In ancient times, dating back to the ancient Greeks, chemistry was not domain specific. The manipulation of oils for medicinal purposes or for artistic purposes utilized the same methodologies and formulas, and were protected by the same groups. Into the middle ages these groups were the monks of early Christianity, who were often tasked with copying and protecting knowledge so that it could persist through time.
In these scrolls were recipes for “drying oils”, which could be used as varnishes to protect paintings from water damage. Thickened oils such as poppy, hempseed, castor, and linseed were used for their varnishing capabilities.
If pigments are added to these thickened oils/varnishes, they could be used as paints, and hence the initial creation of oil paints and the first oil paintings. Early examples of oil paints were 13th century tempera painting, as well as all types of surfaces as described by
Cennino Cennini, a 14th century Italian painter.
The procedures for making these paints was improved over time. Some examples of the improvements include purifying oils, using sunlight to bleach them, additives such as lead and other metals, and mixing/boiling various concoctions.
It is said that a perfected procedure for oil paints was discovered by the Flemish Van Eyck Brothers, in the 15th century. The medium proceeded to gain popularity as it spread through Italy and was adapted by the popular Italian painters of the era.