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 that had been so carefully applied to the ground. 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 final layers of varnish themselves, or advised their patrons and clients to do so.

Three types of varnish have come into common use:

• Drying oil and resin, in effect a resin-rich transparent and unpigmented paint layer, that usually becomes an integral part of that. Some artists have added pigment, perhaps to make a general colour adjustment. There isn’t any clear distinction between that and a final paint glaze.
• Solvent and resin, from 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.

Resins used in varnishes have rich and sometimes strange histories. Most are exudates from trees in exotic locations, and have evocative names like mastic, sandarac, colophony and dammar. They’re usually highly insoluble, either in drying medium that has to be heated to make oil-based varnishes, or in turpentine or similar organic solvents. A great many recipes have been 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.

Any work older than a few decades that has been varnished or had any form of surface treatment is unlikely to appear today with the colours the artist intended. Multiple layers of old varnish and trapped dirt give a misleading impression of what we would have seen soon after the work was completed. Painstaking work by conservation specialists can often restore old paintings to what we presume is their former glory, in full colour again.

Hieronymus Bosch’s Christ Carrying the Cross (1490-1510) is seen above before recent conservation work, and below is the result of thousands of hours of painstaking cleaning and treatment.

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 work is that it was originally far from being so dark. Sadly it’s now 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.

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 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 that were far from dry. In the case of The Opening of the Wallhalla, 1842 (1843), 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 don’t necessarily cause serious ill-effects. But Turner has used Megilp to excess, to produce a soft impasto used in the foreground figures, in particular. This has 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, required for drying oils to polymerise 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 (turn to soap) 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 cleaned periodically 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 also cause colour shifts. 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.

Varnishing Day in the Paris Salon was completely 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 they chose to apply the varnish in person.

Since ancient times people have written, drawn and painted using pigments and dyes in water, often without any binder to adhere them to the ground. These are generically termed inks, and distinct from other media such as watercolours or the temperas. Most common among them is India ink, whose essential ingredients are carbon particles from soot, suspended in water. When applied to a suitably absorbent ground that has stood the test of centuries.

Hieronymus Bosch’s study of The Owl’s Nest from around 1505-1516 is a good example of a pen and ink drawing starting to transcend into painting.

Washes applied using a brush became common in sketches made in front of the motif, such as Egbert van der Poel’s Fire in the Nieuwe Kerk, Amsterdam, in 1645. Known for his paintings of fires, he used these sketches to paint his famous brandjes in the studio. Landscape artists such as Nicolas Poussin and Claude Lorrain used the same sketching media working en plein air to build image libraries for the idealised landscapes used in their finished paintings.

By the early nineteenth century, artists like Samuel Palmer were creating works that are to all intents and purposes full-blown paintings using black and coloured inks, such as this atmospheric nocturne of Cornfield and Church by Moonlight from about 1830.

Used without a binder, inks normally rewet and smudge or blur, so have to be applied after any watercolour. Adding a binder such as shellac to India ink makes it waterproof when it has dried, and this was used in combination with watercolours by illustrators and painters such as the great Arthur Rackham, here in one of his illustrations for an edition of Charles Dickens’ A Christmas Carol.

A related medium that also crossed over from drawings and illustrations into paintings is casein, a protein-based tempera. It too has a long history, but didn’t become popular among artists until the late nineteenth century. Casein is a protein originally obtained from sour milk, but by the nineteenth century it was more usually prepared by the addition of rennet, enzymes used in cheese manufacture, extracted from the stomachs of calves.

Casein powder is then turned into paint by dispersion in an alkaline solution, typically made from lime or borax, and pigment is ground in. When lime is used the paint works best on porous grounds, but must be used fresh. When a little linseed oil is added to borax casein, a shelf-life of several months or more is possible. Casein paints dry as quickly as egg tempera, but can be reworked for a period until their binder has fully hardened. Once that has taken place, their paint film is robust and stable.

The earliest painting I have seen that is known to have been made using casein paints is William Morris Hunt’s unusual view of Niagara from 1879. Hunt deviated here from his customary use of oils, although he still applied his paint to a prepared canvas support.

One of the most famous artists to have used casein paint is Gustav Klimt. In 1902, he used it in a frieze of 24 metres (nearly 80 feet) length for the fourteenth exhibition of the Vienna Secession, his Beethoven Frieze, of which that above is a section known as The Hostile Powers, and that below is Nagging Grief. He applied his casein paint directly onto mortar with added stucco, gold leaf, and other materials. This is known to be a robust form of wall-painting that is much more accommodating than fresco, and just as enduring, but has never become popular.

A few of Colin Campbell Cooper’s skyscraper cityscapes were painted using casein media, including this view of Flatiron Building, Manhattan from about 1908. He was equally proficient in watercolour and oils, and I don’t know why he experimented with casein, nor why he used it so infrequently.

Another Vienna-based artist, Albin Egger-Lienz, also used casein on occasion, here in this fourth of several different versions of Totentanz (Dance of Death) from 1915.

During the twentieth century, casein paints didn’t become popular but continued in use, particularly by those who also made illustrations. Just when they were attracting a following among artists such as Andy Warhol, they were rapidly displaced by acrylics. Despite that, casein paint is still available today, sold by Pelikan under the brand name of Plaka.