The basic techniques employed in gilding have changed little since the building of the pyramids in Egypt. An illustration from a tomb at Saqqara of about 2500 BC shows gold being beaten with a rounded stone, and in the Louvre in Paris there are some leaves af gold found among Egyptian remains which are the same size as those used today, albeit slightly thicker. Over time methods have been developed to vary the colour, texture, lustre and durability of gilding and also to allow its application to different surfaces. The ancient origins of the craft are unknown, but the apparent air of mystery surrounding it in more recent times is largely due to generations of gilders guarding the trade secrets which gave them their livelihood. Indeed some secrecy still lingers, even today.
The possible long-term durability of gilding is demonstrated by Tutankhamun's couch at Luxor. It has a flawlessly burnished head at one end which is in pristine condition, even after the passage of 3,000 years. In our own time however, gilding exposed to greater wear and less protected conditions will not last so long.
In architectural decoration there are two main types of gilding: oil or mordant gilding and water gilding.
Oil gilding: This technique is used for general decoration and with appropriate preparation oil gilding can be applied to most building surfaces, both indoors and out. Gold 'size', a type of glue traditionally based on linseed oil, is applied to a well-prepared, fine ground of either paint or 'gesso', a fine plaster with a size binder. Gold leaf is laid on when the size is touch dry but retains enough tack for the leaf to adhere. Because of the extreme thinness of the leaf it is manipulated onto the sized surface with tools specially developed for the purpose; a gilder's knife, tip and cushion (see illustration). After gilding, loose fragments of leaf are brushed from the surface with a skewing mop and if required, the surface can be buffed with fine cotton wool.
Water gilding: This is a more elaborate process requiring greater preparation, but its elegance and refinement of finish are unsurpassed. Water gilding is used mainly for picture frames, furniture, religious artefacts, sculpture, objets d'art and also for the embellishment of stately buildings. The process essentially consists of applying six to twelve coats of gesso to the substrate to produce a very fine smooth surface, followed by four to eight coats of bole, a refined clay available in various colours. The bole is polished to a fine finish (any flaws or grit would ruin the appearance of the gilding), coated with dilute size and allowed to dry. The surface is then wetted with water and gold leaf is laid onto it immediately; as the water soaks into the gesso it quickly draws the gold into close contact with the surface. When dry, any loose fragments of gold leaf are skewed off as in oil gilding and the surface is given a protective coat of ormolu size to enhance the colour and uniformity of the gilding. Ormolu size is a mixture of weak size and lacquer coloured with a little orange-red resin called 'dragon's blood'.
It is common for different elements of a water-gilded piece to be given contrasting finishes to enhance the design. This effect is called 'bright and matt' gilding. The bright passages are usually gilded twice but they are not ormolu sized. Instead they are burnished with a burnisher, a polished rounded agate set in a wooden handle, to produce a mirror finish. These areas receive no further treatment as any coating would mar the reflective glitter.
In addition to the use of mart and burnished finishing techniques, many other factors affect the appearance of gilding. Gold leaf is currently and has historically been available in a variety of grades and colours. Today purity ranges from 12 to 24 carat, and colour from several shades of gold to tints of red, white and green. These variations are achieved by alloying gold with other metals, mainly silver and copper. In addition silver, platinum, palladium, tin, aluminium, copper, bronze and Dutch metal (an alloy of copper and zinc) have all been used instead of gold leaf for 'gilding'.
Regular gold leaf is approximately 1/250,000th of an inch thick and is translucent if held to the light. The colour of the ground over which it is laid therefore has a significant influence on the colour of the finished gilding. This is most commonly seen on picture frames where, more often than not, a dull rich red is seen in areas where the gold has worn through. This is the bole referred to above, though depending on the date of the piece it may also be yellow, or blue-grey under burnished work. In oil gilding the underlying paint colour performs the same function: deep red for rich, sumptuous gilding, yellow for brightness and glister. Red or white is usually used under silver.
The other main influence on appearance is surface coating. Oil gilding on architectural details is often left unvarnished, but for aesthetic reasons or on areas exposed to wear it may be treated with anything from a very light, clear glaze to a toned varnish. Water gilding will normally have a coat of either size, ormolu or lacquer to serve both as protection and to achieve a consistent appearance across areas of combined water and oil gilding, such as on picture frames with carved or moulded ornament. This may also be toned with pigment to distress the surface or to create a particular effect.
Silver is always varnished to prevent tarnishing, but in recent times it has been largely replaced by palladium and other non-tarnishing alternatives. White metals may also be found glazed with a suitably coloured varnish, sometimes called 'changing varnish', to provide a cheap alternative to gold leaf, a practice dating back hundreds of years.
'If You Know Nothing - Do Nothing!' is a rule that should be observed by anyone contemplating cleaning or 'brightening up' gilded furniture or decoration. More damage is done to sound gilding by inappropriate treatment than by all accidental causes put together. Although gilded surfaces may look solid and metallic they are in fact quite delicate and very easily damaged. Most gold leaf is of at least 22 carat purity, which is very soft and it is easily marked or dulled if not carefully treated.
Water will ravage water gilding because it will dissolve the size which binds the gesso - in seconds rather than minutes. Solvents will attack oil gold size and remove the gold from the surface; or it may dissolve lacquer, removing the intended toning and leaving water gilding unprotected. However, one of the worst causes of damage, simply because of its case of application, is gold-coloured paint. Since it became widely available about 40 years ago it has been liberally applied to all kinds of perfectly sound gilded surfaces that simply required professional cleaning, and in many cases not even that. It oxidises to a dull greenish brown, often within months, entirely destroying the intended effect. With a few exceptions, any object worth gilding in the first place has some intrinsic value and there is no justification for the application of a gold substitute, whether it is paint, aerosol spray, wax or any of the other nostrums currently available. The quick fix these provide is outweighed many times over by the cost of removing them later or the loss of the gilding.
The key to successful conservation is thorough investigation before any work is started. The type of substrate, method of gilding, the type of leaf and any surface treatment must be established so that an appropriate strategy for conservation can be developed. Reverse edges and out of sight corners are useful for experiments; cleaning may well involve using water and solvents, any of which could cause damage, so it is crucial to know how to use them safely. Because of the many variable factors and the need for knowledge of the processes used at different times in the past, this work needs to be carried out by a conservator. The aim should be to retain the original surface, including any glazing and toning as these are just as important to the original work as the underlying metal leaf. An object such as a gilded mirror frame may be valuable, but only so long as it has its original surface and patina. Once these are lost, the artefact itself may be all but worthless in both historic and monetary terms.
Any repair or restoration work which is necessary should be minimal, using the same materials and techniques, and in most cases small areas of repair must be carefully toned and distressed to match the original work. This requires great skill and practice and is not a job for the amateur or the faint-hearted. (In some cases it may be considered preferable to leave new work clearly distinguished from the original, particularly where large areas have been restored.)
Oil gilding on architectural details is often regarded as just another colour to be renewed from time to time. On exterior railings or weather vanes where the substrate is rusting, renewal may be justified, but internally, unvarnished gilding should last almost indefinitely as long as it is preserved from surface wear and deterioration of the substrate. If it is out of everyday reach appropriate cleaning will usually give very satisfactory results, allowing for the passage of time. At lower levels some evidence of wear is often quite acceptable as long as it is not disfiguring. If local damage has occurred in a room, perhaps from water ingress, it can usually be repaired and toned to blend with the original. This is obviously better than renewing all the gilding or, as sometimes happens for economic reasons, painting it out altogether.
In historic buildings gold leaf was always expensive whether applied to furniture or decoration, but nothing else has quite the same reflective quality and glister about it. With appropriate care it will outlast any substitute.
Gilding is literally skin deep, so maintenance is mainly a list of things to avoid: regular cleaning - especially with spray polish and a yellow duster! Also damp, excessive beat, everyday wear and tear, abrasion when dusting and any kind of gold substitute applied to the surface.
It may be safeguarded simply by keeping it dry, out of harm's way and dusted occasionally and lightly with a feather duster. If any damage or defects become apparent professional help should be sought, to preserve both the gilding and the value of the gilded object.
Information reproduced from The Building Conservation Directory, 1999
Bucks Gold Leaf