The Medium That Allowed Second Thoughts

Oil paint is pigment ground in a drying oil, usually linseed. It was known in northern Europe well before Jan van Eyck, who refined rather than invented it, and its adoption took a century and moved at different speeds in different places. Plenty of fifteenth-century pictures are both media at once: a crisp tempera underlayer with oil glazes above, which is a practical compromise rather than a stage on the way to something.

Two properties made it the dominant medium in Western painting for five hundred years.

The first is optical. Oil dries to a transparent film, so colour can be built in layers. An opaque underpainting establishes the light and dark; transparent glazes go over it; light passes down through the glaze, reflects off the layer beneath and comes back out through the colour. The result has a depth that mixing on a palette cannot produce, because the colour is made inside the paint rather than at its surface. It is also why glazed passages shift as you move in front of them, and why reproductions of Titian never quite convince.

The second property is temporal, and it matters more.

Oil dries slowly. Slowly enough to blend wet into wet across an afternoon, and slowly enough that a painting can be worked on for months. Fresco requires the entire design to be settled before the plaster goes up. Oil permits the painter to argue with the picture.

The evidence of those arguments is still there. An arm in an earlier position, a head turned differently, a figure painted out entirely: pentimenti, from the Italian for repentance. Paint becomes more transparent as its oil ages, so covered passages rise slowly back into visibility, and infrared reflectography and X-radiography find the rest. Some famous pictures turn out to have been substantially different paintings first.

This has a forensic use. Somebody copying a finished image has nothing to reconsider, so a canvas full of second thoughts usually indicates invention rather than reproduction. It is evidence rather than proof (a copyist might work from an unfinished model, and a forger who has read the literature can add changes on purpose) but in attribution disputes it carries real weight.

The deeper point is about what kind of thing a painting is. When the medium permits revision, a picture stops being the execution of a plan and becomes a process of discovery, and the idea of the artist as an inventor rather than a contractor grows up alongside it. That shift is usually told as a story about ideas. It is at least as much a story about drying times.

Not everybody wanted the layers. Painting alla prima (wet into wet, finished in a single session) trades glazed depth for immediacy, and the handling stays visible as touch. Frans Hals worked that way in the seventeenth century, and by the nineteenth it was the default for anyone painting outdoors. Academic critics called it unfinished, which it is, and which is the point.

Painting outdoors, however, was barely possible until 1841.

Before then, ground colour was stored in pigs' bladders tied at the neck. You pierced one with a tack to get paint out and plugged the hole afterwards, badly. Bladders dried out, burst in bags, and could not be carried far. Painters made sketches outdoors and finished the picture in the studio, which is why pre-Impressionist landscape has the quality of a remembered scene.

John Goffe Rand, an American portrait painter working in London, patented the collapsible metal tube in 1841. It sealed, it travelled, and it kept paint usable for months. Within a few years the colourmen were selling ready-ground oil colour in tubes, including the new industrial pigments (chrome yellow, cobalt, cadmium, viridian) in intensities that had not existed before.

Renoir put it flatly: without paint in tubes there would have been no Impressionism. It is worth taking literally. A style built on going to the motif, working fast in changing light and finishing on the spot is not available to a painter carrying bladders. The railway took them to the coast, the tube let them work when they got there, and the chemical industry supplied the colours they worked with.