Important Safety Information
Rags with linseed oil or paint stored in a pile should be considered a fire hazard because they provide a large surface area for oxidation. The oxidation of linseed oil is an exothermic reaction (heat producing), which speeds as the temperature of the rags increases. If the heat cannot disperse, the temperature increases and may eventually burst into flames - literally spontaneous combustion. PLEASE TAKE THIS SERIOUSLY. Many paint manufacturers and the artists they supply have suffered fires, many of mysterious origin. Practice extreme safety and avoid a fire.
Soft materials that have absorbed linseed oil should be stored in sealed containers, as the oil will oxidise when exposed to air. This chemical reaction releases so much heat that rags may spontaneously combust. Make sure the lid of the container is closed securely and stored in a cool place. Brushes and tools used with linseed oil should be cleaned thoroughly with white spirit.
The Drying Process of Linseed
"Drying" is a rather inaccurate way of describing the process. It's a term we typically use to describe evaporation, such as with watercolours; once the liquid water departs, we're left with a solid, soluble mass. Linseed oil is different. I sometimes feel the term "cure" would be more appropriate, though even then, from a chemist's point of view, this wouldn't be quite correct either. When we describe the process of oil paint becoming solid, it is actually through the process of polymerization, during which the linseed combines with oxygen to form a solid.
Other factors, such as the chemistry of pigments, can affect the speed. For example, one can speed the process with pigments that are metal based, such as earth colours containing iron. Iodine, which naturally occurs in linseed oil, plays a role in the drying time as well. A number of other variables influence drying speed, including light and warmth. School chemistry taught us that warmth speeds all chemical reactions, and not surprisingly, this is the case with linseed.
Oil paint manufacturers often include artificial drying agents, known in the paint-making trade as siccatives. These can harm the paint structure and cause numerous problems, from embrittlement to flaking to wrinkling. I avoid most siccatives, with the exception of their addition to two products: Paint Medium No. 2 (PM 2), a glaze medium; and Titanium White No. 3, added to the range for the frustrated London artists painting in cold studios. No. 3 includes only a minute amount of siccative. I want your paintings to last for at least 500 years, and if constructed upon sound support, they should.
When paint film containing linseed takes up even a small amount of oxygen, the paint expands, causing problems like wrinkling. As an analogy, imagine coming home to find your carpets had grown, actually increasing in surface area. The carpet would have developed rolls and wrinkles. If the body of the paint is low in oil content, like lead or cadmium-based pigments, the paint will resist this tendency to wrinkle unless an excess of linseed has been added.
To get the best clarity from linseed oil paints, it's always wise to let the painting dry in a good light. Ideally, you should place it near a window with good, strong daylight; even direct sunlight for a few days will do no harm. If you allow a painting to dry in bad light, the painting will develop a dull, yellow cast. This yellow affect will clear when the painting is in normal lighting conditions, but it will never be as bright or crisp as a painting dried in a well-lit environment.
Cold Pressed vs. Refined Linseed Oil
I am often asked whether my linseed oil is cold pressed or refined, and the answer is, both! We've been taught that cold pressed linseed oil (CPLO) is best for oil paint making, but this is not quite true; when the first pressing of flax seeds takes place, the extracted oil is quite dark and contains mucilage and other impurities, all of which tend to darken with age. If I were to make a white oil paint using a CPLO, since it's a darker oil, the resulting colour of the white would be darker. Our linseed oil is initially cold pressed and then even further refined, giving it a very light colour.
I was recently in the studio of my good friend Leo Mancini Heresko. A Boston-based artist and retired Florence Academy professor, Leo was extolling the virtues of his most recent experiments involving the refinement of CPLO. He had acquired a nice, rich-looking CPLO and combined it with water and sand, shaking and draining it. What he achieved is a very pleasing shade of pale linseed. He and I decided to make a further experiment using the linseed I use for paint-making. We shook it up with water for a few minutes and left it to stand a few hours. I was excited about perhaps being able to offer artists an addition to our mediums, but to our surprise, as the hours passed, there was no separation of mucilage. The good news: our oil was pure, and there was no need for further refinement!
Linseed oil comes from the seeds of the flax plant, the Latin name of which is Linum usitatissimum, meaning "most useful". Some claim man's relationship with the plant began 30,000 years ago in the wet and cool climates of Eastern Europe. Through the centuries, it became common in many cultures, mainly for its other major use - making linen. And yes, that also means artist canvasses. We artists owe a great deal to this small, beautiful, and most useful plant.
During the winter months, it is entirely normal for linseed oil to undergo a process known as clouding or thickening due to the lower temperatures. This natural occurrence does not affect the quality of the oil; however, it can make application challenging. To address this issue, simply warm the linseed oil before use. Placing the container in a warm water bath will help restore the oil to its liquid state.