MITRA Forum Question Details

I am not a commercial paint maker, but this is of interest to me. The bias that you mention has been around for quite a while. The idea is that while the alkali process removes more organic impurities, it may leave some residues or somehow weaken the oil in a deleterious way.  Some would suggest that the refinement weakens the oil in some manner creating a product that is lighter and less prone to yellowing, but not as capable of adequately binding a given volume of pigment. We first have to ask, how could an oil that contains more impurities be superior? However, this was certainly true very early on when oils were crudely refined leaving residual acidic or alkaline components. This was compounded by the fact that the oils that were generally refined by these processes were those of the coarsest extraction and were intended for only the crudest uses. This is in no way the state of things today.

 Linseed oil refinement is a carefully controlled process, which allows for cleaner and purer oils than available previously.  We do have evidence that modern quality refined oils yellow less than cold pressed oils. In the recent past, the only real benefit to cold pressed linseed oil was its use in grinding oil colors (as opposed to use in mediums/commercial oil varnishes for wood, etc. where this was less valuable) due to its high acid number. While this does contribute a bit to yellowing, it does allow for the creation of a paint that requires substantially less oil than paint made with linseed oils of lower acid numbers. This is a non-issue today since alkali refined linseed oils are available in any acid number that is required. Additionally, If you read mid-20th century literature on the subject you get the impression that oil paints made from non-cold-pressed oils were more likely to exhibit a suede effect. I am firmly of the opinion that this effect was the result of the commercial use of stabilizers, like wax, aluminum stearate and aluminum hydroxide, which created a paint that was unnaturally short and would not level, unlike more simple oil paints that lacked such fillers/stabilizers. Hand ground paints were less likely to contain these fillers, and therefore, did not exhibit that defect. This dichotomy contributed to the deification of cold pressed oil well beyond the virtues of the material…or at least this is my take

We have a few paint makers on our moderating board and industry cotacts and I look forward to their responses to this thread.

​I have been scouring literature on the differences between non-refined (cold-pressed) and refined oils for many years, and have not found any that informatin that would support claims attributing improved mechanical strength of paint films due to the refinement of the oil. What is well-known is that refined oils contain less impurities so that they tend to yellow less and dry faster. However, this subject has not been studied, so there is no definitive answer.

As has been found in studies by Marion Mecklenburg and others, is that the mechanical strength of paint films is largely due to the pigment, such as lead white. Whether the state of refinement has any effect on this is not clear. However, understanding the nature of the impurities in oils makes its unlikely that they would contribute positively to the mech8ca strength of the dried paint film.

In one study we conducted, however, we found a difference in the tensile strength between types of oils, such as walnut and linseed oil—linseed oil exhibiting higher tensile strength than walnut oil.

I agree with both Brian's and George's assessment. It is also important to mention that various form of refining (including less industrial versions of both acid and alkali processes)  were carried out in the past 

(see the section on Refining linseed oil paints starting on pg 14 of Analytical Chemical Studies on Traditional Linseed Oil Paints 

https://pure.uva.nl/ws/files/3716376/20662_Thesis.pdf

and that what passes as 'cold pressed' both historically and today is not a singular standardized product. While starting out as a raw expeller pressed oil - without the use of heat or chemicals - it ultimately undergoes any number of refining processes that historically changed over time and had wide regional variation. So that would complicate any claim or study as you would need to define what you are comparing to. Also remember that even the genetic make-up of flax had changed over time and it is not at all clear that the fatty acid profiles of todays' oils match that of the past, since strains have been continually optimized and crossbred to have better resistance to disease, draught, yield, etc.

That said, we continue to study this area and are open to the possibility that older processes of refinement might produce some interesting differences in handling and drytime, as Leslie Carlyle's work on the historical reconstructions of lead white using differently processed oils seems to point to. But currently we have not seen any evidence that structurally cold pressed linseed oils are superior and indeed the alkali-refined oils we use seem more consistent and less variable in its batch to batch qualities.

Hope that helps. 

Thanks all.

As an interesting connection to this dialog, I actually spent a summer working with Leslie Carlyle on her HART project in Amsterdam (2005) hand grinding, using a granite muller and slab, hundreds of lead white paints with additives/other pigments added in 5% incriments (barium sulfate, etc) in various mixtures of oils in 5% stepwise additions (eg, cold-pressed linseed oil 95% 5% poppy oil, 90% CP L Oil 10% poppy oil, and so forth). We took these to a few research facilities and paint manufacturers to measure rheology. The last couple of days were spent painting out all of these on multiple substrates. Leslie has used these, and many others, for her years of subsequent research.

​Probably the best way to answer this is to introduce you to AATA. Do a search for Leslie Carlyle. http://aata.getty.edu/Home

​Leslie is an amazing force unto herself and is a major reason why I transitioned from fine art to technical art history and eventually art conservation. In fact, it was my purchase of her "unpublished" doctoral thesis in 1999 that solidified my decision to enter the conservation field.

Hi Elisane -

Would be happy to share what we know and try to respond to your questions.

There is no standard that ties a specific range of acid values to the terms 'low' or 'high', and as you discovered, most linseed oils today will have an acid value below 4. And indeed, ASTM's Standard Specification for Raw Linseed Oil (D 234) actually sets the current maximum allowable acid value at 4. So low and high would be relative to this range, for the most part on a practical level. Certainly in older literature, one can find higher ranges mentioned, but whether that has changed due to modern methods of production -  keep in mind, even cold pressed oils are processed - or changes in the actual cultivated flaxseed, I do not know.  

Within alkali refined oils, however, one can get quite a range dialed in, from extremely low levels below 1, to standard grinding oils in the 2-4 range, and finally oils with very high acid values of 12-15. As for who sells a linseed oil with high acid value, we would recommend looking at the offerings from Natural Pigments. The only colors that would gain much by using a linseed oil with a high acid number would be reactive pigments, the most common of which would be lead white, but also includes cerulean blue, and cobalt green, among others. See this piece by Natural Pigments for more information:

https://www.naturalpigments.com/art-supply-education/reactive-pigments/

Lastly, the better wetting of the high acid value oils can be demonstrated in precisely the way you mentioned - looking at the amount of oil needed to fully wet out a specific quantity of pigment. You also get more viscosity build when combining high acid value oils with a reactive pigment due to the rapid formation of metallic soaps. But to be honest, unless you are planning on making your own lead white, we think you are best off just sticking to one of the standard alkali-refined linseed oils being offered, which should be fine for most cases.

This is still a bit overly simplistic, as we do not know the acid value of the theoretical alkali refined oil in this theoretical instance. I am not sure that the differences would be marked either way, but sure, for lead white, feel free to buy it in CPL Oil. It may yellow a bit more that the alternative but it will definitely take up as much pigment as possible. Unlike many other issues on this forum, you are not going to cause any problems by doing so.