Conserving Decorative Plaster (Part 2)

Conserving Decorative Plaster (Part 2)

Bookmark

By Richard Ireland

Fig. 14 Many months of paint removal and conservation were required for this exquisite 1757 hand modelled lime plaster ceiling at Dominick Street, Dublin.

Paint removal from enriched architectural surfaces is a sensitive area. Firstly, removal erases the historic record and so should only be considered after thorough paint analysis and investigation. ( Fig. 15) Secondly, it can and often does cause physical damage to the underlying material either by 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. Destruction of important underlying decorative schemes and mechanical damage far outweigh any aesthetic reinstatement or dressing up.

Removal can appear an attractive proposition to the private client or institution whose considerations may be of a purely selfish nature and some owners have little regard or respect for the history of the building.

Fig.15 Unknown fictive fluting and gilded enrichment exposed beneath centuries of thick plain paint in the course of a detailed search for evidence of Turner’s 1720’s decorative scheme at St Paul’s Deptford.

tripping, freshening and restoring an ‘as new’ appearance may be driven by the promise of higher rentals or improved image – factors which drive much misguided work on many important buildings and their interiors.

Nonetheless, removal can greatly enhance the long term preservation of a building, and 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. ( Fig. 16) As with all interventionist conservation, perceived benefits and risks must be carefully weighed up.

On outer walls and other surfaces subjected to persistent moisture ingress, applied coatings may be acting as a barrier to evaporation.

As a result, evaporation concentrates at any cracks or gaps 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 salts 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.

Fig.16 Delightful and unexpected hand modelling of a 1740’s cornice smothered beneath heavy layers of thick obscuring paint.

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 rapid evaporation of moisture and crystallisation of salts, hitherto in solution.

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 do not obscure the work in hand, ensuring more sensitive progress.

Fig.18 Tool marks and damage inflicted by the use of inappropriate materials and insensitive labour.

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, the thickness and opacity of the paste blinds the operative to the enrichment beneath, resulting in an increased likelihood of physical damage by tools. ( Fig. 18 )

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.

Some coatings, such as casein bound distempers, have little or no organic binder and may prove particularly arduous and more damaging to remove. ( Fig. 17 )

In these cases water and small tools may prove to be the only effective method. Other techniques such as the use of heat and steam generally lack sufficient control for them to be considered for use on delicate historic fabric. It must also be accepted that no matter how desirable, paint removal may be neither practical nor feasible.

Fig. 17 Casein-bound materials are virtually insoluble to plaster friendly solvents. Mechanical removal with water can work. Alkali pastes should always be avoided.

Other common problems associated with paint removal include the uncovering of poor repairs and previous intervention which must be subsequently dealt with. Examples are poor cast reproductions of existing elements, often reinstated following large scale failure in the past, such as war time bomb damage or other catastrophic failures, and subsequently repaired on a shoe string budget. ( Fig. 19 & 20)

Papier mâché, composition or other fragile and organic-rich applied decorative elements may also be present, especially on 19th century work, which might be destroyed by an otherwise acceptable paint removal technique, so it is vital to investigate what is being dealt with prior to specifying work, let alone commencing it.

Tests areas and trials of any suggested technique are essential to ensure both suitability of any chemical agent, skill of the operatives, quality achievable and effect on the plasterwork and adjacent related substrates and coatings. Programme time available can also be a decisive factor and should not be dismissed from the equation.

Decoration

Decorative coatings fulfil two particularly relevant functions. One is purely aesthetic, relating to colour, texture, and overall visual quality like opacity and transparency. The other is performance related and how the coating functions physically.

This includes permeability, compatibility, durability, flexibility, reversibility, protection, weight, thickness, surface tension, drying, ageing, preparation requirements and number of coats required. Added to this is the means of application and cost.

In the field of conservation it is usual for historic fabric to be repaired and reinstated on a like-for-like basis, using materials and techniques that match those used originally as closely as possible. However, where plaster is concerned, renewing the original finish may be desirable, but it may also be impractical for a number of reasons. In some cases the choice of coating is dictated by circumstance; in others, the options may be less clear cut. In any event, the conservation and preservation needs of the fabric should be foremost.

For example, soft distemper (a chalk-based paint with a binder of size) can be applied by brush to a newly stripped ceiling of especially deep and fragile modelled plasterwork to recreate its original colour and texture authentically, using water soluble reversible materials. The bonus is a surface that can be readily cleaned when its details eventually become clogged with paint again. Such an approach presents a beguilingly straightforward, ideologically sound conservation solution.


Fig. 19 Crude repair of cast enrichment exposed by paint removal.	Fig. 20 Crude cast repair of hand modelling exposed by paint removal.

However, the water soluble stains that inevitably accumulate in the plaster from persistent moisture ingress may be impossible to cover without the use of an impermeable solvent-based sealing coat. Yet impermeable paints are generally undesirable, particularly where future risk of moisture ingress cannot be precluded. Moreover, brush application carries a high risk of further mechanical damage to delicate and fragile ornament. At least three coats of distemper may be required to achieve satisfactory decoration. What of later maintenance or redecoration? Water soluble reversible coatings like distemper will, at some stage, require mechanical removal with brushes prior to further redecoration. This too subjects fragile modelling to the certainty of further damage and loss. It also imposes a responsibility and duty of care that the owner of the building may not wish to shoulder. This can result in the later overcoating of all the good work with a totally inappropriate, easier to use coating that starts off the cycle of decay all over again.

In many instances, it may be more beneficial to spray using a variable ‘high volume low pressure’ (HVLP) turbine. Spraying precludes further unnecessary physical contact. Coating thickness can be better controlled on intricately detailed plaster and redecoration extended by specifying a coating with the most appropriate attributes.

Many specifications needlessly emphasise ‘breathable’ coatings in the misguided belief that this equates to conservation and a responsible approach. Although vital when applied to bare surfaces subject to continued moisture vapour movement, permeability is an irrelevant attribute when applied over impermeable systems.

‘Reversibility’ is also frequently specified, and in general specifications the term is taken to mean soluble in water. This attribute may not be a good thing when used in close proximity to fragile existing coatings that may be easily damaged or altered by water, and it can easily be avoided by using a non traditional coating which is soluble in a synthetic solvent. Advice should be sought to ensure that specifications adequately account for the real needs of the historic fabric.

Oil paints may be less than a fifth of the thickness of modern emulsion paints. At Castletown House near Dublin, microscopic paint analysis has revealed two modern emulsion decorations accounting for more than seventy percent of the total thickness of the previous oil painted schemes since the house was begun in 1722. Generally, much of the heavy paint build up on plasterwork today is a product of the late 19th and 20th centuries.

Limewash, a simple dilution of lime putty with water, coloured with the addition of alkali resistant pigments, is eminently suited to lime renders externally or internally. It should be applied sparingly as a wash as a thick paint-like coating is liable to fail and flake.

Water based and equally vapour permeable is soft distemper which is for internal use only. Though perhaps the most common plaster finish well into the 19th century, it may not always be the best choice for conservation work.

The thickness and difficulty of subsequent removal preclude the use of oil and casein bound distempers. Their additives are intended to decrease dusting whilst increasing permanence and durability rarely suiting them for use on pure conservation grounds.

Fig. 22 Reinstated 1775 Robert Adam scheme, Headfort House, Eating Parlour.

Despite coating thickness, there is much to recommend the use of ‘contract’ emulsion paints particularly when applied to patching of plain run mouldings and flatwork where sharpness is not an issue. These materials have the highest vapour permeability of such coatings and are specifically formulated for use on new plaster. Not only are they relatively cheap and widely available, crucially they are alkali resistant. This makes them eminently suitable where the use of lime repairs precludes the use of oil paints for a minimum of 12 months – the time necessary to ensure sufficient carbonation of lime and reversion of the surface to a neutral ph to avoid saponification. (Fig. 22)

Vinyl emulsions do not have the higher vapour permeable qualities of contract emulsion. Invariably vapour permeability decreases with an increase in gloss and weather resistance for modern water thinned products of this nature.

Oil paints may be restricted by programme time, as noted above, requiring a full twelve months before application direct to any new surface containing lime. This also applies to modern alkali resistant primers whose use will not generally be endorsed by manufactures till a similar lapse of time.

Classidur Super Classic originates from Switzerland. This is a non-conventional solvent based system which has a number of attributes that suit it particularly well as a distemper substitute for interior decorative plaster. Chiefly these include lack of surface tension with high vapour permeability, preclusion of priming or sealing over most water and oil soluble stains including soot and nicotine, very matt finish tintable to the same intensity as distemper, and high opacity allowing strong colour changes in only two coats. The minimal preparation required and lack of tension especially suit it to badly stained stripped substrates and fragile existing coatings. Care should be taken for persistently damp areas as mould resistance may not match that of less organic coatings such as whitewash and limewash especially in relation to large volume spaces like churches.

Silicate paint, Keim mineral paint is another coating originating from overseas, appearing in Germany in the late 19th century where its use has a strong tradition. Though no historic tradition in the UK, the material is widely specified, particularly for use on external renders. It is irreversible and so should be specified with care in conservation situations when its particular attributes will be beneficial and not used purely as a permanent limewash substitute.

Conclusion

The treatment of any culturally important historic building or artefact is necessarily encumbered with a multiplicity of opposing needs. Ultimately preservation is paramount to serve the needs of enlightenment, education and the wider enjoyment and appreciation of the past. Inevitably, the very act of conservation is interventionist having far reaching consequences on the continued survival of an object and so should avoid all unnecessary alterations.

This problem is especially pertinent when considering the removal of paint from plasterwork. An easy decision when coatings are actively assisting decay, the pros and cons are much less defined when removal is proposed for largely aesthetic grounds. Damage is inevitable, yet undertaken sensitively for appropriate reasons, the effects can be startling. Plasterwork detail can be dramatically revealed, enhancing appearance and bringing important attention which may itself be sufficient to ensure continued investment and survival. Conservation should never be purely prescriptive. Each case must be carefully balanced to ensure an appropriate response that best addresses the many conflicting needs of preservation.

RICHARD IRELAND undertakes conservation on buildings as diverse as castles and farm houses to the Entrance Hall decoration and Reading Room at the British Museum. He operates chiefly around the British Isles and Ireland as both consultant and practitioner and lectures widely.

View Richard Ireland's Services in the Heritage Register