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Lime Plaster - A Regional Variation in East Anglia

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(Published in the Journal of the Building Limes Forum, 2003) By Rory Sumerling

At present most practitioners promote the use of sand aggregate rich mortars in the making of lime plasters. The practice of using mixes of 1 part lime to anything from 1 to 3 parts sand aggregate is common and is generally proving successful. The author who trained initially in Scotland with sand lime mixes became aware, on moving to East Anglia, that much of  the vernacular plaster of this area bears little resemblance to the sand rich mixes that are being specified today. These old plasters are significantly more flexible than sand lime mixes and include very high proportions of hair, containing little, if any sand aggregate. This material is so flexible and soft that if carefully removed from lath work, an area of plaster can be rolled up like carpet.

Plaster Sample

A sample of plaster (sample on left) was dissolved in acid which after washing and drying left a very high proportion of hair and virtually no aggregate (sample on right).

Source, The Author

Initial investigation and analysis
From a simple acid dissolution test the sample indicated that the plaster comprised almost entirely of calcium carbonate and hair, but because this test cannot discriminate different forms of calcium carbonate, a sample was sent to the Scottish Lime Centre to provide a comprehensive analysis of the constituents. Examination by petrographic thin section highlighted something already suspected, that the plaster consisted of a proportion of lime and a proportion of chalk aggregate graded from approximately 3mm down to dust (Scottish Lime Centre, 2001). The hair content was extremely high, in excess of 14 kg/m3 and comprised of cow and horse hair.

Literature
There does not appear to be any reference to chalk mixes in historical literature, although, “Haired Putty Setting” is mentioned in Millar (Millar, 1897).  Here a similar material can be created composed of fine lime putty and well beaten white hair, the hair being thoroughly mixed with the putty to prevent it cracking. This produces a very soft and flexible plaster, but usually only a few mm thick.  Ironically, he refers to its use being in areas where local lime was of a strong or hydraulic nature where the mortars are not easily worked when mixed with sand. In East Anglia, where the lime was generally burnt from chalk, it was only slightly hydraulic if at all (Drury, 2001), the lime mortar was therefore high calcium and fatty.

More recently workers have experimented with high calcium carbonate and low sand mortars (Swann, 1999) and have identified that high hair content and low sand aggregate seems to be a feature of Elizabethan and Jacobean plasters which creates a material that bends a great deal and then crumbles along the break line rather than a modern lime plaster that “snaps” like a biscuit.

Plaster Failures
Cracking in sand lime mixes, supported by lath, can sometimes be a problem particularly on historic timber frame buildings, which by their nature tend to flex. Although many factors can be involved in plaster failure, (Swann, 2002) including a lack of hair reinforcement, this type of chalk-rich plaster will accommodate much greater movement without cracking. Much discussion at present revolves around the type of lime used and the effect of hydraulicity, for instance, however in this case it would appear that the type of aggregate being chalk and the very high hair content are as much an important factor as the choice of binder with this plaster.

Test Results and Test Panels

The examination carried out by the Scottish Lime Centre confirmed that the plaster, which was taken from an external plaster on a medieval hall house in Peasenhall, Suffolk, confirmed that the plaster was made from non-hydraulic lime and a chalk aggregate reinforced with a high dosage of cow and horse hair.  Proportions were approximately 3 parts chalk to 1 part lime with a hair content in excess of 14 kg/m3. Following receipt of this information a series of test panels were constructed. Careful selection of a suitably graded chalk aggregate was followed by trial mixes which proved quite difficult to combine due to the high hair content and initially mixes were started in a conventional mixer and then transferred to a plasterer’s bath for finally mixing by foot. However, this has recently been overcome by using a paddle mixer with a specially designed foot paddle.

Test Panel

Test Panels Constructed at Anglia Lime Company

Source, The Author

Site Work
Several large projects have now been completed using this mix internally and externally. The mix is extremely plastic and although the high hair content generally requires the application with a gauging trowel, once the material has been applied it can be finished with steel float or decorated as illustrated below.

Decor

 

Source, Drury S

There are advantages to this type of mix over a sand lime mix. Firstly it produces a highly flexible and soft plaster. Secondly, it can be applied as a single coat up to 20mm in thickness. Finally because the density of chalk is significantly less than sand aggregate the loading on a building is reduced. This is particularly important for historic timber frame buildings where not only is the reduction in weight an advantage, but the remarkable flexibility accommodates movement without cracking and debonding. In ceilings where there is a likelihood of deflection then this material can be ideal.

Lathwork & Plaster

Illustrates lathwork and plaster being protected from drying too rapidly by draping with damp sheets.
The Elms, Walsham Le Willows, Suffolk

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Source, The Author

Further Research and Further Analysis Mortar mixes have been based on the findings from our initial analysis carried out by the Scottish Lime Centre. Experimental panels have been erected incorporating increasing proportions of chalk aggregate to establish effect on workability, flexibility and to monitor any reduction in durability. Investigation of more historic samples may highlight variation in mixes and indicate new factors possibly revealing additional constituents. Experimentation not only with different mixes but also by varying quantities of hair and hair type and length may optimise mix performance. It is relatively early days and durability is still a concern. It may be that this type of mortar will require a higher frequency of limewashing to feed the plaster with fresh lime and to provide an essential sacrificial coating. Using modern fibre reinforcement may make mixing easier and eliminate the problem of hair deterioration while the mortar is in its wet state.

Lathwork & Plaster

Project completion
The Elms, Walsham Le Willows, Suffolk

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Source, Drury S

Conclusion
One question that arises is why did this material fall out of common usage? Perhaps plaster recipes in this area fell into line with practice elsewhere in the country. Certainly it is easy to apply and can result in time saving when applied as a single coat. It may require a longer period of aftercare than some sand rich mortars, to prevent the mortar drying out too quickly and may require more frequent limewashing than sand mixes. However, the material is soft and flexible, dramatically reduces loading on a building, being lighter and uses a local material readily available in East Anglia.

References and Bibliography
Ashurst, J., Mortars, Plaster and Renders in Conservation. Gower, 1988
Bankart, G. P., The Art of the Plasterer. B.T.Batsford, London, 1908.
Buxbaum, T., Pargeting, Shire, 1999
Cooper, A., Our Mother Earth, Bulmer Historical Society, 1998
Drury, S., Attitudes towards East Anglian Lime, Dissertation submitted to Anglian Polytechnic University, 2001.
Historic Scotland, Conservation of Plasterwork, 1994
Holmes & Wingate, M., Building with Lime, A Practical Introduction, Intermediate Technology Publications, 1997
Millar, W., Plastering Plain and Decorative. B.T. Batsford, London, 1897
Oliver, B., Old Houses and Village Buildings in East Anglia, Norfolk, Suffolk & Essex, Batsford, 1912
Sandon, E., Suffolk Houses, A Study of Domestic Architecture, Antique Collectors’ Club, 1977
Scottish Lime Centre., Soft Flexible Mortar, Results from Analysis of Sample AP 653, 2001
Swann, S., Making Lime Mortars Soft, An attempt to recreate “soft” vernacular mortars with high calcium carbonate and low sand contents. Lime News.1999.
Swann, S., Failures in lime plaster on lath-suggested improvements to the general specification. Lime News.1999