Lime plaster

Building conservationists should challenge common misunderstandings about the differences between 16th and 17th century plaster and techniques, and those that followed.

Typical in situ moulding from the early 19th century, including gypsum.

A few years ago I was engaged to advise on the restoration of a Jacobean ceiling in the North of England. The ceiling was being restored by a plastering contractor recognised in the world of historic building restoration, and overseen by one of the best architectural practices in the UK, with a proven track record of repairing some of the UK’s finest historic buildings.

The ceiling had been plastered on to laths that had been fixed to the underside of the floorboards and wrapped around the joists, forming a series of bays. The bays had been ornamented with a typical early 17th-century low-relief repeating motif. The restoration was being carried out using stainless steel wire mesh instead of laths. A layer of a modern gypsum plaster had been applied to the mesh. A rubber mould had been made of the ornament and cast in pure gypsum in a workshop before being dried and glued in place with a gypsum adhesive.

It looked very nice. However, the process and materials being used were completely inappropriate in view of the age of the original plaster scheme. My view was that they might as well have used plasterboard and gypsum finish, as that would have been no less inappropriate than what they had done, and it would have been more easily reversed in the future.

There is a lack of deep understanding of the difference between 16th and 17th century plaster and techniques, and those that followed. The lime revival has been fairly rapid and broad. Inevitably when something moves forward so quickly details can get missed. The revival in the understanding and use of lime has been marvellously supported by groups such as the Building Limes Forum, SPAB (Society for the Protection of Ancient Buildings) and, of course, the IHBC, to name just a few. These groups have spread information on materials and methods be used. This has also been driven by tradespeople in their skill and determination to do things correctly.

There is nothing written that describes the techniques and materials used during the 16th and 17th centuries. Laurence Turner’s wonderful book Decorative Plasterwork in Great Britain (1927) gets closest, with a small drawing of a cross-section through a Jacobean enriched rib. That shows perfectly how it was put together, but there is no technical description.

We have been fed a generic type of lime plastering based on largely 19th-century books such as William Millar’s book Plastering, Plain and Decorative: a practical treatise on the art and craft of plastering and modelling (1899), and other similar publications which explain how the Georgians and Victorians plastered wonderful pieces of the plasterer’s art.

However, it is wrong to apply those techniques and materials to the repair of an Elizabethan or Jacobean ceiling. There needs to be greater understanding of the differences between various periods of plasterwork, so that repairs carried out by our generation will be seen to have been done well.

The plasterers of the 16th and 17th century did not use gypsum to create their beautiful plaster ceilings. They did not need it, and it would have made things more complicated. They had no adhesive with which to glue something in place that had been cast and dried, so this was not something they did. They relied on the inclusion of huge quantities of hair to enable them to manipulate their plaster in the most remarkable ways.

The 18th century saw great changes in fashion, including in plaster work. The new style required things to be more accurate. This precision required the use of rules to straighten plaster, whereas previously things might have been eye-straight. The old plaster could not be ruled, so more sand was added to make it flow easier, and the hair was removed to aid this process. It was now possible to cast in advance in the workshop all of the ornament and glue it in place with gypsum.

In the early 18th century there was a crossover period where both processes were being used together. This should be looked out for: do not assume that 18th-century plaster means cast ornament. During the recent repairs to ceilings at Stowe school it was found that an 18th-century moulding ran in situ with lime and gypsum, as we would expect, had been ornamented using 17th-century techniques.

We, the membership of the IHBC, must be responsible for continuing our education to improve our understanding of the difference between plasterwork from different periods. We must use that understanding to educate others to make sure that inappropriate repairs are a thing of the past.

This article originally appeared as ‘Understanding lime plaster’ in Context 143, published by the Institute of Historic Building Conservation (IHBC) in March 2016, it was written by Philip Gaches, a master plasterer who joined his father in the family plastering business as an apprentice. His projects include work at the Palace of Westminster, Downing Street, Chatsworth, Woburn Abbey and many other fine buildings. He has taught traditional plastering for over 20 years for the SPAB and other organisations.