Jun 052010
 

Decorative Plastering in Devon Image

Lime’s early origins

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.



Mar 152010
 

LIME

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

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.