Long before paintings became movable objects of great value used by the rich as investments, artists and the owners of their paintings wanted to protect the paint layer which had been so carefully applied to the ground and its support. From the early Middle Ages onwards, one popular means of doing this has been to apply some form of protective layer, a varnish.

Varnishes have been widely used not only for protection. Careful choice of their composition can enhance the appearance of a painting, through the optical properties of the varnish medium and its smooth, glossy surface. Until the late nineteenth century, the great majority of painters either applied a final layer (or more) of varnish themselves, or advised their patrons and clients to do so.

Three main types of varnish have come into common use:

• Drying oil and resin, which is in effect a resin-rich transparent and unpigmented paint layer, and usually becomes an integral part of it. Some artists have added pigment, perhaps to attempt to make a general colour correction. There isn’t really any clear distinction between that and a final paint glaze.
• Solvent and resin, in which the solvent will evaporate, leaving a thin surface coat of resin.
• Water-based washes such as egg white, known as glair, vegetable gums like gum arabic, and animal glues.

The resins used in varnishes have rich and sometimes strange histories. Most are exudates from trees obtained from exotic locations, and have evocative names like mastic, sandarac, colophony and dammar. They’re usually very insoluble, either in drying medium which has to be heated to make oil-based varnishes, or in turpentine or similar organic solvents. There have been a great many recipes proposed, and there’s always the lure of the perfect, and inevitably top secret, formula.

The biggest problems with varnishes are their propensity to yellow or grey with age, and their tendency to take up dirt and atmospheric contaminants. Rembrandt’s first painting of Bathsheba at her Toilet from 1643 has sadly lost much of its detail into the gloom of old varnish, which can be almost impossible to clean off when composed of drying oil and resin, without damaging the paint layer underneath.

Another problem for the conservation specialist is a painting like Edward Poynter’s A Visit to Aesculapius from 1880. Although this is little more than a century old, the evidence from contemporary prints made from this painting is that it was originally far from being so dark. Sadly it now seems almost impossible to read as a result of its near-black shadows.

A good varnish should be both colourless and transparent, but painters haven’t always respected that principle.

When finishing his monumental Raft of the Medusa in 1819, Théodore Géricault is thought to have applied glazes or varnish containing asphalt, to give the painting a deep brown tone. Asphalt is not only completely unprotective and almost attracts dirt, but it never fully dries, and can have adverse effects on underlying paint too. It hasn’t helped that this (exactly) two hundred year-old painting was rolled up and stored in a friend’s studio when it remained unsold, and was then transported to London still rolled up the following year.

Conventional wisdom says that it’s best to leave an oil painting to dry for at least six months before varnishing it. JMW Turner sometimes varnished over paint layers which were far from dry. In the case of The Opening of the Wallhalla, 1842 (1843), which was painted on mahogany, Ruskin reported that it had “cracked before it had been eight days in the Academy Rooms”, although this overall view shows little evidence of that damage.

Hellen and Townsend attribute this to Turner’s extensive use of Megilp, here a product sold by his colourman containing leaded drying oil and mastic varnish. Used sparingly and with great caution, such medium modifiers do not necessarily cause serious ill-effects. But Turner has used Megilp to excess, to produce a soft impasto used in the foreground figures, in particular. These have resulted in wide and shallow drying cracks, as the surface has dried quickly and shrunk over trapped layers of liquid paint.

Varnishes do provide mechanical protection to the paint layer, but at the cost of locking out atmospheric oxygen, which is required for drying oils to polymerize properly in their drying process. Applied too early, varnishes can therefore greatly slow drying of underlying paint layers; the danger is that they may saponify instead of drying normally.

Despite these dangers, varnishes can, when used with care by those who understand them properly, be valuable beyond simply providing a protective coat. Kirsty Whiten’s The Quing of the Now People (2015) achieves its superbly realistic effect by the skilful combination of conventional oil paint with varnish.

In the late nineteenth century, attitudes to varnishing oil paintings changed markedly, as Impressionists like Camille Pissarro started to prescribe that their works should on no account be varnished. This was to preserve the soft matte surface of the paint as applied by the artist, and became increasingly popular in the twentieth century.

For such paintings, protection can be provided by glass, when necessary. That isn’t of course an option for many extremely large oil paintings on canvas, which will probably need to be varnished and periodically cleaned well in the future, as they have in the past.

Varnishes, usually of the third type containing vegetable gums or animal glues, have also been used extensively on paint layers other than oils.

These are reported in Samuel Palmer’s Tintern Abbey at Sunset, above, and Dante Gabriel Rossetti’s Lucrezia Borgia, below. Gum or glue varnishes can have impressive optical effects when used carefully on watercolours.

Unfortunately, their tendency to yellow can cause colour shifts in paintings too. William Blake liked to apply glue varnish to his watercolours and perhaps to his glue tempera paintings as well. In the case of his Penance of Jane Shore in St Paul’s Church from about 1793, this has resulted in a generalised yellow shift and loss of chroma.

Other artists appear to have been more successful: Henry Ossawa Tanner apparently applied varnish to this tempera painting of The Destruction of Sodom and Gomorrah almost a century ago, and it doesn’t appear to have suffered any adverse consequences, yet.

Varnishing has become such an accepted process that major exhibitions have incorporated ‘varnishing days’, although what happens on those occasions can be quite different. In Turner’s day at the Royal Academy in London, Varnishing Day was an occasion for artists to make any last-minute changes, and Turner himself seems to have turned up armed with paint and brushes and continued to work on his paintings then.

Varnishing Day in the Paris Salon was very different, attended normally by the artists’ colourmen, who applied a coat of varnish to the paintings for which they were responsible. The artists themselves don’t seem to have been involved, unless performing the varnishing themselves.