18th century plasterer at work
18th century plasterer at work

Plaster or stucco has been used since Roman times to cover and beautify both interior and exterior walls of both public and private buildings. As the techniques and mixes became more sophisticated more and more elaborate and ‘deep’ designs evolved .

Stucco – a plaster mix of lime, gypsum and marble dust perfect for fine work – was developed by the ancient Romans to decorate ceilings, and was widely used in Europe. Influenced by the Italian Renaissance and the subsequent fashion for Baroque style, decorative plasterwork was employed to spectacular effect on the ceilings of some of Britain’s grandest houses. there is a wealth of original plasterwork still existent on the Estate

plastering tools
plastering tools


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.

Cornices serve a practical purpose as an architectural device to cover structural joints between the walls and ceiling, and along with ceiling centres, friezes, corbels, panels, dado rails and skirtings, plaster mouldings became an art form during this era.
Cornices were put up in situ using a ‘running mould’ and plaster ‘enrichments’ (individual decorations) were added to enhance the overall effect. 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.

Ceiling roses and cornice in No 31 Sussex Square
Ceiling roses and cornice in No 31 Sussex Square

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.hman named Desachy patented fibrous plaster, a lime-free mix that incorporated flimsy sheets of hessian to add strength. Much lighter than solid plaster, fibrous plaster makes it possible to cast complete lengths of cornice and various other items together in one prefabricated piece. There is a wealth of original plasterwork still existent on the Estate with some patterns to be seen in several houses.


Running moulds

This modern wood and metal running mould has the shape of the cornice and is used to repeat the style of the moulding that is being copied; dating back to Victorian times, original moulds include popular patterns such as egg and dart, gothic arches, Greek key, dental edging and modillion blocks.

Ornate cornice in No 19 Sussex Square
Ornate cornice in No 19 Sussex Square, the ‘roses’ would have been applied individually.

Skirting not only covered that between the walls and floor but helped protect plaster walls from impact and dado rails from chairs. Although timber has remained the material of choice for skirting throughout the ages Thomas Cubitt favoured stone or concrete with a specific high measurement to be seen in most of the houses he built;

When fibrous plaster was combined with the use of the flexible gelatine moulds, first shown at the Great Exhibition of 1851, a vast array of decorative details could be mass produced

After the introduction of mass-produced reinforced fibrous-plaster mouldings, plasterwork became ever more ornate, as prefabricated mouldings could be made and installed by general tradesmen.

While moulds made it possible to produce decorative runs in quantity, any deeper recessed work had either to be made by hand or by using several small moulded pieces applied in situ by hand.

Placing the design at the start of restoration at Uppark, a National Trust Property
Placing the design at the start of restoration at Uppark, a National Trust Property


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.

Layering up base structure for deeper decorative work
Layering up base structure for deeper decorative work
Ceiling rose in No 1 Sussex Square
Ceiling rose in No 1 Sussex Square


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.

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.