A review of salt transport in porous media : assessment methods and salt reduction treatments

It is an unpalatable fact that while objects can deteriorate through lack of care and attention, they can also deteriorate as a result of inappropriate and misguided interventions. This is particularly the case with regard to salt-related deterioration problems. A successful treatment outcome using aqueous salt reduction methods demands an understanding of the transport processes involved as well as detailed information regarding the characteristics and specific situation of the individual object. The use of poultice materials to reduce the salt content of salt deteriorated objects is a long established technique in conservation. However, due to the complex nature of salt problems within historic structures the result of such interventions can be variable and unpredictable. The amount and depth to which salts are mobilised, and where they are transported to, is dependent on the inter-relationship between the poultice and the substrate, the drying conditions and also the initial salt distribution. This paper examines the current scientific understanding of salt and moisture transport processes, and the extent to which this knowledge can feed back into the practical arena, to aid the conservator. Moreover, areas where further research is required are identified. In particular, the importance of pre- and post treatment investigations is highlighted, showing how, in combination with knowledge of salt and moisture transport mechanisms, these can give useful indications regarding treatment options. The role of selective salt extraction and the post treatment behaviour of residual salts are discussed