Paintings are made of paint applied to a surface, commonly canvas, wood, or plaster. In most paintings, the pigments are suspended in the paint media. Common media include oil and egg yolk. Both substances undergo chemical change in the air, and convert into a plastic-like film. Although called "dying," what is really happening is a chemical change (so called "polymerization"), which makes the media hard.
The painting process for all forms of artist paints is similar in many ways. Generally speaking, first a surface is prepared; next a sketch is often drawn.
An oil painting begins with a base or support, which consisted of canvas stretched upon a frame. Canvas was easier to work with, lent a more natural look to the picture and looked less flat than wood panels.
To prepare a canvas, the artist would first stretch it on a wooden frame, as shown at right. Next, a smooth layer of gesso (white calcium sulfate, plaster of Paris, bound with animal glue) would be applied to seal the canvas. That would be followed by the pigment lead white, to secure the upper pigment layers. Without this priming, paint directly applied would soak into the surface and be difficult to control.
Until paint was produced commercially during the Industrial Revolution (circa 1800), painters had to make their own paints by grinding pigment into oil. The paint would harden and would have to be made fresh each day. Paint consists of small grains of pigment suspended in oil. Although it appears smooth to the naked eye, on a microscopic level, particles of pigment are suspended in oil, as fruit in a Jello mold.
Oil paints do not "dry" by evaporation (as do watercolor paints); rather they harden through chemical reaction, as Jello sets. Contact with the air causes oils to oxidize and to crosslink. The paint sets and hardens over time. Paint of different pigments dry at different rates. Charcoal black retards the drying (creating a slow-drying paint); ochre accelerates the drying (producing a quick-drying paint).
Paint brushes were made with hog''''s-bristle or miniver (squirrel fur). Paintings always consist of many layers of paint, as seen in the cross-section. Colors can be subtly modulated by applying tinted glazes. A glaze is a very thin oily paint that is transparent, enhancing the colors below. In contrast, opaque pigments would block the layers underneath.
Varnish is resin dissolved in spirits (volatile solvents) that evaporate as the varnish dries. It imparts a brilliant finish, intensifying color contrast, as a brick building appears darker after a rain shower.