Decorative Lime Plaster
|Setting out the Rococo ceiling at Uppark|
The subject of decorative plaster is really a tale of two plasters – two plasters that are often confused as one, both having the same appearance but each having very different qualities.
The first, the one plaster that is usually associated with early decorative work is lime plaster. Made from lime putty, lime plaster has wonderful versatility, but its reward is gained at a price, for lime is a deceptively difficult substance to use and its behaviour is often unpredictable.
The other plaster comes from an easier to use, more popular material, a fine white powder capable of a quick predictable set. This is gypsum plaster. It is the most common material used today for plain and decorative plasterwork, but prior to cheap mass-produced gypsum plaster in the late 19th century, both gypsum and lime were used for decorative plasterwork, at times combined side by side in one decorative scheme where the two methods and materials complement each other. Indeed it is highly unusual to find early decorative plasterwork to be the product of strictly one plaster, and lime plaster was often used with an additive of gypsum to aid the set.
Lime plaster remained in widespread use for traditional vernacular buildings beyond the advent of fibrous plaster and cheap gypsum plaster, mainly because of ease of availability in the countryside.
To make lime plaster, a limestone of almost pure calcium carbonate has to be chosen. This is fired in a limekiln at a temperature of about 1,000°C. The burnt stone taken out of the limekiln is quick lime (calcium oxide), a very caustic material that is difficult to keep, so it is almost immediately turned into lime putty (calcium hydroxide) by adding water, a process known as ‘slaking’ which generates a great deal of heat and steam.
Putty lime will harden slowly when exposed to air as the lime reacts with carbon dioxide to form calcium carbonate once again – a process known as ‘carbonation’. Fresh lime putty is therefore protected from hardening by being stored in waterproof containers in a damp state, permanently covered by a thin film of water.
Lime can be used by a mason to bed stones or modelled by a sculptor once the necessary aggregates have been added. (In plasters, aggregates such as sand are added in the proportions of up to around three-to-one for all but the finishing coat, principally to reduce shrinkage.) A modeller using lime plaster, or ‘stucco’ as it is often known, has time to change his mind some time after he has used it, for lime plaster will set over a five to ten day period. During this period it must be protected from drying out too quickly or it will crack. Once set, stucco will last for centuries.
Gypsum plaster behaves very unlike lime plaster. It is made simply by heating gypsum rock or alabaster – both of which are mineral forms of hydrated calcium sulphate – and grinding the result to a fine flour-like powder. At a relatively low temperature some of the water which makes up the crystalline mineral structure is driven off, forming calcium sulphate hemihydrate, which is then ground to a fine powder.
Gypsum plaster will set rapidly – within 15 minutes once it has been ‘knocked up’ with water – forming interlocking crystals of gypsum. This is not a material for modelling with, more a material for casting with, as it sets so quickly. So we have two completely different materials, for different purposes. A slow-setting lime plaster and a fast-setting gypsum plaster.
One of the earliest and most renowned sources of relatively pure gypsum rock was Montmartre, Paris, from which the material takes perhaps its most common name, plaster of Paris.
Plasterers, particularly since the late 18th century, have generally used gypsum plaster both to imitate earlier lime plasterwork and to create their own contemporary plasterwork of varying quality.
Perhaps the earliest known examples of decorative plasterwork are from the Old Kingdom in Egypt. Painted plaster masks adorned the linen wrapped head of a mummy, and stone walls would have had their irregular surface smoothed with plaster before being carved or shaped and painted. This plasterwork was formed with fast setting gypsum plaster.
Roman stucco work, though mainly painted, shows widespread use of lime plaster, for example; as a wall covering for landscape painting, as can be seen in Hadrian’s villa in Tivoli in the 1st century AD; or as a theatrical backdrop of mythological figures and theatrical figures in the upper class Hang houses in Ephesus, c 5th century AD.
Instructions by the Roman architect Vitruvius on the means of ensuring that stucco relief decoration remains sound and firmly attached to the wall are as relevant today as they were in the 1st century BC. His advice on the need for cane and metal support for relief work to prevent distortion is, of course, common sense, as are the rules he describes for obtaining a flat wall surface using three coats of plaster: a coarse base coat of rough sand and lime reinforced with hair to prepare the wall surface, followed by a levelling coat of medium graded sand, lime and hair to level the wall, and finally, a finer finish coat, much thinner than the rest, of fine lime, sand and possibly goat hair.
Vitruvius’ advice on how to make lime plaster adhere to a damp wall has a particular resonance today. To combat wet conditions, he recommended a pozzolanic additive of brick shards and brick powders for the first of the three layers of lime plaster. The combination of brick and lime, well mixed, provides a hydraulic set for the plaster (‘hydraulic’ literally means having the ability to set under water), enabling the mortar to set whilst still wet, without carbonation.
In addition to brick dust, a multitude of other additives were used to accelerate the set of lime, but perhaps the one ingredient that carries the most historical significance must be marble flour. This aggregate was the key ingredient of the finest mid 18th century plaster work, Stucco duro, which was largely confined to Italy and southern Europe. Marble flour allegedly aids both the plasticity and the set of stucco. Although it was never widely used as an additive by English plasterers, the style of the stuccodurists was much admired and imitated. The twists and turns of a fine Rococo ceiling, with all its convoluted curves and intertwining shapes, could not have been easily made without the setting properties of marble dust or, as was later discovered by the English imitators of the stuccodurist, a lacing of gypsum plaster.
|Work on the overmantle in the King’s Dining Room at Edinburgh Castle, modelled in lime plaster|
Modelling for decorative work is made up in many layers in much the same manner as for flatwork. To minimise shrinkage, graded sands of various particle sizes are added to lime putty for each layer: coarse sand is used for the hidden core and a very small proportion of fine sand is added for the top layer. On the Continent, hair of differing strength and thickness is also added for reinforcement; coarse cattle hair for the base layer and fine goat hair for the finish coat.
Plaster additives used for decorative work are legion. The setting time of lime plaster can be speeded up with crystalline additives of alum and potassium sulphate, or retarded with animal glues and urine, and its strength can be increased with the mineral additives, magnesium and fluorosilicate; but there were many others, and those found in historic plasters can be difficult to identify from analysis, particularly if they are organic in origin.
As stucco is pliable while it cures and hardens, it generally requires some kind of support or reinforcement. This may simply be the wall itself or an armature set within it, particularly where the modelling is in high relief.
Lengths of ornament, or ‘runs’ are made by pushing a metal form cut to the profile of the moulding required through wet lime plaster. This profile is carried on a simple wooden frame called a ‘horse’ and it is guided by battens set out in the ceiling or walls. Alternatively, moulding can be run in much the same way but on the bench, for fixing to the ceiling or wall later. Sections of runs are then cut for corners, mitres and awkward returns, and fixed in position with nails or screws and fresh plaster used as an adhesive.
Repeated ornament is cast in the workshop using moulds, traditionally of hard material such as lead or boxwood, lead moulds being cast from a hand-modelled plaster original, boxwood being carved in the reverse. The moulds, which are usually of one piece, are coated with a releasing agent such as olive oil. The stiff but pliable lime plaster is then forced into it and left until firm enough to be removed. If no gypsum has been added, this may take around five days. On partially setting, the ornament is pulled out for final attention with the modelling tool.
Confirmation that these working methods are the same as those used in the 18th century was given by the discovery in 1983 of a selection of tools, moulds and trial casts left under the floorboards at Audley End in the 18th century by Joseph Rose, the travelling Yorkshire plasterer.
Choice of form and material are closely linked in decorative plasterwork. Jacobean plaster has a coarser, less intricate appearance than later work, partly because it also involves coarser materials. In 1997 and 1998 two ceilings in the Royal Apartments at Edinburgh Castle were reinstated by Historic Scotland.
In 1617 a suite of five rooms was hastily made ready by imported London plasterers for the inauguration of James VI of Scotland. Cromwell made short work of the ceilings later, but examples of the plasterers’ work survive at contemporary houses elsewhere, including the Scottish castles of Muchalls, Glamis, Thirlestane and Graigievar. This evidence together with the account of the ‘Master of Works’ which details the tradesmen, plasterers and materials of the decorating programme gave Historic Scotland more than a glimpse of the missing plaster scheme. Historic Scotland’s aim was to reconstruct the missing plaster scheme, in both technique and material, to match those used by the original plasterers.
|The decorative plasterwork at the Royal Apartments at Edinburgh Castle was made using a ‘horse’ (above left) to create the basic pattern of long ribs of lime plaster onto which ornamental casts were applied (above right).|
Uncovering their methods and materials proved more difficult and time consuming than had been envisaged. Ordinary chemical analysis of surviving contemporary plaster from Thirlestane failed to extract and differentiate the amorphous mixture of lime and lime-based aggregate and additives. However, petrographic analysis under a polarised microscope was more successful. This method of analysis is founded on the principle that each known mineral has different optical properties which enable them to be identified under cross-polarised light. Using this technique, Professor Graham Morgan of Leicester University was able to determine that the plaster contained up to 50 per cent aggregate and other ‘rubbish’, including kiln ash, old plaster and limestone sand. Kiln waste, like the brick shards recommended by Vitruvius, promotes a reactive set in lime plaster, allowing fast curing of the hundred or so moulded pieces that made up the ceilings, and for the large and weighty mouldings of the ribs and cornices. The old lime plaster was introduced as an aggregate, supplementing the limestone sand. As both these materials and the proportion of the lime that was active when added all contain calcium carbonate, the three materials were indistinguishable by chemical analysis.
Time constraints of the project prevented suitable experimentation for strict use of this plaster mix, and so the mix was adapted with the more conventional materials chalk and gypsum. However, four lengths of cornice, several casts and the modelling of the overmantle in the King’s Dining Room were executed in a mix similar to that used at Thirlestane, containing lime, coarse local limestone dust, old lime, and smithy waste of clinker and iron filings. The mix was so thick that the decorative modelling had to be almost pressed out of the lime or even carved. The result was a highly successful copy in the style of the original, confirming that the original Jacobean plasterwork really was formed from plaster with seemingly impossible quantities of lime – impossible, that is, until you discover how much of the lime present in samples is derived from limestone and recycled old lime used as aggregate. This plaster could be described as particularly rugged, its component aggregates being somewhat too large for fine modelling, and its rather thick consistency and quick setting time ruling out elaborate designs.
THE DECORATIVE CEILINGS AT UPPARK
|The neo-classical ‘Adam style’ saloon ceiling at Uppark (above). Its geometric and ordered patterns are made possible by the use of gypsum plaster, and are in complete contrast to the earliest style of plasterwork to be found in the Red Drawing Room (below), which is hand modelled in lime plaster.|
The reinstatement of the fine decorative stucco work at Uppark, West Sussex by the National Trust after a disastrous fire in 1989 provided essential information on the craft of traditional decorative plastering. Here was a house of great importance that had a decorative plaster scheme spanning a cross-over in styles in the late 18th century.
On the west side, three ceilings survived in the earliest style used here, the flamboyant Rococo style, for which the plasterers used a little fine local ‘Harting’ sand (less than five per cent) together with a small amount of gypsum to make the plaster mix flow and remain fluid for several hours. Thick egg and dart runs are intersected by modillions and dentils along the cornices with sunburst and Apollo masks, together with grape, sunflower and goat motifs for the Little Drawing Room, and two large masks with cornucopia basket hats for the Red Drawing Room Ceiling. The Staircase Hall has what must surely be one of the largest acanthus style roses which is set down from the ceiling centre in a cone shape some half metre in depth.
On the east side two ceilings had survived that had been undertaken twenty years later, in 1770, by Sir Matthew Featherstonehaugh to designs by Paine. These ceilings, which included the Saloon, were in the neo-classical style that had become fashionable by then; a ‘tighter’, more repetitive form of decoration, later to be known as the Adam style. The thinner, linear design of harebell swags, numerous paterae and arabesques owed much to the craft of setting out with chalk lines and trammel (netting), as well as intricate modelling. The central elipse was a triumph of flexible casting: lime and sand together with considerable amounts of pearl glue enabled the cast of egg and dart to be curved to fit the changing shape of the coffers.
And finally, there was the dining room. This room had been altered and decorated in fine painted wainscot and plaster statuary by Repton in 1812-13. Here the plasterwork is almost purely cast. Gypsum casts painted bronze form overdoors of hind and horse with busts of Napoleon, Fox, Bedford and Bathine by George Garrard.
|Gypsum casts of modillions for the Staircase Hall, Uppark|
|Gypsum casts of paterae for the saloon, Uppark|
The National Trust bases its approach to building conservation on the principle that the cause of the decay must be identified properly first before treatment is embarked on. It also adheres to the two dictums to ‘repair like with like’ and to ‘preserve as much of the original as possible’.
Recently, repair techniques have been devised in particularly sensitive locations such as the Jacobean decorative plaster scheme in Chastleton and the ceiling in the medieval chapel at Petworth. In these cases, wherever surviving plaster had been severely weakened by material or structural decay but remained in position, the repair programme was designed to provide hidden structural support. The consolidation work was designed to secure plasterwork to sound building fabric using mechanical ties, without compromising the flexibility of traditional building material and building design. For example, ceilings are tied with flexible anchors set in the back of the plasterwork, connected to metal bars fixed to the ceiling joists above, and loose plaster walls are fixed to repaired stud work behind with penny sized washers and tie rods.
The bottom line, as in all conservation work, is minimum intervention.