Sold under a variety of names that may or may not contain the word alkyd, these mediums are synthetic resins that are excellent for oil painting because they dry quickly.
They work as a binder that encapsulates the pigment and speeds the drying time. Varieties made with safflower, soy beans and tobacco-seed oils hold color better than those made with linseed oil. Some alkyds look thick and tan colored in the container, but they become smooth and transparent when added to paint.
The term alkyd was introduced in 1970 by Winsor & Newton who applauded its virtues of being similar to acrylic paint but drying faster.