PDA

View Full Version : Technical question about accelerating drying oils


plnelson
03-24-2019, 01:37 PM
No everyone thinks watching paint dry is boring. Recently someone wrote to me on another forum saying that "fat over lean" doesn't apply if you use a drying accelerator like a cobalt dryer or galkyd on the lower layer. Then he said you can paint "lean" over that.

And this got me wondering about the chemistry. Normal drying oils dry by a process of oxidation. They react with oxygen in the air via a process in which chains of unsaturated fatty acids get some of their bonds replaced with molecules that are hydroperoxides, and these cause cross-linking to occur between adjacent chains, forming polymers.

So far so good; we all know this. But after this stage, it's not over - these molecules can then form esters which continue to react with other substances in the paint (e.g., pigment or other medium molecules) to provide further chemical changes over many years. The result is that the dimensions and weight of an oil painting change over years - first going up as oxygen is is added, then doing down as volatiles are created in this latter process. Much of the yellowing we see seems to be due to the long-term breakdown of the hyperperoxides from the first stage. (https://www.jstor.org/stable/1506842?seq=1#page_scan_tab_contents)

So MY QUESTION is how do drying accelerators affect this? Do they catalytically compress months and decades into a much shorter time (and therefore make it more exothermic?!) or do they replace some stages in the normal drying process with different chemistry, in which case how does this affect the weight and dimensions of the paint as it dries? Is it true that drying accelerators let you ignore fat over lean?

Thanks in advance for any insights.

Pinguino
03-24-2019, 04:34 PM
That level of technical expertise is probably well above the level of nearly all members here, including myself (and I know a lot of stuff).

It is my understanding that some catalysts, such as Cobalt and Manganese and Lead) assist in the Oxygen absorption, then quit. Other catalysts, such as Zirconium and Calcium, do not assist in Oxygen absorption, but do assist in the cross linking, after initial reaction with Oxygen.

I use a product named CoZiCa, which has it all. I expect that this should be helpful for reducing the fat over lean problem; but then, I only paint thin layers on board, where there is always less of an issue.

Antonin
03-24-2019, 05:44 PM
......"fat over lean" doesn't apply if you use a drying accelerator like a cobalt dryer or galkyd on the lower layer. Then he said you can paint "lean" over that.
The result is that the dimensions and weight of an oil painting change over years - first going up as oxygen is is added, then doing down as volatiles are created in this latter process.
....do drying accelerators affect this? Do they catalytically compress months and decades into a much shorter time (and therefore make it more exothermic?!) or do they replace some stages in the normal drying process with different chemistry, in which case how does this affect the weight and dimensions of the paint as it dries?
Is it true that drying accelerators let you ignore fat over lean?

There are several things that make lean paint over dried fat oil paint not workable.
First non starter is that the oil in freshly applied paint is mobile.
It penetrates porous surfaces.
All the inhale and exhale that goes on in drying oil paint makes the dried paint somewhat porous.
So a portion of the oil in that already lean overpainting is going to be sucked into and through the porous dried skin of the fat layer, plumping up both that dried skin and the less dried interior of the fat layer (making the fat layer even fatter).
Next problem is that the remaining oil left in the already lean overpainting may not be enough to keep that layer's pigment particles well bound.
Adhesion of lean layer to fat layer is going to be poor.
Flexibility of the lean layer is also poor in relation to the fat layer.
At some point much or all of the leaner layer will scale off of the fat layer.

sidbledsoe
03-24-2019, 10:12 PM
So MY QUESTION is how do drying accelerators affect this? Do they catalytically compress months and decades into a much shorter time (and therefore make it more exothermic?!) or do they replace some stages in the normal drying process with different chemistry, in which case how does this affect the weight and dimensions of the paint as it dries?
I don't know the specifics of the chemistry of catalytic driers, but when an excess is used it can result in the paint film being prone to increased brittleness, cracking, and yellowing/darkening, so it can't be a particularly good thing, if used in excess. An example would be "boiled linseed oil" which dries nicely but darkens as nicely too. Hardware stuff isn't really boiled, it just has driers added, and the name is just a traditional term since it used to be heated, but driers are much cheaper.

Gigalot
03-25-2019, 02:34 AM
Cobalt or Manganese driers are hydroperoxyde decomposition accelerators. That increase polymerization speed at the first part of drying process. Actually, cobalt works during short time.
So, oil oxidize without any help of cobalt, but after that it polymerize with the help of it's ions. Until hydroperoxydes completely decompose.

"Abstract

The cause of yellowing of oil-based paints has been investigated by analysing drying oils simultaneously by iodometry (to determine the degree of oxidation) and by colorimetry. It was found that yellowing of drying oils can be attributed to co-oxidation reactions of contaminants."

Gigalot
03-25-2019, 03:44 AM
Thermooxidation at 100C and photooxidation at wavelengths above 300 nm of dried oil films were evaluated. The chemical modifications of the networks were determined by infrared analysis coupled with gaseous treatments (NO, SF4, and NH3). The dried films are rather stable in thermooxidation, whereas in photooxidation, important degradation of the network occurs with many chain scissions. This photoinstability results from the presence of crosslinks that are sensitive to radical attack because of the lability of the hydrogen atom on the tertiary carbons. Yellowing of the cured samples, observed with ultraviolet-visible and fluorescence spectrometries, rapidly is decreased by irradiation because the oil contaminants that are mainly responsible for the yellowness are photooxidized. On the contrary, yellowing slowly but continuously increases during thermooxidation at 100C.

DebWDC
03-25-2019, 02:25 PM
Hi plnelson

Philosphical overview:
Why do you ask the question? Do you want your paintings to dry faster? Or is it an interesting mental-exercise problem that you enjoy thinking about? I have a long list of interesting problems I enjoy thinking about but never would want to do anything about (for example, why US peer-reviewed scientific journals in the social sciences pretty much quit publishing accounts of failures after WWII).

Short answer operationally:
If you want your paint layer to dry within 1 week at 70 degrees Fahrenheit and sea-level atmospheric pressure and 50% relative humidity and also not wrinkle or crack too much, then I suggest using a commercial alkyd medium such as Galkyd or Liquin, painting in thin layers, and not using additional dilutant such as OMS or turpentine. Do this for every layer, and you will not have to worry about fat over lean. In other words, operationally, there are ways to paint soundly and in a reasonable amount of time while ignoring fat over lean because you are painting kinda fat over kinda fat.

Long answer for interesting mental-exercise problems:
1. I dont know the answer to your question.
2. I suspect that drying accelerators, as you term them, vary in their effect on paint layer surface and interior, and that variance depends on a multitude of contributing factors: the type and amount of drier, the type of drying oil, whether the paint pigment itself is a metal (manganese, lead, zinc, etc.), the thickness of the paint layers, AND the temperature and humidity of the studio.
3. All the pesky confounding variables I listed in #2 above mean, to me, that an accurate answer would be difficult to determine. The precision needed to control the confounding variables would be hard to achieve. That is why the published conservation papers have such long descriptions of the set-up, materials, time elapsed, etc.

BTW, Galkyd is composed of alkyd, a dilutant (petroleum naptha), and I suspect a small percentage of drier because I have read it dries faster than linseed oil; it is NOT a drier itself. You may wish to verify this yourself from the MSDS information.

Ps I suspect that some of the cracks in painting surfaces I have seen in national galleries are a result of a lean layer painted over a fat layer at a time in which the fat layer is still adding oxygen molecules and therefore expanding. They look like expansion cracks to me, not shrinkage cracks. Imagine painting on a balloon, waiting for the paint to dry, and then blowing up the balloon. I expect to be pilloried unmercifully for this conjecture...:)

Deb

Harold Roth
03-25-2019, 07:06 PM
So MY QUESTION is how do drying accelerators affect this? Do they catalytically compress months and decades into a much shorter time (and therefore make it more exothermic?!) or do they replace some stages in the normal drying process with different chemistry, in which case how does this affect the weight and dimensions of the paint as it dries? Is it true that drying accelerators let you ignore fat over lean?
How about asking this on the MITRA forum? It's art conservators, and they will answer artists' questions. A search for "driers" turned up a bunch of stuff:
https://www.artcons.udel.edu/mitra/forums

It's a fun rabbit hole for the dedicated paint nerd too.