On the face of it, watercolour painting should be the simplest of all the more popular painting media. The paper is both support and ground, the paint layer is thin, with a little binder of gum arabic and a diluent of plain water. Although some have tried using varnish and other protective layers, in the great majority of watercolour paintings, that’s all there is.

In practice, though, as anyone who has taken watercolour painting seriously knows, there’s a great deal more to it. Much of that is hard to see and can only be inferred from images of watercolour paintings.

Raphael’s The Miraculous Draft of Fishes (c 1515-16) is among the earliest large paintings made using opaque watercolour. Because of limitations in the manufacture of paper at the time, it was made on many sheets that have been mounted on canvas. At around 3 x 4 metres (10.5 x 13 feet), it’s huge by any standards, and even today would probably need to be painted on multiple sheets and mounted on added support.

Over three centuries later, the Pre-Raphaelite artist Marie Spartali Stillman painted her watercolours in layers, using a combination of transparent and opaque watercolour on paper that had been hot-pressed to give it a satin-smooth finish. When re-wetted with another layer of wet paint, watercolour usually becomes liquid again, resulting in the layers mixing. Great care and skill is required to apply wet watercolour on dry paint in the way that she did. Even then some paintings simply don’t work, the layers mix and that attempt has to be discarded. When successful, the end result can resemble layered oil paints, which don’t suffer the same problem when dry paint is overpainted.

The Childhood of Saint Cecily (1883) was one of the last paintings Stillman composed when she lived in Florence, as reflected in its idealised Tuscan background. As with so many paintings of saints, it departs from even the most inventive of hagiographies, here for the patron saint of music, Cecilia. The saint is shown playing a harp-like psaltery, while an angel adjusts the garland on her head. She has worked subtle patterns into the blue fabric on the arm of the angel, and in the side of the instrument. Flowers and hair have fine detail with sharp highlights, just as might be seen with oils.

Love’s Messenger (1885) is probably the finest of her single-figure paintings, and was her most successful ‘problem picture’. The woman stands by her embroidery at an outside window. On her right hand is a messenger dove/pigeon, to which a letter is attached. She clutches that letter to her breast with her left hand, implying that its contents relate to matters of the heart. The dove is being fed corn, which could either be its reward for having reached its destination (thus the woman is the recipient of the message), or preparation for its departure (she is the sender). Despite being exhibited at the Grosvenor Gallery in 1885, and elsewhere, this painting didn’t sell until after she had reworked the background in the 1890s, which must have been a feat in itself.

A detail view of Love’s Messenger shows how her watercolour technique results in a facture more closely resembling that of oils, although this painting is an extreme example with its use of tempera too. This was accomplished by using transparent watercolours more like oil glazes, and gouache (opaque watercolour) for details.

Her portrait of Beatrice (1896) refers to Dante’s Vita Nuova, through Dante Gabriel Rossetti’s popular translation into English. In this her first version, she shows Dante’s beloved Beatrice lost in contemplation while reading, an intimate insight set firmly in the Pre-Raphaelite mediaeval. This was exhibited at the New Gallery later that year.

This detail view of Beatrice provides more insight into her exacting technique.

Over the same period, other artists were developing techniques for use on cold-pressed paper with its coarser texture.

At a time when most watercolour painting was still disparagingly considered to be ‘drawing’ for routine topographic views, Alexander Cozens developed specialist techniques, such as keeping ‘reserved space’ to let his white paper ground show through, wet on wet as well as wet on dry application of paint, and scratching out. He also employed both transparent and opaque paints for different effects.

JMW Turner did a great deal to advance both technique and the critical reception of large watercolour works painted in the studio, although even today his oils are much better known, with a few exceptions such as his sublime paintings of the Rigi.

Towards the end of the nineteenth century, Winslow Homer earned his place as one of the greatest watercolour painters of America during a period spent in a fishing community in north-east England. The advanced techniques which he used are shown well in The Watcher, Tynemouth (1882), and include both transparent and opaque paints, rewetting and blotting to remove paint for highlights, scraping, application of wax to resist the adherence of paint, and the use of pure gum solution as a glaze. In this case, his paper is only lightly textured, though.

Perhaps the greatest exponent of these techniques was another American, John Singer Sargent.

Sargent’s most radical watercolours were painted when he stayed with various friends in the Bellevue Hotel, at the top of the Simplon Pass, in the summers before the First World War. While his family and friends whiled away their days in leisure, Sargent got them to pose for a unique series of informal portraits. They may have been reclining at leisure, but Sargent took those watercolours very seriously, and deployed an amazing array of techniques. Among the finest is his Simplon Pass: The Tease from the summer of 1911.

One of his most unusual techniques, used extensively here, is wax resist. Before applying paint, Sargent scribbled over areas intended to be vegetation, using a soft wax crayon, probably made from beeswax. On fairly rough paper, that wax is deposited unevenly, and when painted over using watercolour it shows the white paper through. This creates disruptive patterns of near-white in the midst of the greens, and a superb textural effect, as shown in this detail.

In complete contrast, Edward Robert Hughes’ Night with her Train of Stars of the following year has been painted in a controlled and meticulous manner, although it too uses advanced techniques, particularly in the sky and background. I suspect that some of the latter may have relied on the scattering of salt grains to impart its fine texture. In other passages he has allowed different colours to rewet and blur what would otherwise be sharp edges.

Of course, AI can now mimic these when provided with suitable original material to plagiarise. But there’s still nothing like an original painted by a master.