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The modern science of organic chemistry proves that linseed oil is the least stable of all vegetable drying oils and turns rancid far more rapidly than any other. For this reason, unlike Walnut Oil, Linseed Oil cannot be used in the food industry even with large quantities of preservatives. |
How do the two oils compare chemically?
All drying oils depend upon the amount of unsaturated fatty acids in them to dry and form films. The composition of these fatty acids determine how stable the oil is, how much yellowing will take place, how brittle the film will become over time and how rapidly the film will dry. The composition tells us how well an oil will perform as an artist's vehicle.
A comparison of linseed and walnut oil fatty acid composition indicates that linseed oil is less stable, turns rancid more readily, yellows badly and becomes brittle rapidly leading to cracking.
The primary fatty acids that make up walnut oil and linseed oil are the same. What is different is the amount occurring in each. Following is a list of the primary fatty acids' typical percentages found in walnut and linseed oils and the effect of each.
Acid
Walnut
Linseed
Effect
Oleic
17%
14%
Insures stability. The higher the percentage, the less likely
the oil will turn rancid or crack upon drying.
Linoleic
60%
42%
A stable component responsible for the drying of the oil to a
tough, flexible film. The higher the percentage, the better the
film.
Linolenic
12%
38%
An unstable component responsible for drying, rancidity, yellowing,
embrittlement and cracking due to the rapid oxidation of the
paint film. The higher the percentage, the more rapidly the oil
changes.
| Acid | Walnut | Linseed | Effect |
| Oleic | 17% | 14% | Insures stability. The higher the percentage, the less likely the oil will turn rancid or crack upon drying. |
| Linoleic | 60% | 42% | A stable component responsible for the drying of the oil to a tough, flexible film. The higher the percentage, the better the film. |
| Linolenic | 12% | 38% | An unstable component responsible for drying, rancidity, yellowing, embrittlement and cracking due to the rapid oxidation of the paint film. The higher the percentage, the more rapidly the oil changes. |
Because of its high linolenic acid content, linseed oil forms what is known as conjugated fatty acids which, when combined with oxygen from the air, form polyunsaturated hydroxy compounds causing both extreme yellowing and brittleness. Linolenic acid, unfortunately is one of the most unstable fatty acids in the vegetable world. As the technical staff at one of the largest suppliers of linseed oil in the world, point out: "linseed oil is not well suited for artists color because it yellows badly, becomes brittle and turns rancid due to its high linolenic acid content. In fact the characteristic odor associated with linseed oil is caused by its decomposition or rancidity".
Where did the rancidity controversy begin?
A woman named Mrs. Mary P. Merrifield was hired by the English government in 1845 to procure information on the traditional oil painting methods, of the Italian Renaissance, from manuscripts and interviews. In every manuscript Merrifield quotes, walnut and linseed oils are used interchangeably with walnut oil being preferred because of its lack of yellowing. In only one instance, during an interview, does she quote a mid 19th century painter, "Sig. C." as saying that Titian painted both with nut oil and with linseed oil but that he (Sig C.) found linseed oil was better because the "Nut oil soon becomes rancid".
So there we find it. One quote, opposing scores of others, gets repeated throughout history even into modern literature including the well respected work of Ralph Mayer.
It is interesting to note, at the time Titian painted, artists manufactured their own color and mediums. One recipe for "boiled oil", included the addition of garlic cloves and was tested for cooking completion by dipping a hen's feather into the roiling liquid to see if the feather burned. Is it any surprise that oils occasionally became polluted or rancid?
After much research,
we find that one opinion and findings from some primitive chemistry
are wrongly repeated, even today, in spite of the facts proven
by modern technology.
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Why do some contend that linseed oil is superior to walnut oil?
Linseed oil is commonly available and very inexpensive (one third or less the cost of walnut oil) due to its extensive use in commercial housepaints and varnishes. For obvious reasons manufacturers using linseed oil would proclaim a superiority.
Because of the lack of commercial importance of drying oils in artists applications, and lack of funding for dedicated fine arts research, many authors of today's painting manuals rely on research done in Germany during the 1920's by Alexander Eibner.
Eibner studide the drying of linseed, walnut and poppyseed oils, without pigments, when applied on glass plates. Eibner assumed from variations in the weight of the dry oil films that linseed oil would shrink less over time and therefore be more stable. He failed to take into account the effect of pigments, supports and application methodology normally used by artists, as well as, variations in temperature and humidity. Most importantly, he never continued his tests to verify or disprove his conclusions.
Modern chemists, familiar with oil and coatings technology, characterize Eibner's work as untested, inconclusive, full of unsubstantiated assumptions and of little practical use.
According to industrial paint and coatings specialists, familiar with the properties of both walnut and linseed oils, they typically substitute one for the other in practical applications except where the cost of walnut oil or the yellowing of linseed oil is a factor.
-An "F.Y.I." publication from M. Graham & Co.; used with permission.