Page 182 - The Building Conservation Directory 2013

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T w e n t i e t h a N N i v e r s a r y E D I T I O N

5

INTERIORS

Scagliola

David Harrison

T

he term

scagliola (pronounced scălliōla,

with a silent g) derives from the Italian

‘scaglia’, a local name used in the Italian

Alps for limestone (Oxford English Dictionary,

2012). Scagliola is a form of plaster, traditionally

gypsum, which imitates decorative stone and

has been widely used for the decoration of

walls, columns, floors, fireplaces and table tops.

Artificial forms of decorative stone

based on gypsum or lime plaster have a

long history and were widely used in the

ancient world. Recipes for their production

can even be found in the works of the

great Roman architect, Vitruvius.

The technique was rediscovered in the

Renaissance and flourished on the continent

in the Baroque and Rococo periods as a

medium for the imitation of exotic marble

and precious stone inlays. First introduced to

Britain during the 17th century, rare surviving

examples include a fireplace in the Queen’s

Closet at Ham House, Surrey.

In Britain, however, the use of scagliola

is more commonly associated with the

imitation of the finest marbles on a much

larger scale, without any joints or defects,

and for columns and pilasters in particular.

This use reached its zenith in the Regency

period, typified by the extravagant bright

scarlet and imitation lapis lazuli columns

introduced by John Nash at Buckingham

Palace in the 1820s (illustrated overleaf) and

by the spectacular staircase hall of Charles

Barry’s Reform Club, London in the 1830s.

In the latter part of the 19th century

marezzo scagliola was developed in the

US. It advanced the traditional methods

of making scagliola by using Keene’s

cement, a gypsum-based cement patented

in 1838, rather than plaster. Both marezzo

scagliola and traditional scagliola in

America was prolific in the 19th century,

although their use was generally confined

to public buildings, such as state capitols,

courthouses, churches and railway stations.

Recognition

The difference between marble and scagliola

can be established by feeling the surface – if

it is cold, it is likely to be marble. Scagliola

also produces a hollow ring when tapped.

In appearance, marble is more translucent,

although scagliola sometimes has pieces of

alabaster or other semi-precious stones added

to the mix to give localised translucence.

Looking carefully at any damage holes or at

the rear surface if this is accessible, a marble

piece will look like marble throughout, whereas

scagliola will have a distinct plaster backing

behind the colour coat. The presence of

masonry joints would also normally indicate

marble, as scagliola is often used expressly to

create continuous surfaces. Scagliola can also

be identified if the surface has hairline cracks.

Chemical analysis may be used to confirm

whether or not the material is made of gypsum,

but it should be borne in mind that it will

reveal information about the current material

as it is now, rather than what it was. Mass

spectrometry for example will reveal whether

it is a gypsum plaster by showing up calcium,

sulphate and oxygen. In the case of with

Keene’s cement, there will also be a percentage

of aluminium, potassium, sulphur and oxygen.

Chemical analysis would also reveal the

presence of pigments, animal glue, isinglass,

linseed oil or some other organic material.

Care and maintenance

Scagliola is vulnerable to water ingress. Being

made of gypsum, it will quickly return to

its natural state when water is added. Water

running over the surface will etch runs and

pitting as it washes out the gypsum. If soaked,

salt crystals will effloresce on the surface of

the scagliola as it dries, as crystals of calcium

sulphate migrate and form on the surface.

Water will also loosen the bond between the

surface and deeper layers of the build-up,

leading to delamination and an abnormally

hollow sound when tapped. Prolonged

exposure to moisture will eventually weaken

and destroy scagliola.

Scagliola is quite brittle. Because it is

made up of many separate elements joined

by variable strengths of adhesion, scagliola

is not a structural material and will crack

along the line of least resistance. Care should

be taken to allow tolerance for thermal

expansion and contraction when restoring a

piece, for example between a scagliola column

and the superstructure. Cracking caused by

compression is often seen in columns and

frequently occurs at the base. Superficial

cracking can be considered to be part of the

patina of age, which gives scagliola part of

Wyatt’s spectacular Egyptian dining room at Goodwood Park, which was completed by 1806 and covered up

in 1906, reputedly following the objections of Edward VII. The scheme was preserved behind panelling and

over-painting, and has recently been restored. (Photo: Courtesy of the Trustees of the Goodwood Collection)

A 17th century scagliola fireplace in the Queen’s

Closet at Ham House, Surrey: this is one of the

earliest known examples of the technique in Britain.