The re-appearance of lime as a building product during the last twenty or so years has clearly brought many advantages especially in the restoration of historical buildings. However, it has also raised a number of problems that we at Papers and Paints are consulted about on a daily basis. The most frequent question is – "What paint can I use on fresh lime plaster."
I see this blog as being a work in progress; one that I shall tweak and amend when the opportunity presents itself. Sitting, as I am on the sun-drenched terrace of a Portuguese villa, without access to my extensive library a few of my points could do with a little 'firming up'. No matter. Having just received an email from an interior designer who now realises why I had been cautioning about the use of lime plaster and the problems of a short redecoration programme I feel that the time is right.
A difficulty with writing short pieces on complex subjects is that one must generalise and qualify all those statements for which an exception immediately springs to mind. Lime and its use in historical buildings is certainly one those subjects.
The use of lime in building, at least in the form of plaster, was the norm until the twentieth century. Thinking of our big cities alone, huge numbers of houses still retain lime-plastered walls under many layers of subsequent paint. Whilst one gives very little thought to the type of paint that is used on these it is certainly a consideration when it comes to the redecoration of newly refurbished historical buildings. For, although most plaster repairs carried out in the typical 1880s terraced house will be in the form of gypsum plaster, those buildings that have been the subject of extensive restoration – often with the involvement of outside agencies such as English Heritage - will almost invariably involve the use of new lime plaster.
When lime plaster was the norm it was well-understood that a period of time must elapse before walls could be decorated – or, at least, before anything other than a temporary coating could be applied. The main reason for this was the length of time that it took the water component to be released in the form of moisture vapour. This problem can better be appreciated if it is realised that a square yard of lime plaster contains nearly half a gallon of water in each coat. However, the sort of lime generally used in plastering hardens by a slow process of carbonation, reacting with carbon dioxide in the atmosphere over a period of time. Another possible problem is the high alkalinity of fresh lime. This diminishes with carbonation but must be considered especially if an oil-based paint is desired for the decoration.
Peter Nicholson, in his An Architectural Dictionary, of 1819, spoke of (soft) distemper as a stop-gap measure:
Painting in distemper, or water-colours mixed with size, stucco, or plaster, which is intended to be painted in oil when finished; but not being sufficiently dry to receive the oil, may have a coating in water-colours, of any given tint required, in order to give a more finished appearance to that part of the building
He continued:
It will not require less than two coats of any of the foregoing colours in order to cover the plaster, and bear out with an uniform appearance. It must be recollected, that when the stucco is sufficiently dry, and it is desirable to have it painted in oil, the whole of the water colour ought to be removed; which may be easily done by washing; and, when quite dry, proceed with it after the directions given in oil painting on stucco.
Nicholson was also aware of the implications of this long drying-out time, and in a comment on speculative building said:
Perhaps, in general cases, where persons are building on their own estate, or for themselves, two or three years are not too long to suffer the stucco to remain unpainted; though frequently, in speculative works, as many weeks are scarcely allowed.
Dr Ian Bristow refers to the common practice, until quite recently, of decorating the plasterwork in newly finished interiors first in a porous water-based paint (soft distemper), pending final decoration in oil a year or two later. He cites the accounts for Henry Pelham's house in Arlington Street, London, which show that the whole of the staircase had been painted in this way by January 1746 and was not decorated in oil until about two years later. While carrying out the paint analysis in a number of early nineteenth century houses I have invariably found evidence for an initial scheme of soft distemper under subsequent layers of oil paint. Thomas Cubitt, the builder, advised his clients to live "under builder's finish" for two years to prevent expensive decoration from being spoiled.
In a perfect world nothing would be applied to the walls for as long as a year, depending on the depth of plaster and the nature of the substrate. However that isn’t always practicable. The application of a true soft distemper is also not the most realistic option as it is fragile and the necessary skills to make it are rarely to be found.
Whatever you do, take care to avoid the so-called traditional paint route.
Richard Ireland, the expert on the conservation of decorative plaster has dealt with the subject of relevant paints in a very good blog on the ProjectBook website.
He is one of the few specialists who have both the practical and theoretical knowledge to write on the matter.
If early decoration is unavoidable there are two main options:
a) The first is the use of a ‘contract’ emulsion. These paints have the highest moisture vapour permeability rates of all emulsions and are specifically formulated for use on new plaster. Once the plaster is dry in depth more sophisticated emulsions can be used on top.
b) Another option might be Classidur Golden Classic. This is a non-conventional solvent based system with properties that make it particularly suitable as a substitute for a soft distemper. These include high vapour permeability and a lack of surface tension.
In both instances there are limitations on the range of colours. Papers and Paints is one of the few specialists who are able to tint both these products.
Although not recommended for application to large areas, oil paints may be employed, but only once an alkali resistant primer has been applied.
Lime plaster is an excellent material in the restoration of historical buildings, but do remember that its use will require careful thought in terms of timing and its subsequent decoration.
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Whenever i see the post like your's i feel that there are helpful people who share information for the help of others, it must be helpful for other's. thanx and good job.
ReplyDeleteNewlath