How Do You Cover Something in Gold? Beating Metal Thinner Than Anything

Gold is the most malleable metal known, meaning it can be deformed further without fracturing than anything else, and skilled beating reduces it to leaf around a ten thousandth of a millimetre thick, thin enough to transmit a greenish light when held up. A single gram covers something close to half a square metre. It also does not tarnish, since it reacts with almost nothing, so a gilded surface applied centuries ago looks as it did when new provided the material beneath has held. Those two properties together explain the whole practice, since a durable, non-tarnishing decorative surface that uses a negligible quantity of an expensive metal is an attractive proposition for anyone decorating a building, a manuscript or a frame.

Two main methods exist and they produce different results:

• •Water gilding, laying leaf onto a prepared ground of gesso and clay wetted with size, which can be burnished to a mirror finish
• •Oil gilding, laying leaf onto a slow-drying oil-based adhesive, which is more durable outdoors and cannot be burnished
• •The ground beneath is what produces the finish, since the leaf is too thin to hide anything
• •Leaf is handled with a flat brush charged with static rather than with fingers
• •Loose leaf is used by experienced gilders and transfer leaf on a paper backing by everyone else
• •Excess is brushed away and collected, since even the scraps are worth recovering

Producing the leaf is a separate trade with a long history. Gold is alloyed slightly for colour and workability, cast, rolled into a ribbon and cut into squares, and then beaten in packs interleaved with a material that will not stick, historically a membrane taken from cattle intestine and now synthetic film. The pack is beaten, the enlarged sheets cut into quarters, repacked and beaten again, through several cycles lasting hours, with the final stage requiring the greatest skill because a sheet that thin tears at nothing. The work was traditionally done by hand with hammers weighing several kilograms and is now partly mechanised, though hand-beaten leaf is still produced and preferred for some applications. Leaf comes in books of small squares interleaved with tissue.

Cheaper materials imitating the effect have their own long history and their own problems. Imitation leaf made from a copper and zinc alloy is sold by the book at a small fraction of the price and looks convincing when fresh, and it tarnishes, so it must be sealed with a lacquer that changes the surface and eventually yellows. Silver leaf tarnishes badly and is usually sealed or used deliberately for the darkened effect. Aluminium leaf substitutes for silver without tarnishing. Powdered metal in a binder produces gold paint, which never matches leaf because the particles scatter light in every direction rather than presenting a continuous reflective surface, which is why gold paint always reads as paint. Restorers can identify which material was used by examining a tiny sample, which matters for dating and attributing objects.

The applications span an enormous range of periods and purposes. Illuminated manuscripts used it on raised gesso grounds so that the gold stands proud and catches light, which is why such pages are luminous in a way reproductions never convey. Icon painting and altarpieces used it as a ground representing divine light rather than as decoration. Architectural gilding covers domes, spires, railings and lettering, and outdoor work is expected to last decades. Book edges were gilded to seal against dust. Picture frames are the commonest surviving domestic use. Food-grade leaf is edible and passes through unchanged, being chemically inert. And modern uses include spacecraft thermal shielding and coatings on optical instruments, where the same non-reactivity and reflectivity are what matter.

Gold is the most malleable metal and beats to about a ten thousandth of a millimetre, so a gram covers roughly half a square metre and does not tarnish. Water gilding over clay can be burnished to a mirror, while oil gilding is more durable outdoors. The leaf is too thin to hide anything, so the surface beneath determines the finish.