External Rendering

CLEANING LIME DECORATIVE PLASTER

Paint removal from enriched architectural surfaces is a sensitive area. Firstly, removal erases the historic record embodied in the paint layers. Secondly, physical damage to the underlying material can and often is caused by either the cleaning agent or the tools used. Where plasterwork is concerned, no matter how sensitively undertaken, removal of softened paint with conservators’ tools inevitably damages surfaces. Leaving well alone is often more beneficial to overall preservation. The harm caused to important underlying decorative schemes and mechanical damage far outweigh the benefits of any aesthetic reinstatement or dressing up. Nonetheless, excessive paint build-up obliterates fine decorative detail, transforming the finest ornament to dull and pedestrian shapes of little interest to all but the most knowledgeable viewer. Removal can revitalise and uncover technical virtuosity and re-ignite both public and scholastic interest in decorative interiors that might otherwise languish in obscurity or, worse, fall prey to neglect and decay. As with all interventionist conservation, perceived benefits and risks must be carefully weighed up.

On outer walls and other surfaces subjected to persistent condensation or moisture ingress, applied coatings may be acting as a barrier to evaporation. As a result, evaporation concentrates at any cracks or gaps in the paint and at the edges of the painted area, causing high local concentrations of salts. Sulphate salts, for example, can be leached from later cement renders and internally from gypsum, leading to eruption of salt crystals at the plaster and paint interface.

Increased moisture levels in the plasterwork may also lead to rusting and expansion of ferrous armatures and possible fungal attack of organic materials. In cases such as these, paint removal can be essential to arrest the rate of decay and reduce loss of plaster enrichment. Before undertaking such an irreversible step, it is essential that a responsible programme of recording be undertaken using photography and microscopic paint analysis. A small section of the full paint thickness should also be retained in an appropriate area of the plasterwork for future reference and investigation.

Where salt attack is particularly severe, it is worth undertaking salt analysis prior to paint removal. In one instance the removal of paint from early 17th century plasterwork led to the sudden crystallisation of salts, hitherto in solution as the structure dried out. The result was the rapid erosion of the plaster surface and the irretrievable loss of decorative plaster. In such cases paper pulp poulticing may be the only means of ameliorating damagingly high salt concentrations without physical removal of the plaster.

Where paint removal is undertaken, the use of methylene chloride (such as Nitromors Green Label) is controllable and effective on organic coatings such as oil based paints and modern coatings, leaving the inert lime and gypsum plaster chemically undamaged. The translucent liquid and thickening waxes incorporated in these cleaning agents do not obscure the work in hand, enabling the conservator to proceed with care. Importantly, if this process is carried out with the necessary care and skill, any particulate residues may subsequently be removed with nothing more than a gentle dusting with a soft bristle brush.

Alkali pastes in particular should be avoided, especially on porous historic fabric. They can inflict great damage to plasterwork. Not only does the plaster absorb potentially harmful soluble salts, but there is also an increased risk of physical damage from tools due to the thickness and opacity of the paste which blind the conservator to the enrichment. Lack of chemical control and the necessity to wash away residue with water should be enough to suggest their use has no place in the treatment of historic surfaces.

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