Scagliola

David Harrison

The Egyptian dining room at Goodwood Park  
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)
 

The 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

  Richly decorated scagliola fireplace
  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.

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 its attractiveness as a finish. It is important to identify the type and extent of cracking and to determine whether the cracks indicate pieces have delaminated and are ready to detach, or whether it is just superficial ‘crazing’. Establishing a benchmark for future comparison in this regard is one of the most useful conservation techniques.

As the skills required for the manufacture of scagliola had largely died out by the 1930s, a number of fine examples have been inappropriately restored using plain plaster or other fillers and by painting in the colour. Care needs to be taken not to expose such patches, by abrasion for example, unless this is the intention. Where over-painting has disguised the presence of a decorative finish, extensive damage has often been caused by contractors who are unaware of the sensitive nature of the underlying material. The filling of cracks with inflexible fillers can also lead to deterioration if done in such a way as to restrict movement.

MANUFACTURE

In order to conserve and maintain this sensitive material, it is useful to have a general knowledge of the methods of construction. The first and older technique is described as fine or traditional scagliola, the other is referred to as marezzo or American scagliola (the latter because of the large amount of marezzo scagliola seen in the US).

Area of exposed pink scagliola surrounded by later overpainting  
Pink Regency scagliola which had later
been painted to look like Sienna marble
 
A pair of rich blue scagliola columns with gold capitals  
The restored lapis lazuli scagliola columns
in the Music Room at Buckingham Palace
 

With fine or traditional scagliola which imitates marble, the craftsman tried to imagine how the earth compressed, broke and twisted the various elements to produce the appearance of a particular type of stone. Geological processes which took millions of years were recreated on the bench by making large lumps of soft marble which are arranged to reflect the required brief and then allowed to set.

This method is further expanded to produce a mix of soft material with lumps of the hard set material within it. The simplest example of this is a Porphyry type scagliola which is made by mixing a thick slurry of red pigment with small chips of hardened plaster. A Verte Antique scagliola is made by combining lumps of set plaster and pieces of alabaster into a slurry mix. An experienced scagliolist will have many different recipes and techniques for creating different types of decorative stone.

Marezzo scagliola is made wet, getting the effects required by fixing the flow of pigments and plaster mixes at a particular stage in the manufacture. Marbling and veining is produced by dipping tangled lengths of silk thread in a liquid pigmentation then stretching them over a bench. Over these coloured silk threads a thin skin of coloured Keene’s cement is poured or spattered, transferring the pigment from the silk threads to the thin skin of cement. This form of scagliola is usually made to the required thickness right from the start, and this is often as little as one eighth of an inch thick.

Both traditional and marezzo scagliola are finally cut back to expose the different colour changes within the body of the material, then smoothed and polished with progressively finer abrasives. Finally, the surface was usually sealed and polished with oil or a mixture of oil, wax and sometimes other materials.

VISUAL INSPECTION

Inspection should include a close examination of the surface, noting any efflorescence, signs of mould or runs of varnish, and identifying the various coatings that may have been added such as polyurethane. At this point the careful removal of a surface sample should help to determine the most appropriate treatment.

It is essential to make sketches and scale drawings to record the condition of the piece so that any defects can be measured and recorded accurately with comments. Attention should be paid to the thickness of the colour coat, as this is often not great and may already have been polished to negligible thickness. Further polishing may result in non-reversible blank patches.

Chemical analysis of both the surface and the substrate can help, but there is no substitute for a trained eye in identifying a particular scagliola recipe. A knowledge of the pigments, materials and techniques available when a particular piece was made will aid both an understanding of how it was made and how it could be replicated.

With experience, a scagliola piece can be sounded with a gentle tap to identify the differences between delaminated and firmly attached areas. As scagliola is a surface application it has a variable thickness, usually physically keyed into the background by scratching and application of a slip of plaster, if this adhesion is failing, sounding will give a different tone. To a trained ear this can help determine whether cracks go right through the piece or are superficial. When the sounding is complete, it is helpful to use the drawing as a map to indicate suspect areas. This should give an indication of how much of the piece is still securely attached to the substrate.

It is important to treat the inspection as part of an ongoing maintenance programme, providing a snapshot of the condition at a particular time. This allows others following on to gauge rates of deterioration and the success or otherwise of any intervention.

CONSERVATION

  A conservator cleans a pair of Marezzo columns
  Marezzo marble columns at Stamford Old Town Hall, Connecticut: over-painting has been stripped and the underlying surfaces are being cleaned and repaired prior to polishing.
   

As with the conservation of any historic material, the intervention should always be fail-safe and should only be attempted after non-invasive testing and close inspection, or preferably a full condition survey, including visual inspection, sounding and historical research.

Where surface delamination has occurred it may be possible to re-attach the layers by injecting one of a variety of specialist modern adhesives, selected according to the properties required, such as gap filling, reversibility and, of course, adhesion.

Different depths of separation need different treatments, but the principle is to clean out the void either by flushing or blowing, inject a suction-reducer (which may be water or diluted adhesive) and then inject sufficient liquid adhesive to reattach the surface layer to the substrate. Any holes made in the process are then plugged with appropriately coloured material prior to refinishing and polishing.

Colour fading can occur when heat sources are too close to scagliola or where regularly exposed to bright sunlight. Fading is usually almost impossible to reverse, although minor surface fading can be rectified by cutting back the surface and refinishing.

Scagliola has often been over-painted at some time, either to match a new colour scheme in the room or because the client disliked the original colour. To revive a scagliola surface, for example after paint stripping, it is advisable to carefully start with a small trial area to find the best restoration method for a particular scagliola. Most chemical strippers can erode gypsum, lime or Keene’s cement very quickly if applied in the wrong concentration.

When the most suitable cleaning and restoration methods have been chosen and the paint removed, the surface can be inspected again for further defects. At this stage any hidden filling will come to light. Dirt on the original surface can now be carefully removed.

Hayles and Howe, Scagliola Catalogue  
A new catalogue of Hayles and Howe’s work in this field is available from the author (click image for details).  

If the decision has been made to cut back the surface of the scagliola and restore it, the next step is to start cutting back the surface with a coarse abrasive paper, gradually moving to fine. As the work proceeds, all loose and friable material is removed, and damaged areas are repaired with fresh scagliola mix to match. These repairs must be done wet to reduce suction and to enable adhesion. Once set, the process of cutting back restarts, gradually moving to the finer grit and even finer powders if required, retracing the final finishing process of the original manufacture as far as can be determined. This cycle is repeated as many times as necessary until a sheen starts to appear, before carefully and delicately applying the specified finish.

Matching the highly polished surface of scagliola requires careful selection of abrasives with the appropriate particle size for each stage of the procedure, as well as a suitable polish. The abrasives range from very rough plaster planes of 80 grit through to 1600 grit, finishing with fine powders. French polish, linseed oil, milk and a number of modern waxes have all been used to achieve a high gloss. Sadly, modern urethane coatings have also been widely used in the past, and these may need chemical removal.

The stunning lapis lazuli scagliola columns in the Music Room at Buckingham Palace posed a very interesting problem. The columns which were originally made in 1836 by William Croggon and his team included a mix of filings of brass, tin and copper, to give a gold and silver flecked appearance to the blue sheen. The filings immediately began to corrode and expand, blowing pits in the surface. This process continued slowly for 150 years, but when the Palace State Rooms were recently opened to the public, the decay began to accelerate as a result of the additional moisture generated by visitors.

To restore the columns to Nash’s original design it was decided that a treatment using beeswax mixed with some rare, specially selected pigment and finished with a lighter grade of wax to give a shine would protect the columns without obscuring them from view.

 

~~~

Recommended Reading

A Zecchini, Arte Della Scagliola sul Lario, Hoepli, Milan, 1997

G Beard, Craftsmen and Interior Decoration in England 1660-1820, Bloomsbury Books, London, 1986

W Millar, Plastering Plain and Decorative, 1st Edition 1897, facsimile edition, Donhead, Shaftesbury, 1998

G Worsley, ‘Buckingham Palace, London’, Country Life, 5th August, 1993

 

 

 

 

The Building Conservation Directory, 2013

Author

DAVID HARRISON is managing director of Hayles and Howe Ltd in the UK and Hayles and Howe Inc in the US. Working in traditional fibrous plaster as well as specialist scagliola enables the two companies to provide a wide range of design specifications and finished products. A book by his colleague David Hayles, The Magic of Scagliola, is being prepared for publication.

Further information

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Plasterwork, scagliola

 

 

 

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