Poultices
The true or plain poultice and the cleaning and desalination of historic masonry and sculpture

Catherine Woolfitt and Graham Abrey

Cleaning and desalination of an outdoor marble sculpture using a plain clay poultice

The term poultice has its origins in the field of medicine with the application of a cleansing pack to the body to relieve infection. The notion of the poultice has been adapted for the cleaning of historic buildings and a true poultice is intended to draw out deep-seated contaminants and staining from the surface of masonry and sculpture. In current practice the word poultice is extended to a wide range of cleaning materials and techniques, not all of which achieve a true poultice effect on the substrate.

What might be termed the true or plain poultice contains water and the poultice medium only, relying on these ingredients to achieve the mobilisation and removal of the contaminant. The most common poultice medium is clay, although paper and cotton fibres are also used, and talc, chalk and even flour are traditional poultice materials. A mixture of clay and paper fibre produces an absorbent and plastic mixture that is often favoured by conservators of stone sculpture.

This plain or true poultice is normally used for desalination, to draw out soluble salts, or as a cleaning method on substrates such as limestone that respond to water cleaning. In these cases the poultice is allowed to dry out and the soiling and/or salts are drawn into the poultice by capillary action with the moisture. Multiple applications may be necessary to draw the salts from within the surface pores. Whatever the medium, the poultice is mixed with water to form a material that will adhere to the substrate. Clay forms a sticky mass that adheres well to stone and other surfaces. These plain poultices can be conveniently mixed by hand as required on site with the addition of water to the poultice medium.

CLEANING PACKS

The plain clay poultice may be modified with the addition of certain compounds to target particular stains or surface coatings. These 'active' or 'chemical' poultices are designed for the removal of the various types of soiling and contaminants that are insoluble in water and for those which have penetrated deep into the surface pores. Proprietary cleaners and strippers are specifically designed and formulated for certain applications, including degreasing surfaces and paint stripping. Poultices containing sequestering agents, such as EDTA, are available for the removal of metallic stains, the copper and iron stains which frequently disfigure masonry subject to rain water run off from bronze sculpture or iron fixings. Sequestering agents chemically isolate specific staining material such as metals, forming compounds which are soluble and can be removed from the surface.

Trial cleaning of a detail on a 19th century cement stucco facade using ammonium carbonate in a clay and paper fibre poultice.

Alkaline poultice cleaners and strippers are commonly used for cleaning or degreasing masonry surfaces and for paint removal. Sodium hydroxide is the most common alkaline cleaning agent in proprietary cleaners for a range of masonry substrates, including limestone, sandstone, brick and terracotta and is the most common ingredient in proprietary paint removers. Care must be taken in the use of sodium hydroxide based cleaners to minimise risks to the building and the user. Sodium hydroxide based cleaners and strippers must be neutralised with acid afterwash. Adjacent, dissimilar building surfaces must be protected and personal protective equipment worn by the cleaning operative. In the field of stone conservation ammonium carbonate is added to clay and clay/paper poultices to remove soiling from limestone. Ammonium carbonate is a less alkaline cleaner than sodium hydroxide. It works by reacting with calcium sulphate on the soiled surface to form calcium carbonate and soluble ammonium sulphate that can be rinsed off with water.

These 'active' or 'chemical' poultices are all applied to a pre-wetted surface to minimise penetration of the chemical into the masonry surface and covered with plastic film to prevent the poultice drying out. The cleaning additives in these mixtures chemically dissolve the soiling or staining which is held to the surface of the poultice, and then both the cleaning agent and the contaminant are removed with the clay. Rinsing with water and, where necessary neutralisation, follows to remove any soiling that remains on the surface and also to remove residues of the chemical cleaners. Strictly speaking these materials are clay-based cleaning packs rather than true poultices, but the word poultice is now widely used in the building cleaning industry.

OTHER 'POULTICES' AND GELS

Application of a hot lime poultice to heavily soiled limestone sculpture

Cleaning agents are often mixed in a gel medium, normally based on CMC (carboxymethyl cellulose). Gels containing alkaline cleaning agents, such as ammonium carbonate, are covered with plastic sheet after application to the surface and in this way function like packs, not true poultices. Other more specialist cleaning packs may be developed for particular cleaning problems on sculpture or decorative masonry detail. One poultice developed for the cleaning of limestone is the hot lime poultice credited to Professor Robert Baker and initially used at Wells Cathedral. Hot slaked lime is highly alkaline and softened the black soiling deposits and the 'sulphate' crust on the limestone. Although successful to some degree this method is difficult to control and non-selective and is now rarely used, even in the field of stone conservation. The 'Mora Poultice' including ammonium and sodium bicarbonate, EDTA and a surfactant in a CMC gel is still sometimes used for limestone.

DESALINATION

Clay poultices, traditionally of either sepiolite or attapulgite clay, with fine particle sizes in the range of 50mm, have been the usual choice for desalination of historic masonry suffering from soluble salt-related decay. The depth and degree of salt contamination should be understood at the outset of the operation through drilling of masonry to obtain samples at various depths for analysis of the types of salt present and their content. Pre-wetting with sprays must be sufficient for water to reach and mobilise salts in the heart of the masonry, depending on the depth of contamination. The clay will need to be applied with wire mesh or other reinforcement. Once the poultice has dried out the clay can be removed and a sample must be checked for salt content. This procedure should be repeated until the salt levels are significantly reduced. This procedure can take a long time and periods of months rather than weeks should be anticipated for large-scale and thorough desalination of masonry walls. It should be noted that dry clay powders are potentially hazardous substances and must be used with adequate personal protection, mainly to prevent inhalation of the fine particles.

Trial cleaning of a detail on a 19th century cement stucco facade using ammonium carbonate in a clay and paper fibre poultice.

Desalination of sculpture and architectural detail on a much smaller scale is used to draw salts from vulnerable porous surfaces. In certain cases, for example where the surface has open pores or has badly deteriorated (often the case with limestone or sandstone sculpture), clay may be inappropriate and paper or cotton fibres may be preferable and more easily removed. Decisions regarding the cleaning and treatment of sculpture should rest with an appropriately qualified and experienced conservator.

CLEANING WITH POULTICES/PACKS

Poultices are normally associated with small scale cleaning but they are potentially useful for the general cleaning of building facades. Commercial demands to bring the costs of poultice cleaning in line with less expensive methods, such as traditional water cleaning, have led to the development of spray applied poultices. In certain cases clay based poultices or packs may be the best cleaning option.

Poultices can be used in combination with other cleaning methods. For example, a sodium hydroxide based poultice might be used to soften and remove heavy soiling and black 'carbon' deposits on the underside of cornices and other projecting detail before using a gentle wet abrasive, or possibly a water cleaning method, to clean the remaining areas of lighter soiling on the facade. This combination of cleaning methods on one facade can often provide more control, removing heavy soiling to decorative mouldings and enrichments with less risk of damage or over cleaning than using a single cleaning method.

Poultices can also be used as the principal cleaning method to remove or reduce soiling on a facade. Poultice cleaning is often selected for very detailed facades where more control and care is required in the cleaning procedure. Soiling that has penetrated deep into the masonry substrate is often more easily removed by a poultice than by other cleaning methods. Poultice cleaning can also be used as a post-cleaning treatment to remove areas of staining that may have emerged following the general cleaning of the facade. Plain clay poultices will usually mobilise and remove water soluble staining such as the brownish staining or discoloration sometimes left on Portland and other pale-coloured limestone facades by water cleaning methods.
Incorrect use of chemical poultices can cause damage, for example by the mobilisation of new staining material in the substrate or through inadequate neutralisation or rinsing. Adequate neutralisation or rinsing with clean water to remove potentially damaging residues must follow any chemical cleaning procedure. All poultice materials must be used in accordance with COSHH (Control of Hazardous Substances to Health) and CDM (Construction [Design and Management]) regulations. Many cleaning products and items of cleaning equipment are available to registered or approved users only and material data sheets for all products used must be obtained and consulted.

Site trials are essential prior to any facade cleaning programme to establish which method will be most effective and to indicate the 'level of clean' that can be achieved without risk of damage to the masonry surface. In inexperienced hands any cleaning agent or piece of cleaning equipment can cause damage to surfaces. The cleaning of historic masonry facades should only be carried out by suitably qualified masonry conservation specialists, from the specification stage through site trials and execution of the work.

Recommended Reading

• C Andrew et al, Stonecleaning: A Guide for Practitioners, The Robert Gordon University and Historic Scotland, 1994
  
• C Andrew et al, Stone Cleaning in Scotland, 3 volumes with Research Summary and Literature Review, Historic Scotland and the Robert Gordon Institute of Technology, 1992
  
• J Ashurst and F G Dimes, Conservation of Building and Decorative Stone, 2 volumes, Butterworth-Heinemann, 1990
  
• N Ashurst, Cleaning Historic Buildings, 2 volumes, Donhead Publishing, 1994
  
• MJ Bowley, Desalination of Stone: A Case Study, Building Research Station, April 1975
  
• RGM Webster, Stone Cleaning and the Nature, Soiling and Decay Mechanisms of Stone, Proceedings of the International Conference held in Edinburgh, UK, 14-16 April 1992, Donhead Publishing, 1992
  
• C Woolfitt, 'Lime Method Evaluation in English Heritage Research Transactions', Research and Case Studies in Architectural Conservation, vol 2, Stone, James and James, London, 2000
  
• British Standards Institution, BS 8221-1:2000 and BS 8221-2:2000, Code of practice for cleaning and surface repair of buildings; Part 1 Cleaning of natural stones, brick, and terracotta, BSI, London 2000