process of salt damage and to examine how salt crystallization might be inhibited.
A nyone who has visited Petra in Jordan will have been struck not only by the rock cut tombs for which Petra is famous, but also by the severe deterioration fr om which some of the tombs are suffering (Fig. 1).The deteriora tion is caused in part by the growth of salt crystals within the pores of the rock.The salts, which are naturally present in the rock, repeatedly dissolve and crystallize under the influence of fluctuating temper ature and atmospheric humidity.The rock cannot withstand the pressure of the grow ing crystals, and the surface gradually powders away.
One of the Institute's research students, Fadi Bala'awi, is looking at the effect of wind speed on decay rates at Petra.His research will show whether decay could be reduced through the provision of shelter, either natural or manmade.The Institute is also involved in a pan-European project, funded by the European Commission, that is investigating another approach to the control of salt damage, which could have worldwide application.Salt damage is not unique to Petra; indeed, it is one of the main causes of decay in porous building materials such as stone, brick and unfired earth.It is responsible for severe damage to buildings and monuments throughout the world, and also to wall paintings, sculp ture and museum objects.

Salt crystallization
Salt crystallization takes place when the limit of saturation for a salt solution is exceeded.This is brought about in two main ways: through water evaporation, which leads to an increasingly concen trated solution, or through a fall in temper ature, because the solubility of many salts decreases with temperature.But the pro cess of crystallization is reversible.Under sufficiently humid conditions, salt crys tals can deliquesce by taking up water vapour from the air to form a solution once again.Similarly, an increase in tempera ture can cause precipitated salt crystals to re-dissolve.Fluctuations in environmen tal conditions can thus cause repeated cycles of salt crystallization and deliques cence.As a result, salt damage is not usu ally caused by a single event.Instead, it is frequently a continuing process that leads to far greater damage in the long term.

Tr eatment options
Salt damage presents a particularly diffi cult challenge for the conservator, not only because the mechanisms by which the damage occurs are still not fully under stood, but also because the present options for treatment are extremely limited.One can opt for a policy of salt removal, in the hope that reducing the amount of salts present will reduce the rate of damage.However, it is impossible to remove all salts from monuments, masonry, large sculptures and wall paintings (Figs 2, 3).Indeed, it may be neither easy nor desira ble to exclude the access of moisture to the object, which provides the means for mobi lizing and transporting further salt.So, over time, salts have a tendency to re-accu mulate.Moreover, under certain circum stances the selective removal of part of the salt system can lead to an increase in the rate of damage because of an alteration in the behaviour of the salts remaining after treatment.So, less salt does not always necessarily mean less damage.An alterna tive approach is to moderate the damaging behaviour of the salts, either by passive means or by direct treatment, and it is in these aspects of the problem that the Insti tute has been actively involved in new research.

Environmental control
Changes in relative humidity (RH) and tem perature play a key role in the activation of salt damage, and so environmental control has been increasingly cited as a desirable means of reducing salt deterioration, espe cially in wall paintings and museum objects.1• 2 • 3 However, this form of passive   amelioration is difficult to get right, and, like most conservation interventions, car ries the risk that, if it is applied incorrectly, the damage to the object could be exacer bated.It is therefore very important that the environmental conditions selected to limit the salt damage are appropriate for achieving this end.
A significant recent advance in this area of research at the Institute has been the development of an environmental control of salts (EGOS) software program.4Other research, also undertaken at the Institute, has investigated the kinetics of salt behav iour to establish the timescale over which damage to wall paintings can take place, and the role that the various environmen tal, salt-and object-related factors play, in order to facilitate the application of envi ronmental-control measures.5But, despite the advantages of environmental control as a passive form of treatment, in many situ ations (e.g. in the open air or when there are severe conflicts of building use) it is not a sufficient or indeed feasible means of pre venting further damage.Another approach to the reduction of salt damage that has often been suggested, but remains as yet unrealized, is the use of crystallization inhibitors.

The current European project
Crystallization inhibitors have a long his tory of use in industry, for example for the prevention of barite crystallization in off shore oil extraction processes, but their application in the field of conservation has not yet been fully explored.Therefore, a new project of the European Commission, with the acronym Saltcontrol, is currently being undertaken as part of the FP6 Sixth Framework Programme, to tackle this problem.The project, coordinated by the University ofGhent, aims to investigate the use of crystallization inhibitors as a direct means of altering salt behaviour to prevent damage occurring.6 The role of the Insti tute within the project is to assess the influ ence of environmental conditions on the process of salt damage, and to examine how this affects the inhibition efficiency of crystallization inhibitors under consider ation.

Influence of environmental conditions
At present there is a lack of general agree ment concerning the exact mechanisms by which salts cause damage within porous materials, but there are strong arguments supporting the view that it is attributable to the development of crystal-growth pres sures when the salt crystallizes from a supersaturated solution.
Salt-crystallization inhibitors are effec tive in delaying the onset of crystallization, allowing the solution to become progres sively more supersaturated.Nevertheless, under conditions of continuing evapora tion or if the temperature falls to suffi ciently low levels, it is inevitable that, even in the presence of inhibitors, at some stage the system will start to crystallize.The amount of crystal-growth pressure gen erated has been related to the degree of supersaturation at which crystallization occurs.7 Consequently, the use of inhibi tors carries the risk that, if crystallization does take place, the solution will be more supersaturated than it would otherwise have been, and so greater damage could result.It is therefore most important to establish the limits of environmental tol erance under which inhibitors can be applied safely.
A key aspect ofthe role the environment plays in precipitating salt damage in cul turally important buildings and objects is the fa ct that climatic conditions are not constant.Levels of RH and temperature undergo significant variation, resulting in cycles of salt dissolution and re-crystalli zation.In this way, changing environmen tal conditions can cause multiple cycles of damage.Nevertheless, such changes may facilitate the use of crystallization inhibi tors.When applied in the context of a fluc tuating environment, inhibitors may have the potential to act as a holding measure, preventing the onset of crystallization for a sufficient length of time to allow envi ronmental conditions to return to levels under which damage is not a threat.Fur thermore, in some situations a delay in the onset of crystallization can allow mecha nisms by which salt in solution is trans ported to proceed, such that crystallization takes place at the surface where damage is less severe.Indeed, this alteration of the transport properties of the salt-water sys tem indicates that crystallization inhibi tors could also be a useful adjunct to salt reduction treatments to improve their extraction efficiency.This particular aspect is a focus for research by other partners in the Saltcontrol project.
In order to reduce the potential dangers associated with the use of inhibitors, we therefore need to know the extent and timescale ofthe fluctuations in RH and tem perature that can occur without failure of the inhibitor.Previous work has shown that the speed of air movement is also an important factor in determining the rate of moisture loss from porous materials, and hence the time interval before the onset of crystallization and the type of damage caused.8•9 Therefore, airspeed also merits attention, together with RH and tempera ture, as an environmental factor that may affe ct the rate of salt damage.

Salt supersaturation
Supersaturation of a salt solution, and ulti mately damage, is caused by fluctuations in both RH and temperature.However, it should be stressed that changes in these two factors do not exclusively determine the onset of crystallization; the sizes of the pores, and the ways in which the pores are interconnected, may also play a significant role in precipitating crystallization.10 It is well known that the water-vapour pressure of a salt solution is strongly affe cted by its composition, and it has been shown that the presence of salt mixtures influences the rate of moisture uptake and loss by altering the difference between the ambient RH and vapour pressure of the solution.4•11 If crystallization is inhibited, the solution will continue to evaporate, and its water-vapour pressure will drop as it becomes increasingly supersaturated.Therefore, at constant RH and temperature, it is expected that the evaporation rate should fall as the difference in water vapour pressure between the ambient RH and the salt solution decreases.Further more, at moderate levels of supersatura tion, salt-crystallization inhibitors can delay the onset of crystallization indefi-Fig ure 4 Cleeve Abbey, Somerset: the exterior of the sacristy and cloister viewed fr om the north, 1998.
nitely, and so it is possible that a supersat urated solution can reach equilibrium with its surroundings at RH values below that at which crystallization would nor mally be predicted to take place.Thus, it appears that salt-crystallization inhibitors may help to reduce the incidence of salt damage in objects exposed to a fluctuating environment by slowing the rate of evap oration and extending the limits of envi ronmental conditions under which damage does not occur.
The precise way in which changes in RH and temperature affect the kinetics of salt solution supersaturation and crystalliza tion in porous media has not yet been determined.Nor do we understand how this behaviour is mediated by the presence of inhibitors and their influence on the water-vapour pressure of the solution because of the promotion of higher levels of supersaturation.Also, it is becoming increasingly apparent -as our research at Cleeve Abbey in Somerset (Figs 4, 5) sug gests -that kinetics may play a significant role in determining not the only threshold supersaturation reached prior to crystalli zation, but also the pressure generated by the growing crystals, and, in the case of sodium sulphate, the identity of the crys talline phase that is formed.

Conclusion
In our current work at the Institute we are addressing these issues in the hope that we will be able to clarify the dynamic nature of salt damage and the environmental condi tions under which inhibitors, iffound to be suitable for application in conservation, may be used.One of the aspects of crystal lization inhibitors that makes them partic ularly attractive is that their potential use is not limited solely to a straightforward means of preventing salt crystallization.By delaying the onset of crystallization, so that salts remain in solution in the object for longer, crystallization inhibitors have the capacity to enhance the effectiveness of salt-reduction treatments.Consequently, if the results of this study do show that inhibitors can be used successfully and reliably without risk to the object, they will increase the treatment options and will potentially offer enormous advantages for countering the notoriously difficult prob lem of salt damage.

Fi gure 1
Rock-cut fa r;; ade of a tomb at Petra, Jordan, showing severe salt damage along the base.

Figure 2
Figure 2 Th e interior of the nave of Hardh am Ch urch, West Sussex, 1997.The twelfth century wall paintings at Hardh am form the most complete cycle of Romanesque wall paintings surviving in England.Sadly, they are in extremely poor condition, primarily because of the effe cts of salt crystallization.

Fig ure 3
Fig ure 3 Detail of one of the wall-painting scenes in the nave ofHa rdham Ch urch, 1997, showing severe disruption of the painted surfa ce caused by salt crystallization.

Fig ure 5
Fig ure5Detail of salt damage to part of a thirteenth-century wall painting on the south wall of the sacristy of Cleeve Abbey, Somerset, 1997.