Artists are often instructed to paint what they see, and not what they think they can see, as a key step in realist painting. Understanding the science of surface optical properties isn’t required to depict the world realistically, just the combination of good observation, appropriate media and skilful technique. For many centuries, this knowledge and skill was passed on by masters to their apprentices, then with the rise of academies, they were taught.
The decisive step for European painting was the adoption of suitable media.
The skilled craftsman-artist who painted The Wilton Diptych in egg tempera and gold leaf in about 1395-99 worked for hundreds of hours to distinguish flesh from hair from clothing, but you can still see the fine brushstrokes giving the skin a slightly unreal texture. This is because egg tempera paint dries too quickly to depict it completely smoothly.
Michelangelo’s Virgin and Child with Saint John and Angels, popularly known as The Manchester Madonna, from about 1497, shows his skills in modelling flesh and fabrics using egg tempera, but even here the medium makes the expression of surface texture very difficult.
Michelangelo switched to painting in oils, shown here just a few years later in another uncompleted painting of The Entombment (c 1500-01), which made all the difference.
The advantages of oil paint in the depiction of surface texture include:
- slow drying, enabling the use of wet-on-wet as well as wet-on-dry techniques;
- a wide range of paint viscosity, enabling different application from thin glazes to thick impasto;
- good paint polymerisation to form a robust paint layer;
- a wide range of pigments, many of which are lightfast and durable;
- little or no colour shifts with drying.
Even after nearly ninety years of intensive industrial chemical development, acrylic paints are still unable to match the properties of the oil paints used with great success six hundred years ago.
This paint section cut by Elizabeth Steele from a painting by Honoré Daumier made on wood panel shows layers of pigment-rich oil paint over a relatively thick white layer of ground. The craftsmanship of oil painting lies in knowing how to construct the paint layer, so as to convince the brain of the viewer of different surface textures. It was this that was developed during the Renaissance, and passed on from master to apprentice.
Jan van Eyck and other masters of the Northern Renaissance acquired these skills early, here in about 1435, as oil paint had been developed over the previous couple of centuries in northern Europe. He captures perfectly the different textures of Chancellor Rolin’s face and hands, those of his clothing, and the stone wall behind.
When oil paint made its way to Italy, it was adopted by masters like Leonardo da Vinci, who developed techniques further. Leonardo used multiple thin glaze layers consisting largely of drying oil with just a little pigment, to develop the subtle shadows of the flesh. This was so admired by other masters of the day that it was named sfumato, Leonardo’s smoke, for its subtlety.
Raphael moved on to the depiction of other types of surface, ranging here from bright reflected light on glass and metal, to delicate fur trimming on the clothing of Pope Leo X. At this stage, most of the effects were accomplished using fine and precise application of details in paint.
Later in the sixteenth century, painters in Italy like Veronese discovered how they could ‘fool’ the viewer with more painterly techniques, in these brisk strokes of white paint almost daubed onto the red cloak.
Rembrandt and Rubens developed control of paint viscosity so that encrusted detail of metal and jewels would stand out.
Rembrandt used coarse marks with impasto and extensive gestural highlights to impart surface texture, here in the skin, clothes and jewellery of The Jewish Bride from about 1667.
Best practices in oil painting were secrets passed on in the master’s teaching, and remarkably few were recorded in books. The scientific basis for these practices wasn’t established until well into the twentieth century, so it’s unsurprising that some artists either never learned or deliberately ignored those best practices.
One example is what’s now known as the fat over lean rule, intended to ensure that the paint layer dries from bottom upwards. This is important as oil paint normally shrinks slightly as it dries. This process of drying doesn’t involve any loss of liquid, but is the chemical formation of a polymer, requiring oxygen from the atmosphere. If upper layers of paint dry by polymerisation before lower layers, then those lower layers may not polymerise properly, and in the presence of even small amounts of water can saponify instead, turning to something resembling soap, which never dries, leaving the whole paint layer unstable and weak. In worst cases, cracks open in drier surface layers, and liquid paint from lower layers exudes onto the surface.
The fat over lean rule was observed by the masters of the Renaissance, both Northern and Southern, and by most until the late eighteenth century. When artists like Sir Joshua Reynolds sought to discover the ‘secrets’ of Rembrandt’s techniques, it was ignored, and he used resin additives that slowed drying in thick lower layers.
Fortunately, the rise of academic training by the nineteenth century restored better practice, for example in the paintings of James Tissot, who studied at the École des Beaux-Arts in Paris. Further progress was made in pushing the boundaries of what the viewer’s brain perceived.
While oil paint has proved the most enduringly suitable for the depiction of surface textures in European art, there are alternatives. In the right hands, watercolour can be its equal, but historically its main competitor have been pastels.
A succession of masters in pastels, including Jean-Etienne Liotard, have demonstrated this, although pastel paintings are fragile and lack the robust paint layer of oil paints.