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.

Sirens are mythical woman-like creatures with alluring voices, best-known from their appearance in Homer’s Odyssey, but also featuring in other tales including that of Jason and the Argonauts. Typically their singing lures sailors to their death, and that reputation has led them to represent anything that’s dangerously attractive. Originally they weren’t described in any physical detail, but visual representations soon envisaged them as having the upper bodies of beautiful young women, and the lower bodies and legs of birds, and that has been incorporated and elaborated in later accounts and retelling.

At the end of the year that Odysseus and his crew stayed with the sorceress Circe, she helpfully advised him that he would have to sail past the sirens, two to five creatures who lured men to their death with their singing. In preparation, Odysseus got his sailors to plug their ears with beeswax before they reached the sirens, so they couldn’t hear their song, and to bind him to the mast. He gave them strict instructions that under no circumstances, no matter what he said at the time, were they to loosen his bonds, as he would be listening to the sirens’ song.

As the group reached the sirens, Odysseus told his men to release him, but instead they bound him even more closely to the mast. Once they had passed safely from earshot of the sirens, Odysseus used his facial expression to inform his men, who then released him, and they sailed on.

William Etty’s The Sirens and Ulysses from about 1837 is one of the pioneering accounts in paint of this story from the Odyssey. His three naked sirens are all woman, one playing a lyre, another holding double pipes aloft, all three doing their best to draw the sailors from Odysseus’ ship to a shore where there are the remains of earlier victims.

Edward Poynter’s The Siren from about 1864 has Aesthetic overtones in the lyre she is playing.

Arnold Böcklin takes an unusual approach of almost dereferencing Odysseus in his painting of Sirens from 1875, although there is an approaching vessel that could be his. The two sirens filling the canvas are very human down to the waist, below which they resemble birds. One sits facing us, clearly in full voice, and highly alluring in looks. The other, her back towards us, is playing an aulos and looks rather obese, to the point of almost being comical, her right breast laid upon a flat-topped rock. At their feet are three human skulls and other bones to indicate their graver intentions.

Gustave Moreau’s The Sirens (1882) shows them as beautiful figures in a static scene, with a saturnine setting sun. There is, though, a lone sail on the horizon that hasn’t yet attracted their attention. Their lower legs turn into the writhing coils of sea serpents.

Moreau’s slightly later group portrait of The Sirens from about 1885 is more complete, with Odysseus sailing past, but its three figures are clearly all woman and no bird.

Eight Dancing Women with Bird Bodies (1886) is one of Hans Thoma’s unusual mythological paintings. The best-known women with bird bodies were the sirens, who range in number from two to five. In another painting showing the sirens trying to lure a passing ship, Thoma paints similar figures, suggesting these are intended to be sirens.

John William Waterhouse’s Ulysses and the Sirens (1891) is closer to the Homeric account, although he provides a total of seven sirens, shown as large eagle-like birds of prey with only the head and neck of beautiful women. He has added bandage wrappings around the head of each sailor to make it clear that their ears are stopped from hearing sound, a visual device that links neatly with the text. His sirens are clearly singing, particularly the one closest to the viewer, who is challenging the hearing protection of one of the sailors. Another sailor, at the stern of the ship (left of the painting), is seen clutching his ears.

Almost a decade later, Waterhouse painted this non-narrative portrait of The Siren (1900).

The Sirens (1903) marked Henrietta Rae’s return to painting narrative works featuring classical nudes. Odysseus’ ship is in the distance, as three beautiful sirens use their aulos and lyre to lure its occupants.

Late mythology suggests an unpleasant end for these sirens: Hera challenged them to a singing contest against the Muses. When the latter won, the penalty they exacted of the sirens was to have all their feathers plucked out to turn into crowns. As a result of that disgrace, the sirens turned white, fell into the sea, and formed the islands including modern Souda, on the north-west coast of Crete in the Mediterranean.

Sirens have steadily spread their presence into other paintings, particularly during the twentieth century.

Georg Janny’s fantasy painting of Sirens Bathing by the Sea from 1922 is throughly other-worldly, and there’s no trace of their bird legs.

More cryptic is Paul Nash’s Surrealist Nest of the Siren (1930), which brings together the incongruous, and hardly refers to Homer’s story. The painting is framed by brightly-painted walls with pillared decorations, perhaps ornate wainscot panelling. In the middle of these is what might be a painting, but also seems to be a three-dimensional plant trough containing sinuous shrubs. In the middle of those is a small nest, like an acorn cup.

Standing in front of this is a structure resembling a weather-vane, mounted on a turned wooden shaft. At the weather end of the vane is the faceless figure of a siren; the leeward end appears purely decorative. Three red rods appear to have detached themselves from the walling, two protruding from the plant trough, the third resting on the floor.

They even manage to sneak symbolically into other classical stories.

In Girodet’s ink and chalk drawing of The Meeting of Orestes and Hermione (c 1800), Hermione is seen at the right, her arms folded, looking coy as Orestes approaches her. The second woman, with Orestes, is presumably Hermione’s maid. This is one of a series of illustrations made by Girodet to accompany Racine’s play, and has subtleties you might expect from a great narrative artist. Visible in the gap between the figures is a table-leg in the form not of a Fury foretelling Orestes’ fate, but of a siren, implying that Hermione is luring Orestes to her. Hermione, for all her apparent coyness, has let the right shoulder-strap of her robe slip, in her enticement of Orestes. She has assumed the role of femme fatale, as portrayed by Euripides and Racine.

In more recent literature, sirens appear in the less-known second part of Goethe’s play Faust.

Margret Hofheinz-Döring is one of the few artists who has painted from this second part. With the Sirens from 1962 is her pastel painting showing the sirens among rocky inlets of the Aegean Sea, a sub-scene concluding the second act.