Alkyd is a synthetic resin that was first developed more than a century ago for industrial use. Alkyd is a term coined from the original name 'Alcid', which indicated that it was derived from alcohol and acids. The use of alkyd as a binder in artist materials such as paint, medium, ground, and varnish is fairly recent. Alkyds used in artist paint and medium are made through a special reaction caused by combining polybasic acid, polyhydric alcohol, and the fatty acid of drying vegetable oil (soy, linseed, safflower). During production, the fatty acid becomes an integral part of the alkyd molecule; consequently, this type of alkyds is often referred to as 'oil-modified alkyds'.
There are many types of alkyds available to manufacturers of artist materials. The fatty acid content in each type of alkyd can vary greatly. Oil-modified alkyds used in artist paints and mediums require at least 60% fatty acid (i.e., long oil alkyds) to prevent the dried paint film from becoming too brittle. Driers and silica are often added to oil-modified alkyds to speed up drying time and give extra body. Generally, alkyds made with linseed oil are the most durable and weather-resistant, but on the downside, they are more likely to yellow than soy and safflower.
The main benefit of using alkyds is the fast drying time and uniformity of palette. A thin layer of alkyd paint (or oil paint used with alkyd medium) will touch dry within 24 hours. Additionally, alkyd paint tends to level more than oil paint and produces a fairly glossy, firm, and durable paint film. Alkyds may yellow slightly depending on the type of drying oil used in the production.
Alkyd-based materials are compatible for use with traditional oil-based materials. Alkyd paints can be intermixed with traditional oil paints and mediums; however, this will slow down the drying time. Likewise, traditional oil paint can be mixed with an alkyd medium to speed up drying time. Alkyd paints and mediums are soluble in mineral spirits or turpentine. Alkyds are sensitive to over-thinning. Too much thinning will lead to loss of adhesion. No more than 25% of mineral spirit or turpentine should be used to dilute alkyd paints or mediums. Once an oil paint is used with an alkyd medium, there should be no going back to a traditional medium; otherwise, there may be problems with adhesion. The fat over lean rule should be observed whether alkyds are used on their own or intermixed with traditional paints and mediums.