To hell with “buttery consistency”

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.

Detail of Rembrandt’s so-called “Jewish bride”: The impasto struc­tures in the sleeve defy all attempts of reverse engineering.

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.

My test paint­ing to explore lead qual­it­ies. Canvas, 20x64cm

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.

My approach to the “Stone Bridge”.

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