The case against fat over lean

[contumacious]

Now, that’s what I dispute! Seems to me that 2 or 4 should be more fat (oily) than 1 or 3. But the depth of the layer (after solvent evaporation, if any) should be less for 3 or 4, than for 1 or 2.

No doubt the fatness (oiliness) and layer depth are both factors that affect such things as drying time and elasticity.

Forget about the thickness of the two layers for a minute. The key terms I am using here are the VOLUME of paint and the SURFACE AREA they cover. If both are identical, Vols 3 and 4 can never even be the same fat / lean level as Vols 1 and 2, let alone more fat. It is physically impossible.

[Don Ketchek]

Now, that’s what I dispute! Seems to me that 2 or 4 should be more fat (oily) than 1 or 3. But the depth of the layer (after solvent evaporation, if any) should be less for 3 or 4, than for 1 or 2.

No doubt the fatness (oiliness) and layer depth are both factors that affect such things as drying time and elasticity.

In 2010 I had a series of emails with both technical support from both Gamblin and Winsor & Newton. Here is what they told me:

The basic structure of an oil film as it dries is this: through exposure to the atmosphere, oxygen molecules come into contact with the paint and link up with individual oil molecules, stimulating the formation of an oil chain. The result is a lattice-like structure that locks the pigment securely into place. You can think of it as the oil paint film “breathing” as it dries. [B]Now, if you start adding solvent to your paint, you push the oil molecules far apart[/B] – you start messing with their ability to form a lattice-like structure that has strength and flexibility. So, you make the film less flexible (leaner) by changing the physical structure of the film – altering its ability to form a secure, flexible structure, by thinning it, diluting it.

[B]It will be less flexible upon drying because the oil molecules have been separated, allowing greater space between the oil molecules[/B]. [B]When you add solvent to your oil paint, you are thinning/diluting the paint – you are pushing the oil molecules further apart, thereby creating a film with less flexibility upon drying. Less flexible=leaner. Adding solvent causes oil content to be dispersed. [/B]

Hope this helps clarify.

Don

[Anonymous]

adding solvent does not make the paint any leaner.

Don’s post contains the correct information.

I cannot think of any reason why solvent (evaporated or not) would make paint dry faster.

drying will be affected.
as Don quoted above you start messing with their ability to form a lattice-like structure that has strength and flexibility.
they are talking about the intermolecular forces. This is the physics behind why oil is thick and viscous and solvent is the opposite. Solvent addition drastically reduces intermolecular forces. This physically changes the rheology of the paint as I stated earlier,

existing oil molecules flow together to fill the voids, since the oil is still in liquid state at the time the solvent evaporates. So the voids are filled by inflow. This does mean that the residual paint film is thinner (in vertical dimension) and thus subject to whatever advantages and disadvantages come from a thin paint film….The remaining oil will flow together to fill the voids left by the lost solvent.

It can’t physically do that, the intermolecular forces are no longer there, the oil doesn’t return to it’s original state. You can see a physical change in the paint film surface. As more solvent is added the surface becomes more and more matte, dispersing the oil that was covering the pigment, and oil is what makes the surface more glossy because the particles are surrounded with oil. The solvent addition also makes the paint film more prone to absorption into lower paint and ground layers.
Solvent addition weakens the film and leads ultimately at some point to an underbound condition. These physical changes occur irrespective of the subsequent complete evaporation of the solvent. The change and/or damage has already been done.
Paint making involves laboriously grinding oil and pigment together until the pigment particles are fully dispersed, surrounded, and joined closely at the hip, so to speak, with the binding oil. Add solvent and you rapidly and efficiently break that apart.
As I said earlier, there is a lot more to it than the very simplistic thought that lean only means the ratio, the ratio is just not enough information to completely rely upon.

[contumacious]

Thanks for bringing it all together Sid and Don.

I guess was using the wrong terminology and confusing myself. 10 ML of paint / medium mixed with solvent added, obviously has to have less oil in it by volume than 10 ML of otherwise identical ingredients without solvent so in my mind that meant it had to be more lean, but it isn’t.

[Gigalot]

Why nobody consider, that solvent addition makes paint film to be porous, weak, brittle and chalky?

[laika]

Pinguino, I think of it in terms of physical thinness: if you could somehow work a dab of oil paint over a large area you would have a “lean” layer that could interact with the atmosphere to dry quicker than a big pile of paint spread thickly over the same area. Up to a certain point, solvent facilitates spreading a dab over a large area, but you can paint relatively thin areas by physically doing the work of scrubbing the paint onto to the surface.

Oil paint doesn’t dry by evaporation: it reacts with the air. Thick impastoed layers can take years to dry. Thin layers over thick are prone to cracking because of the difference in drying times.

Lamar

Art is life's dream interpretation.
- Otto Rank

[Anonymous]

Why nobody consider, that solvent addition makes paint film to be porous, weak, brittle and chalky?

The rule only speaks to oil content.
Many only consider one thing to mean lean, and that is “the ratio of oil to pigment”.
But that is not an all encompassing definition, it only describes one single condition that is specific regarding oil content, and it doesn’t describe other factors like solvents, alkyds, waxes, thickness, nor where the layer is on the drying curve. Even the type of binding oil differs with regard to dimensional change.
Lean really means less oily, less fat. And with less oil there is less dimensional change and dimensional change is the reason for, and the crux of the rule, fat over lean. As mentioned, a thinner, solvent dispersed layer is a leaner layer, it contains less oil and will undergo less dimensional change, and also do it faster, than a thicker, fatter later.

[Pinguino]

… It can’t physically do that, the intermolecular forces are no longer there, the oil doesn’t return to it’s original state. …

Now, that’s the only clear evidence that counters my argument.

[Richard P]

I’m glad I just use Walnut oil as a medium then

[Pinguino]

I’m glad I just use Walnut oil as a medium then

The issue wasn’t regarding oil, because adding any oil would make the paint “fatter” than it was before. The issue was whether adding solvent actually made the paint “leaner,” for any fixed amount of oil.

[Richard P]

What I meant was that adding solvent can weaken the paint. The amount that is ok to use or not depends on a lot of factors I’d imagine.

[Watercollar]

Doesn’t the “oil absorption” of each pigment type play the most crucial role in establishing “fat over lean” principle?

Sebastian.

(C&C Welcomed.)

[Anonymous]

Doesn’t the “oil absorption” of each pigment type play the most crucial role in establishing “fat over lean” principle?

Yes, any factor that contributes to less or more oil content (thus more or less dimensional change) in a paint layer has some role, not sure about how crucial that consideration is, and would vary.
Since the mixing of pigment/paints is so variable, convoluted, and nebulous, no one I ever heard of pays any attention whatever to relative oil absorptions.
That would take it to a way more extreme level than people already make out of it. There are constantly so many questions about this and that factor, that I think some people say, whatever, I am just gonna paint.
It is just a general guideline to attempt to strive toward, and just to minimize issues like cracks.

[Anonymous]

I’m glad I just use Walnut oil as a medium then

I wouldn’t be, it makes you more prone to want to paint lean over fat.
Using only solvent is the easiest way to follow fat/lean.
You naturally start with solvent thinned layers, use less until none is used. Bam that is fat over lean no matter how you look at it.
But if you only use oil, and you need or want to start with thinned paint, you are stuck with using only oil. This is AnnieA’s dilemna as she searches for some way to get thinned lean, washy paint at first, but without solvent, which is nigh impossible.
You can overcome this oil only, fat first issue by allowing early oil thinned layers to completely dry, because completely dry also means less dimensional flux, which is the same condition as lean paint. It is really all about, and it is only about dimensional change.

[Richard P]

I paint wet in wet never wet on dry.

[Author]

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