Fat over lean?

[DAK723]

I think there is a lot of confusion on this subject for a couple of reasons. One is that the drying of oil paintings due to the variety of additives (oils, solvents, sometimes resins, driers, etc) is a very complicated thing that is not completely calculable.

The other - as has been mentioned many times - is the semantics. Fat over lean, thick over thin, flexible over less flexible. Every one means something slightly different.

I had a long email discussion with Winsor & Newton about this issue 3 years ago. Keep in mind that the answers were provided by a single individual - and even among the literature provided on w&N's website, there are differing opinions and definitions of "fat over lean."

In this person's opinion, the best way of looking at it is "flexible over less flexible." And both the oil content and the solvent play a part.

Really though, the thing to focus on is that fact that fat over lean means more flexible over less flexible. Lean = less flexible. How does an oil film become less flexible? Well, it can be less flexible when you take it in the context of one layer among many in a painting - if it has less oil in it than other layers, it will be less flexible, or "leaner" than the other layers. This context is created by adding more oil to the other layers - making the other layers more flexible, fatter.

The other way to make an oil paint film less flexible, is to add solvent to it. In another of my emails to you, I believe I said that adding solvent does not make oil paint leaner all on its own - I"m sorry about that, I think this was a confusing statement - my concern was that you understood that the film really really must be seen in context, within the painting - the film is leaner in comparison - hope this makes sense? 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. Now, if you start adding solvent to your paint, you push the oil molecules far apart - 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, rather than subtracting oil from it. It will be less flexible upon drying because the oil molecules have been separated, allowing greater space between the oil molecules.

In emails with Gamblin, I also received the same basic response. Increasing oil content increases flexibility. Adding solvent spreads apart the oil molecules making the layer less flexible. And the most important aspect is that flexible and less flexible are in comparison to the other layers in the painting.

Now, a person might ask, "If I use a 50/50 medium of oil and solvent, which has a greater effect on flexibility? The oil making it more flexible or the solvent making it less?" In every correspondence I have had, the answer is basically if you add oil in almost any percentage, you make the layer more flexible. So the effect of adding oil seems to play a bigger role in increasing flexibility then the effect of adding solvent does in decreasing flexibility. But again, it is the comparison between layers that is important, not some "theoretical" degree of flexibility.

Don

[sidbledsoe]

That is very good practical information and guidance Don. It describes the the physical reason why adding solvent changes the paint matrix.

Wait, do I read rightly? That it is now the general consensus that adding
mineral spirits to my paint DOESN'T speed its drying? I can't be reading
this...

[sidbledsoe]

I don't see a consensus at all, but maybe we could take a poll. This argument has been going on for years now. the very same members are saying the same things over and over again time and time again.
Here is a thread from 2005, continued into last year with the very same members arguing about it then and now, saying the same thing.
Reread the first page of this thread and you will see the same things being said there as what has been said in this last page or so. Good luck!

[Ron Francis]

Thank you Don. That reasoning makes sense to me.
I does seem to contradict Mayer's statement:
"It should be clearly understood that the sole purpose of thinners is manipulative and that none of them have any reactive effects on the paint."
Although maybe the word "reactive" is debatable.

Plog,
Well, Mayer certainly says that solvent doesn't increase the rate of drying.
I'll repeat it here:
"and none [solvents] has any drying affect on paints and varnishes except as it allows the paint or varnish to be spread in a thinner film".
It certainly isn't common consensus though.
If it does increase drying rates (apart from just increasing surface area), I would like to hear some reasoning behind it.
Don's 'porous' idea sounds good but isn't backed up by AMIEN. However, Mayer does say that excessive amounts of solvent does make a layer porous. (I read it yesterday but didn't mark the page and can't find it again.)
If that is true, then it does lend credibility to the theory.
It seems that there is much contradictory information around.

[sidbledsoe]

Thank you Don. That reasoning makes sense to me.

Yes, I also fully agree with Don's post above.
Don and I were saying this on page 1 of this thread:
Don's first post #12:

If your add solvent to your mix - it reduces the amount of oil molecules by spreading them farther apart (to make room for the solvent molecules). So even after the solvent evaporates, your paint layer has less oil molecules then a layer with no solvent added. Therefore, more solvent equals leaner.

and then followed by my post #13:

When you add a solvent then you will effectively make that paint less flexible and thus in practice it will behave as a leaner paint film and that is important

These are saying the same things Don just posted today, and previously, the same thing he posted in the 2005-2011 thread which I linked in my post above, the thread where we have discussed this same thing, as we have a number of other times, over the past several years.
I believe that this is basically the same reason a spirit solvent thinned mix will dry faster but I also believe that turpentine and oil of spike are two solvents that contain many chemicals and resins which may also interact chemically with the oil and speed drying. I have read this in a number of places but can't provide links other than this one by David Rourke. Most anyone in the real world who has painted with a turpsy wash or solvent thinned paint can attest to the resulting faster drying, regardless of the physical, chemical, absorptive, evaporative, or whatever other reasons behind it.

Personal experience tells me this about mineral spirit's effect on drying time:

Slow drying pigments dry in a day or two (if painted thinly), as opposed to
a week with no thinner. Fast drying pigments begin to dry on the palette
after a few hours of using them, as opposed to a day or two.

Apologies to Mayer, but I'm not telling my paints they have been behaving
wrongly for these last few decades.

[Ron Francis]

Yes, thank you Sid.
So in short, adding solvent reduces the amount of cross linking between polymer chains because the chains are further apart?
If that is true then it is reasonable to say the film will be less flexible.

I would be interested in reading any papers on this if anyone knows of any.

LOL plog.
Maybe one day I'll do some measured experiments.

[SSB]

Hello all. I would like to admit i asked the question somewhat rhetorically for the sake of momentarily amusing myself. I hope it has amused you all as well. I hope we have all learned something. I would like to note that when painting an actual painting fat over lean and thick over thin is useless distraction. Whatever time one spends determining how fat the paint should be is better spent analyzing other things. I like my paint the same consistency every time. So does everyone I have ever seen actually paint a painting. Other than adhesion wet into wet what does it matter? Nothing. Absolutely nothing at all. The smartest thing I have ever heard said about all this is it doesn't matter at all, or at most very little.

PS: I like science, and these questions are interesting and useful to ask and answer!

[sidbledsoe]

Originally Posted by Ron Francis

So in short, adding solvent reduces the amount of cross linking between polymer chains because the chains are further apart?
If that is true then it is reasonable to say the film will be less flexible.
I would be interested in reading any papers on this if anyone knows of any.

Other than the pages that the chemists at WN, Gamblin, and Utrecht, etc. have out there, I don't know of any scientific study papers or where to find them.
But just consider the fact that the polymerized oil is the binder holding it and the pigment together in the paint matrix, and more binder= more oil= fatter = more flexible.
Now it is also a fact that when you add solvent, there is a certain point where this matrix is so weakened and spread apart that the film loses it's integrity and does not hold anymore and that is where it is dubbed "underbound" (despite the oil to pigment ratio being the same). To me and to my satisfaction, this verifies that what you are saying above is what is really happening. It logically should happen to a small degree with a small amount of solvent and increases in a linear fashion until it is underbound.
I think that this is the big important thing that Amien is missing and ignoring when they only consider the oil ratio after drying business. It reminds me of the engineers that built the Titanic and said it can't sink or the guy back in the thirties that calculated on paper that bumble bees can't fly because they shouldn't be able to generate enough aerodynamic lift, they left out some important factors.

[sidbledsoe]

Originally Posted by SSB

Hello all. I would like to admit i asked the question somewhat rhetorically for the sake of momentarily amusing myself. I hope it has amused you all as well. I hope we have all learned something. I would like to note that when painting an actual painting fat over lean and thick over thin is useless distraction. Whatever time one spends determining how fat the paint should be is better spent analyzing other things. I like my paint the same consistency every time. So does everyone I have ever seen actually paint a painting. Other than adhesion wet into wet what does it matter? Nothing. Absolutely nothing at all. The smartest thing I have ever heard said about all this is it doesn't matter at all, or at most very little.

PS: I like science, and these questions are interesting and useful to ask and answer!

We would rather talk fat and lean here than paint anyday so thanks for this weeks' fat over lean confusion thread, we usually have one or two up and running at all times, if you do a thread title search for "fat lean" you will find three full pages of these type of threads loaded with questions, confusion, explanations, arguing, opinions, and controversy, dating all the way back to the early days here .

[sidbledsoe]

Time for another thread bump. After more thought, yet another effect of adding solvent to binding oil is the possibility of volatilizing and stripping some of the components present in the oil. Linseed oil is a very complex mixture of a number of fatty acids and other components. I have spoken to a couple of analytical chemist colleagues of mine here at my work who regularly do this kind of thing and typically solvent extract and analyze oils on a daily basis. They are also of the opinion that it is entirely possible and indeed probable that some components of the linseed oil can be volatilized by the solvent. They said it was likely and that it depends upon solubility coefficients and affinities for the components vs the solvent in question, thus different solvents would likely have a different effect. This means that some of the shorter chain components that do not polymerize to the degree that the longer chains do, may be removed thus rendering a change in the drying profile for the oil.
This isn't a study or something that they have data on, it is an opinion, it is based upon their knowledge base regarding lipid chemistry.
I am aware also that this has been discussed at Amien, I have read the thread and found their answer to be non-specific and not definitive and also without any data and this prompted me to consult with some chemists here myself.

Hurray, science is finally confirming that which life experience
has already shown me to be true! (Much better than confirming the
opposite, which it often does).

[Ron Francis]

I was the one who questioned AMIEN about volatile components of oil. There is a reasonable amount of data about solvents on a dry film, but not when added to a drying film.
I suspect (or hope) that defects caused by adding solvents would have been manifest by now as solvents have been used for quite a while now in mediums etc.
At any rate, I couldn't find any evidence that components in oil volatilized with the solvent when it evaporated, and AMIEN seemed to think that if there were, it would be negligible.
So the jury is still out on that one.
Here is the thread:
http://www.amien.org/forums/showthre...ght=volatilize

I have also posed the question about solvent making the bonds between oil molecules weaker and therefore less flexible.
http://www.amien.org/forums/showthre...9053#post19053
Two answers so far ...
AMIEN:

OK, we agree with you. On the other hand, the oil molecules could migrate back together ...

And Gamblin:

We agree, and recommend to painters that when adding more than a “small” amount of artist’s thinner to oil colors, that some binder (oil or alkyd painting medium) be added as well, to maintain the integrity of the resulting paint layer. For instance, a 50/50 mixture of alkyd medium and thinner will have a nice, fluid consistency for thin washes and fluid mark-making, but will have more strength and flexibility compared to using thinner alone.

I have asked if this has been proved or is theory.

[sidbledsoe]

thanks ron, i agree with that stuff, except for the oil migrating back together once the film is spread out, again, the underbound condition indicates that this just ain't happening.