Drying oil is a natural oil that oxidizes when exposed to air, causing it to contract and harden into a solid layer or a film that binds pigments together. oils used in oil painting such as Linseed oil, Poppy, Walnut, and Safflower, as binders, each oil brings its own characteristics to the personality of the paint.
The Fat Over Lean Rule
The oxidization process of oils is very slow. As oil paint oxidizes it also contracts. If anything, that dries faster is put on top of an incompletely dry oil paint layer and it will simply crack.
Oils should always be painted fat over lean: the more oil (or fat) in the paint the longer it will take to dry, so, always put paint with more oil over the top of paint that has less oil in it (more diluted with solvent).
Notice: Poppyseed oil dries much more slowly than linseed oil. For this reason, poppyseed oil should not be used for a ground layer of a painting, and linseed oil should not be painted over a layer of poppyseed oil.
Binders or Drying Oils Used in Oil Painting
Linseed Oil
Linseed oil dries very thoroughly and so creates very stable paint. Both Refined Linseed Oil and Cold-pressed Linseed Oil are known to be used in oil paint manufacture.
The oil is obtained by pressing, sometimes followed by solvent extraction. For artistic grade linseed oil, the extraction process may include the addition of petroleum which makes it only really suited for art purposes.
Linseed oil will have to be purified from impurities in it because that will cause a greater degree of yellowing over time. Refined linseed oil, with the clearest look in the bottle, is likely to have less impurities and so will have less tendency to yellow over time.
Cold pressed linseed oil has not undergone any chemical treatment and so is therefore often considered the more stable of the two varieties of oil.
Linseed oil is a colorless to yellowish oil obtained from the dried, ripened seeds of the flax plant used by artists as a polymer-forming pigment binder in oil paints,
Linseed oil as a triglyceride is distinctive for its unusually large amount of α-linolenic acid, which oxidizes in air turning to a water-repelling (hydrophobic) hydrocarbon-based polymer.
Having a high content of di- and tri-unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air.
This polymerization, which is called autoxidation, results in the rigidification of the material. To prevent premature drying, linseed oil-based paints are stored in airtight containers.
Linseed oil as a binder used in oil paint makes oil paints more fluid, transparent and glossy. It is available in varieties such as cold-pressed, alkali-refined, sun-bleached, sun-thickened, and polymerized (stand oil). The introduction of linseed oil was a significant advance in the technology of oil painting.
Another usage of boiled linseed oil is for sizing in traditional oil gilding to adhere sheets of gold leaf to a canvas. It has a much longer working time than water-based size and gives a firm smooth surface that is adhesive enough in the first 12–24 hours after application to cause the gold to attach firmly to the intended canvas.
The fatty acids in a typical linseed oil are of the following types:
The triply unsaturated α-linolenic acid (51.9–55.2%),
The saturated acids palmitic acid (about 7%) and stearic acid (3.4–4.6%),
The monounsaturated oleic acid (18.5–22.6%),
The doubly unsaturated linoleic acid (14.2–17%).
By heating linseed oil near 300 °C for a few days in the complete absence of air.
The product, which is highly viscous, gives highly uniform coatings that "dry" to more elastic coatings than linseed oil itself.
Coatings prepared from stand oils are less prone to yellowing than coatings derived from the parent oils.
Notice: Rags soaked with linseed oil pose fire hazard because they provide a large surface area for rapid oxidation.
Poppy Oil
Poppyseed oil or poppy oil is obtained from poppy seeds (specifically seeds of Papaver somniferum, the opium poppy).
Poppy Oil is a very pale, more transparent drying oil that is less likely to yellow than linseed oil.
Due to Poppy oil drying properties, it's used in oil painting for binding pigment, thinning paint, and varnishing finished paintings. Poppy oil is used especially in white paints.
While poppyseed oil does not leave the unwanted yellow tint for which linseed oil is known, it is much weaker in the test of time than the contemporary linseed oil.
Poppy Oil is much slower drying than linseed oil – on average 5-7 days – which makes it ideal to use when working wet-into-wet.
Recently the use of less expensive Safflower oil has become more common.
Poppy Oil has little or no odor and a pleasant taste, and it is less likely than some other oils to become rancid.
Poppyseed oil dries much more slowly (5–7 days) than linseed oil (3–5 days). For this reason, poppyseed oil should not be used for a ground layer of a painting, and linseed oil should not be painted over a layer of poppyseed oil.
Walnut Oil
Walnut oil was one of the most important oils used by Renaissance painters. Its short drying time and lack of yellow tint make it a good oil paint base thinner and brush cleaner.
Walnut oil is oil extracted from walnuts, Juglans regia. The oil contains polyunsaturated fatty acids, monounsaturated fatty acids, and saturated fats. Walnut oil can turn rancid.
The film formed by walnut oil when dry is stronger than Poppy oil but still not as strong as linseed oil film.
Walnut oil yellows less than linseed oil, as a result, Walnut Oil is used to make whites. It is a great oil to use when painting detail and it has a similar drying time to linseed oil.
Walnut oil paints rich finish thought to have helped Renaissance painters achieve high quality oil paintings with soft light.
Walnut oil is composed largely of polyunsaturated fatty acids (72% of total fats), particularly alpha-linolenic acid (14%) and linoleic acid (58%), oleic acid (13%), and saturated fats (9%).
Cold-pressed walnut oil is typically more expensive due to the loss of a higher percentage of the oil. Refined walnut oil is expeller-pressed and saturated with solvent to extract the highest percentage of oil available in the nut meat. The solvents are subsequently eliminated by heating the mixture to around 400 °F (200 °C).
Safflower Oil
Chemical analysis of ancient Egyptian textiles dated to the Twelfth Dynasty (1991–1802 BC) identified dyes made from safflower, and garlands made from safflowers were found in the tomb of the pharaoh Tutankhamun.
There are two types of safflowers that produce different kinds of oil: one high in monounsaturated fatty acid (oleic acid) and the other high in polyunsaturated fatty acid (linoleic acid).
Currently, the predominant edible oil market is for the former, which is lower in saturated fats than olive oil. The latter is used in painting in the place of linseed oil, particularly with white paints, as it does not have the yellow tint that linseed oil possesses.
Safflower Oil takes 2-3 days longer to dry than linseed so is recommended only for use in the final layers of a painting.
The oil content of the seeds varies between 20 and 40%. Safflower petals contain one red and two yellow dyes. In coloring textiles, dried safflower flowers are used as a natural dye source for the orange-red pigment carthamin.
Oils Used as Solvents in Oil Painting.
Turpentine Oil
Turpentine or the spirit of turpentine, is a fluid obtained by the distillation of resin harvested from living trees, mainly pines. Principally used as a specialized solvent, it is also a source of material for organic syntheses.
As a solvent, turpentine is used for thinning oil-based paints, its use as a solvent in industrialized nations has largely been replaced by the much cheaper turpentine substitutes obtained from petroleum such as white spirit.
Turpentine is composed of terpenes, primarily the monoterpenes alpha- and beta-pinene, with lesser amounts of carene, camphene, dipentene, and terpinolene.
Notice:
As an organic solvent, turpentine vapor can irritate the skin and eyes, damage the lungs and respiratory system, as well as the central nervous system when inhaled, and cause damage to the renal system when ingested, among other things.
Ingestion can cause burning sensations, abdominal pain, nausea, vomiting, confusion, convulsions, diarrhea, tachycardia, unconsciousness, respiratory failure, and chemical pneumonia.
The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for turpentine exposure in the workplace as 100 ppm (560 mg/m3) over an 8-hour workday. The same threshold was adopted by the National Institute for Occupational Safety and Health (NIOSH) as the recommended exposure limit (REL). At levels of 800 ppm (4480 mg/m3), turpentine is immediately dangerous to life and health.
Clove Oil
Clove oil is used as a diluent (solvent) to delay the oxidation process of the binder, dry film-forming oils in oil painting.
The oil of clove, also known as clove oil, is an essential oil extracted from the clove plant, Syzygium aromaticum, which is native to Southeast Asia. It contains eugenol, a major component that has high free radical scavenging activity and is a powerful antioxidant.
Eugenol may have some antioxidants and ranks high in ORAC (Oxygen Radical Absorbance Capacity) value, which means that using it in a specific amount will delay the oxidation process of linseed oil which is the main oil used in oil painting as a binder.
As the percentage of Eugenol increases the reducing power of the clove oil increases, and that's directly proportional to the delay time of the oil painting.
There are three Grades of clove oil:
Bud oil is derived from the flower buds of S. aromaticum. It consists of 60–90% eugenol, eugenol acetate, caryophyllene, and other minor constituents.
Leaf oil is derived from the leaves of S. aromaticum. It consists of 70–82% eugenol and some amounts of beta Caryophyllene and alpha Humulene.
Stem oil is derived from the twigs of S. aromaticum. It consists of 85–92% eugenol, with other minor constituents.