Gilding is the art of applying metal-leaf to a surface. Techniques were developed thousands of years ago to gild wood, metals and other materials. Because of the comparative thickness of the gold leaf used in ancient gilding, the traces that remain are still remarkably brilliant. All through history gilding has occupied an important place in the ornamental arts of countries around the world; and the techniques practiced today may be taken as typical of the arts as practised from the earliest periods.

                  Metals that are made into leaf need to be highly malleable. They can be pounded into sheets well below a micrometre in thickness without breaking or tearing. Today, the typical thickness of gold leaf is about 100 nanometres or 0.0001 mm. When made by hand, small pieces of annealed metal are placed between sheets of parchment and pounded repeatedly with wooden mallets. As the metal thins out, it forms large sheets. These sheets are divided and the process repeated. The final sheets of metal are trimmed, cut to various sizes, and sandwiched between sheets of paper to protect them.

At a thickness of 100 nm one square metre of gold leaf corresponds to 0.1 cubic centimetre or less than 2 grams of gold. One ounce of gold would cover at least 14 square metres.         

                   Oil gilding is a very traditional process. After the substrate surface is prepared by cleaning, sanding, sealing, priming, and coating with an impervious lacquer or enamel, the ground is ready for a "size" of linseed and other oils similar to varnish to be applied. The size cures until it comes to tack (that is, it has dried enough to hold the leaf on to the surface but is still "tacky" enough for the leaf to adhere to it). This can vary between a 3, 12 or 24 hour size. The larger the area being gilded, the longer “open time” needed to complete the transfer of gold.

The leaf is cut to the appropriate size with a gilder’s knife on a pad of calf’s leather and transferred with a tip (a very soft squirrel hair brush) and static electricity (which picks up the gold). The surface is then skewed with soft brushes (a gilder’s mop of squirrel hair), cotton wool or sable hair to fully press the leaf against the size and also remove any loose fragments of leaf. Oil gilded surfaces are not burnished any further but a final polishing removes all traces of gold dust fully revealing the beauty and lustre.

Any draughts or movement of air including heavy breathing and the gold leaf is likely to fly off the pad! Outdoor projects such as the domes of buildings and large statues require a temporary structure around them to eliminate any wind as well as control the temperature and humidity.

 

                  Many different materials can be gilded including concrete, wood, brass and copper, glass, ceramic, plaster and various synthetics. The techniques are generally the same and the gilt can last for generations if not millennia but only as long as the preparation is done properly. Domes outside need re-gilding every few decades or so due to damage by the elements and birds, etc. Sculptures and especially picture frames need to be handled with great care or the gilt distresses and wears off. Some pieces are varnished to seal and protect the finish and can last for a very long time.

 

                  Metal Leaf is available from specialist art suppliers in booklets of 25 leaves, 80 or 85 mm square, and either loose or attached to a backing paper for easy transfer. While pure gold is yellow in color, gold can also appear to have other colors. These colors are obtained by alloying gold with other elements in various proportions. For example, alloys which are mixed 14 parts gold to 10 parts alloy create 14 karat gold, 18 parts gold to 6 parts alloy creates 18 karat, and so on. This is often expressed as the result of the ratio, ie: 14/24 equals 0.585 (rounded off), and 18/24 is 0.750. There are hundreds of possible alloys and mixtures, but in general the addition of silver will color gold green, and the addition of copper will color it red. A mix of around 50/50 copper and silver gives the range of yellow gold alloys the public is accustomed to seeing in the marketplace.

 

           

Different colors of Ag-Au-Cu alloys

White gold is an alloy of gold and at least one white metal, usually zinc, nickel or palladium. Like yellow gold, the purity of white gold is given in karats.

 

Rose gold is a gold and copper alloy widely used for specialized jewellery due to its reddish color. It is also known as pink gold, red gold and Crown Gold. Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content–the higher the copper content, the stronger the red coloration, as pure gold is yellow and pure copper is reddish. A common alloy for rose gold is 75% gold and 25% copper by mass (18 karat). Since rose gold is an alloy, there is therefore no such thing as "pure rose gold". The highest karat version of rose gold is also known as crown gold, which is 23 karat.

 

Green gold alloys are made by leaving the copper out of the alloy mixture, and just using gold and silver. It actually appears as a greenish yellow, rather than as green. Eighteen karat green gold would therefore contain a mix of gold 75% and silver 25%.

 

Grey gold alloys are made by adding silver, manganese, copper or palladium in specific ratios to the gold.

 

Purple gold (also called amethyst or violet gold) is an alloy of gold and aluminium. Gold content is around 79% and can therefore be referred to as 18 karat gold. Blue gold is similarly an alloy, this time between gold and indium. However, both are highly brittle as alloys therefore cannot be manufactured into goldleaf but can be cast or fabricated into jewellery.