Pigments are ground and mixed with a drying oil, commonly known as a vehicle or binder. They are called binders because they suspend the pigment in the oil, making it easy to apply the paint on the painting surface. Eventually, the oil dries as it absorbs oxygen, sealing the pigment to the surface. There are several types of organic binders used in painting, but I will review the most common of them. These include linseed, stand, sun-refined or sun-bleached, poppy seed, walnut and safflower oils.
Linseed Oil
The most commonly used binder and oil medium is linseed oil. Linseed oil is made by pressing the ripe seeds of the flax plant. There are two types of linseed oil, hot pressed and cold pressed. Inexpensive linseed oil is made by applying extreme pressure and heat. After hot pressing, any oil left in the seeds is extracted using solvents. The solvent is evaporated, the remaining oil is then added to the hot-pressed oil which undergoes a refining process. Cold-pressed linseed oil is made with some pressure, but no heat. It is the purest form of linseed oil because it has superior binding ability and is less likely to become brittle with age compared to hot-pressed oil. Refined linseed oil is oil refined with sulfuric acid and water, removing more of the impurities. It does not bind as well as cold pressed.Linseed oil is used as both a binder and as a painting medium. It is usually the preferred choice for clear painting oils because of its durability. Manufacturers usually base their binder on linseed oil and may mix in or substitute poppyseed oil according to their preference. Though linseed oil dries quickly at first, the complete drying process takes several years. As it ages, linseed oil dries into a tough, leather-like film that hardens and becomes more transparent with age.
Stand Oil
Stand oil is made by heating linseed oil to 525-575° F and holding that temperature for a number of hours. A molecular change occurs, though it doesn’t change the physical properties of the oil. Stand oil is a heavy oil, similar in consistency of honey. It is usually mixed with several parts of turpentine, resulting in a paler color than other linseed oils. Artists prefer stand oil mixed with thinners as a glazing medium, a paint mixing medium, and with varnishes. Because of its unique ability to dry to a smooth, enamel-like film free of brush strokes, it is a superior binder for glazing. And, mixed with other ingredients, the resulting medium ages well and is nearly non-yellowing.Sun-Refined or Sun-Bleached Oil
Sun-refined or sun-bleached oil is made by combining oil with an equal amount of water and then exposing it to sunlight and limited air exposure for several weeks. At the end of this period, the impurities will settle to the bottom and the oil is filtered and separated from the water. The result is a thicker, more viscous oil, which hampers its binding abilities, but speeds up drying time. This type of oil is more suitable for a clear varnish, glaze, and painting medium. However, it is more prone to yellowing than cold-pressed and refined oils.
Poppyseed Oil
Poppyseed oil is a colorless to straw-colored oil pressed from the poppy seed. It is most often used as a binder mixed with whites and pale colors as it is less prone to yellowing than linseed oil. However, compared with linseed oil, it dries much slower. The film is spongy and has a tendency to crack, especially when the pigment is layered. Poppyseed oil is most often used by artists in direct or alla prima single layer painting.Walnut Oil
Walnut oil is derived from the stale kernels of the common or English walnut. The film of walnut oil when dry is stronger than Poppy oil (though still not as strong as linseed) which makes it a better oil to use in the initial layers of paint. It is a great oil to use when painting detail and it has a similar drying time to linseed oil. Though it dries as quickly as linseed oil, it has a tendency to grow rancid with storage and is not recommended as a paint medium.Safflower Oil
Safflower Oil is also used to make whites in some brands because it is bright and clean with less tendency to yellow than linseed oil. It takes 2-3 days longer to dry than linseed so is recommended only for use in the final layers of a painting.More on Binders
Linseed, poppyseed, walnut and safflower oils may be mixed with one another. You can find binders mentioned here at most art supply stores. You should experiment with them, but keep in mind the properties of each and their shortcomings.For more information about binders, check out The Artist’s Handbook of Materials and Techniques by Ralph Mayer.
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