The appeal of decorative plasterwork lies not solely in aesthetics. It is cheaper and more versatile than carved wood, and can last for centuries. The use of lime plaster reached a peak in Britain in the 18th and 19th centuries; today, it is enjoying something of a renaissance. ‘Lime has a wonderful, flexible quality and it allows a building to breathe,’ says Jon Joy, who now heads A. G. Joy & Son, a small firm of plasterers set up by his father and grandfather in the 1950s and based in Suffolk and north London. ‘Now, people have begun to realise how important it is to treat buildings properly.’

Mr Joy works with plasterwork ranging from contemporary schemes to the 15th century, but specialises in traditional methods and materials. His firm is one of a few dozen that still make (or ‘run’) moulded ceilings in situ. ‘My father was trained on bomb damage from the war, which gave him the opportunity to work on some top-quality projects with first class craftsmen.’

Working with lime can be a complicated and scientific business, not least because the term ‘lime plaster’ can refer to quite different materials with varying properties. They all have an effect on the methods employed and the final look of the decoration, so it is vital for those doing historical work to glean as much information as possible about the original. Mismatched plaster will not only look wrong, but can damage surviving work.

The two most common forms of traditional lime plaster are raw lime and gypsum (or plaster of Paris). Left to dry naturally, raw lime will take several weeks to set, but stays malleable so that the plasterer can continue to sculpt it long after it has been applied. Generally, it will contain a form of aggregate such as sand or animal hair, to speed up setting and reduce cracking.

Gypsum was not readily available in Britain before its industrial manufacture in the early 19th century. Its versatility and quick setting properties it will go from wet to rock hard in 10 minutes make it ideal for casting details such as leaves and flowers, but it is also used to mix with or ‘gauge’ raw lime. It is this gauged form of lime plaster that is most common today. The faster drying time reduces the inconvenience, and leaves time to finish detail.

Any plaster scheme usually begins with three coats: an initial scratch coat, a floating one which straightens the walls and ceilings, and a finishing coat. In situ cornice mouldings of the type Mr Joy works with are run after the second coat is applied. A mould or ‘horse’ of the desired profile is run over the raised plaster, guided by a straight timber rule fixed to the wall. As the mould cannot be run right into the corners, the mitres are worked in by hand, with straight steel tools called joint rules. Any fine decoration, such as flowers or leaves, is applied later, although for very complex work, a professional modeller is drafted in. ‘Once everything is set up, it is quite a quick process you can run a 10ft cornice in about half an hour.’

In his 28 year career, Mr Joy’s most memorable job was working on the restoration at Windsor Castle after the fire. ‘My father went straight down there. He was picking up pieces from the floor and salvaging what he could to try to do some research. In the end, the work was awarded the Humber Silver Salver, having won the Plaisterers Trophy for solid plasterwork.’

A. G. Joy & Son 07768 510895

Top tips

  • If restoring plaster, get an assessment of any original work. You may need to replace less than you think
  • Not enough gypsum in the initial mix can result in a fragile moulding
  • Clean paint-clogged plasterwork with special poultice strippers (check this is compatible with the plaster first)