Watching Paint “Dry”
Watercolour and acrylic paints have water as part of their medium — they dry by evaporation. But oil paints don’t. They dry by what’s called a siccative quality. That is they absorb oxygen from the air. This has the undescriptive definition of:
(Chemistry / Elements & Compounds) a substance added to a liquid to promote drying: used in paints and some medicines
[from Late Latin siccātīvus, from Latin siccāre to dry up, from siccus dry]
Essentially, oils have a rate of autooxidation from the air, they absorb oxygen and harden. I’ve often described this as putting Jell-o into an enclosed container and adding tons of pineapple chunks to it: the oil is the Jell-o and the air is the pineapple — you can only add so much to the enclosed bowl and it will stop jiggling. Perhaps I haven’t got this analogy quite right. But now I want Jell-o.
As oils harden, there’s an interesting problem: oxygen is absorbed through the paint surface, meaning if the paint is very thick, you can see a different “drying” rate on the paint’s film than on the first layer’s applied on the canvas. The surface could be hard and the oils underneath still squishy like yummy lemon Jell-o. (Warning: oils processed as art supplies are not cleared for human consumption.)
Fat Over Lean
One of the main appealing properties of oil painting are the glazes. By adding a small amount of pigment to the relatively clear oil medium, you can very subtly tint an image. This is called glazing. Most Renaissance Old Masters (think the Ninja Turtles and their peeps — Artemisia Gentileschi not April O’Neil) used a toned underpainting and then built up several of these thin glazes of colour on top to create astonishingly realistic figures and scenes. The translucence of the paint film allows for sophisticated ranges of flesh tones. But then we hit the problem of the upper layers of oil glazes drying before the lower (first) ones do — and this is where cracking comes from.
Okay, another analogy: imagine the top (newest) layer of oil is stretching as it dries out hardens, and it stretches to the max. Its surface is expanding because it is absorbing oxygen (not evaporating water). Now, they oxygen eventually begins to hit the layer below. And it stretches and expands to the max. But they layer above is already dry, how can it expand any more with the one below pulling it!? >crack<
Like a big cookie on a pan. Slide an uncooked cookie under a cooked cookie, a bigger one and stretch and heat up that dough: as the bottom cookie dries and expands its surface, it will crack the smaller cookie it is now stretching on its surface. >crack< Nomnomnom.
To get around this, painters developed the Fat Over Lean rule. With each layer of glaze, add an increased amount of oil paint to the layer. (Less pigment, more oil.) This way, the rate of oxygen begin absorbed by an oily (fat) top layer will be slower than the hidden lower, less oily (lean) layers, and hopefully they will saturate with oxygen and harden at approximately the same time.
This leads to other tricks and techniques too. If you use too little oil in an early glaze, it can obliterate the drawing or painting underneath that you want to show through all the thin transparent glazes. It also can make the paint too pasty and thick, which is unworkable for fine detail. So, in the early, lower glazes, sometimes solvents such as turpentines are added. The loosen the paint, disprese the pigment particles, and then kindly evaporate in a big hurry leaving the old that’s left to be covered by another turp+pigment+oil layer that has a little less turp and a little more oil. And so on.
To answer the question above about the dandelion salad, oils themselves are not harmful (though not processed to be safe for food). An open container of say, safflower oil on the table will do no more harm to breathe in than some extra-virgin olive oil with Balsamic vinegar and a few chili flakes on your table for bread. It’s the solvents you have to be especially wary of. Even some of the odorless ones have harmful vapors, although it’s possible nowadays to buy non-toxic alternatives. I’d be happy to recommend some I’ve tried if anyone has email requests (this is not an infomercial).
Patron Saint of Pigments
In Renaissance Italy, the patron saint of painters was St Luke — who was also the patron saint of doctors. Painters didn’t have a Guild of their own, they belonged to the same as doctors. Why? Besides the mythology of the saint himself, it was for the practical reason of painters and doctors both frequenting apothecaries for medicinal and artistic ingredients.
The pigments in oil glazes add another property and challenge to the artist who up until about 150 years ago, had to mix each batch of paint by hand. The pigment particles are not all the same size, and do not all disperse at the same rate within the oil medium. What this means is some colours will have more oil, and others less. Yeah you see it coming: the glazes following the fat over lean rule are best applied in certain orders to reduce cracking upon hardening.
As an example, let’s say you’re painting a red rose, with all it’s subtle shadows and highlights. To get ideal results in your glazes, you may want to apply the glazes in this order: manganese blue, cadmium red, quinacradone red, alizarin crimson. Mostly this will not matter to modern oil painters, but it can still have an effect even today. Most true alizarin crimsons will have up to twice as much oil content as a lead-based white.
What’s the point in all these complicated glazes? Just to mix colour? Not just — they add luminosity to the painting. You see, when light enters the hardened oil paint film, it passes through several distinct layers of mostly transparent paint. And sometimes, before being reflected back out to the surface, it bounces off of one of the colourful pigments, and back down to the layers below, and then out. Sometimes it will bounce on the boundaries of the separate glazes before bouncing out to meet your eye. And this is what gives oil paintings their glow and their deep deep blacks. The dancing behaviour of the light in the complicated multiple layers and their colour pigments.