Sunday morning: Darn, my go-to bakeries are closed and I, breadless, have to resort to the chain around the corner. I know exactly what to expect there: Smooth, even, uniform loaves that are pretty much exactly like the one I’ve bought some weeks ago when it was fresh. The baker down the road, on the other hand, who can’t afford Sunday wages and so gets to sleep in, might rather surprise me, throwing in some nuts here or changing some time variable there. Fortunately his business is going well enough for this sort of variety to stay around for a while to come.
I’m taking my breakfast to the studio, have a cup or two and slowly get to starting today’s work. There’s paint tubes of all sources lying around between the different tables, only rarely a matter of personal preference and rather of availability. Lots of the cheap Boesner brand, some Lukas and only few by Schmincke; then again plenty of old tubes passed down from some older colleagues or painting parents, usually encrusted in leaked oil and spilled stains.
The price range for the freshly-bought stuff is considerable: The larger size of Boesner costs about 3€ per 100ml, the cheaper Schmincke prices start around 10€, but depending on the pigment might go up much more. Now, shouldn’t oil paint be an especially simple product? (Apart from very few actually expensive pigments,) what justifies the price beyond pigment and linseed oil, which is all you’d theoretically need in paint?
Firstly, pigment load: High-quality paint typically won’t contain much more oil than the minimum required to bind the pigment, whereas a standard grade might be a lot more liquid. Moreover, there will be an amount of filler, such as chalk, which has a refraction index very close to that of linseed oil and hence turns near-colourless in suspension, not significantly influencing the tone but of course the strength of a colour.
But then there’s a lot more, though the specific recipes aren’t usually publicised. There’s no knowing what strange additives especially the cheaper brands might contain. Mainly, there is an ambition to provide paint of a uniform quality, especially consistency-wise. Mixtures of just pigment and oil will behave vastly different depending on each pigment’s rheological properties.
Some of these differences remain: Most notably, the different drying times, ranging from about a day for earth tones to several, if not a week or more, for notorious colours such as ultramarine or many reds. As the drying of oil paint is a chemical process of polymerisation rather than merely a physical one of evaporation, it benefits of catalysts such as heavy metal salts, which are a natural constituent of the usual earth tones but not, for instance, of synthetic organic pigments. Notably, the two main whites, titanium and zinc (oxides), don’t dry quickly, which can be pretty bothersome, considering their ubiquitous use.
Much more variation occurs with consistency. A decent number of pigment/oil suspensions forms non-Newtonian liquids.
Newtonian liquids are what from practical experience we’d consider the standard case: Like water (or ethanol, or turpentine, or ether, or sulphuric acid …), viscosity increases linearly with shear stress. Imagine swimming: The harder you fight against the water, the more resistance there is. Moving slowly you hardly feel any, while trying to move fast you’ll find it quite an obstacle. Once you slow down, however, water is instantly back to its most welcoming nature.
Of non-Newtonian liquids there’s very different kinds. There can be more or less resistance with increasing shear force than you’d expect on a linear scale, and furthermore the effects can linger after the stress subsides or instantly cease. The famous example of a dilatant liquid is a suspension of starch and water: Liquid and flowy when at rest, its viscosity increases so much under stress that you can shatter a drop with a hammer – the shards then instantly “melt” to liquid again. If you fill a pool with that mixture you can walk on it, provided you don’t stand still, in which case you slowly sink. On a less dramatic scale, titanium white in linseed oil also exhibits dilatancy. Stiff, pigment-heavy mixtures tend to stick to the brush instead of gracing the canvas, and it doesn’t hold the finest brushstrokes very well, as it becomes more liquid after application.
An opposite of dilatancy is thixotropy, wherein a normally stiff substance becomes more fluid under stress – and that’s a property hard to get by in paint, as it occurs mostly with heavy metal pigments, all of which are banned or very much banned, out of production and toxic as hell. The toxicity of lead, arsenic or cadmium compounds isn’t the kind of exaggerated warning that’s there just to keep the children away, no, this stuff is genuinely nasty to work with.
That being said, in terms of painting properties they’re wonderfully rewarding. Heartfelt thanks to the friend who gave me his small, old GDR-manufactured supply of lead white tubes, which for a long time I have been so eager to try out.
It didn’t disappoint. No, in fact it has all the idiosyncratic merits that I hoped to see. It’s warmth, opacity and drying behaviour are unique amongst whites; and then there’s its thixotropy.
Both other whites are rather cool – titanium more so than zinc –, which isn’t so significant in pure application but heavily so in mixed tones. Especially white skin tones tend to appear fairly dull unless there’s glazing involved. Lead white is a bit on the warmer side of the spectrum and goes along wonderfully with oranges and browns.
Opacity: Somewhere in the middle between titanium and zinc. A mixture of the latter two could probably approach any degree of opacity in between, so this is hardly a unique advantage.
Drying time: Heavy metals tend to act wonderfully as catalysts, lead white dries tremendously fast, usually in under a day even when applied somewhat generously. Titanium may smear for days even when superficially appearing dry, as does zinc, whose dried films also tend towards a certain brittleness.
Now for the interesting bit. What is thixotropic paint good for and how does it work? To sum it up: It allows a much greater variety of textures than a single type of paint would otherwise.
It allows long lines: A brush loaded with lead white can be drawn out forever, as only the paint at the tip liquefies. In that regard it feels like any other paint strongly diluted with turpentine – only that it retains its full tinting strength, while the diluted paint would of course contain a lot less pigment and would appear as transparent as a layer of ink.
It allows overwriting: Paint a thick trace of lead with the brushstroke well-preserved (which, by the way, it does perfectly). Carefully paint a second trace across – the first stroke’s texture will survive instead of smudging; essentially, to an extent (certainly not if you’d like to apply fine scumbles) you can treat it as if it had dried already.
The more idiosyncratic textures too easily become clichés just for their own sake; but skillfully used they’re a powerful tool. Abruptly pulling the brush from a puddle of lead white produces a long, delicate threads or, given sufficiently much, even veritable curtains, where other paint would just break off.
My lead white supply isn’t large, and once I’m out of it I might seriously contemplate making my own. The process isn’t too complicated and mostly involves some patience, the problem is just how to avoid a major lead spill and right now I’m not quite ready for that enterprise. No hurry either, for now having tried out the thing satisfied me enough for a while.
Self-made white would probably have yet more of a thixotropic behaviour than the tubes I got now. That’s because they’re still industrially ground, so much finer than you’d do it manually. Optimal particle size is different for each pigment, and in quite a few cases too fine is bad. Contrary to what you might sometimes read, paint manufacturers account for this. The blue pigment smalt is a particularly well-understood example. Historically it was used as a replacement for the exorbitantly expensive ultramarine and azurite. It’s less intense but works well for most purposes. Today synthetic ultramarine or cobalt blue (cobalt aluminate) are a better alternative to smalt because of the latter’s extraordinary hardness.
Smalt is a cobalt compound, too: cobalt oxide, a mineral glass. It was produced synthetically since the 16th century, by adding cobalt ore to a glass melt. Recently I had the opportunity to use some, and was in fact handed an amorphous blob of blue that I still had to grind.
But imagine finely ground glass: What used to be transparent on a macroscopic scale turns white and opaque as fine particles. So does smalt, which is intensely blue as a lump but loses more chrominance the finer you grind it. There is a sweet spot that’s fine enough to use but not too fine to spoil the colour. I needed two attempts to reach it, the first time the paint was still way too harsh to use it. But even at its best, this is a harsh pigment to deal with. I tried using it on a panel painting and was seriously afraid that because of the aforementioned hardness I would scratch my beautifully polished chalk ground. The painting was to be a copy of a Rembrandt landscape. Although thanks to my newly-found lead supply I had the possibility to use only historically accurate pigments, in the end I used cobalt blue instead of smalt, with which neat gradients were unnecessarily difficult to paint. I did have quite some problems with the lead white, too, which I found particularly difficult in thin layers. No, zinc and titanium are very useful things to have, I’m just concerned that perhaps two whites could sometimes feel a bit alone with just one another.