Oil Paint

Oil paint is a type of slow-drying paint consisting of small pigment particles suspended in any light carrier other than water. Oil paints have been used in England as early as the 13th century for simple decoration^[1], but were not widely adopted for artisic purposes until the 15th century. The most common modern application of oil paint is domestic, where its hard-wearing properties and luminous colours make it desirable both interior and exterior use.

History

The slow-drying properties of organic oils were commonly known to early painters. However, the difficulty in aquiring and working the materials meant that they were rarely used. As the public preference for realism increased, however, the quick-drying tempera paints became insufficient. Flemish artists used mixed tempura and oil painting during the 1400s, but by the 1600s easel painting in pure oils was common, using much the same techniques and materials found today.

Carrier

When exposed to air, vegetable oils do not undergo the same evaporative process that water does. Instead, they oxidize into a dry solid. Depending upon the source, this process can be very slow, and it is this property which gives oil paints their unique characteristics.

This earliest and still most commonly used vehicle is linseed oil, made from the seed of the flax plant. The seeds are crushed and the oil extracted. Modern processes use heat or steam in order to produce a larger volume of oil, but cold-pressed oils are generally considered superior for artistic use^[2]. Other sources of carrier oils exist. poppies, walnuts, and soy beans, are often used as a substitute for the relatively expensive linseed.

Once the oil is extracted additives are sometimes used improve its chemical porperties. in this manner the paint can be made to dry more quickly if that is desired, or to have varying levels gloss. Modern oils paints can have vey complex chemical structures due to this, for effects such resistance to UV or a suede like appearance.

Pigment

The colour of oil paint derives from the small particles mixed with the carrier. Common pigment types include mineral salts such as white oxides: lead, zinc and titanium, and the red to yellow cadmium pigments. Another class consists of earth types, e.g sienna or umber. Synthetic pigments are also now available. Natural pigments have the advantage of being well understood through centuries of use but synthetics have a greatly increased the spectrum available, and many are tested well for their lightfastness.

Usage

The main disadvantage of oils paints is the relative complexity of use when compared to acrylic paint or tempera. The carrier is usually highly resistant to water and requires some sort of solvent such as turpentine or benzene to clean up. These are toxic must be handled with care. The pigments may also be dangerous. Lead is toxic, which is an important reason why it has mainly been replaced with zinc and titanium. Cadmium can cause cancer with prolonged inhalation. Both the carriers and the materials used to clean them are also highly flammable. Paper or rags soaked in pure linseed oil are known to spontaneously ignite.

Oil painting is done on surfaces with pigment ground into a medium of oil — especially in early modern Europe, linseed oil. Other oils occasionally used include poppyseed oil, walnut oil, and safflower oil. These oils result in different properties in the oil paint, such as less yellowing or different drying times. The oil usually takes weeks to dry.

It was probably developed for decorative or functional purposes in the High Middle Ages. Surfaces like shields — both those used in tournaments and those hung as decorations — were more durable when painted in oil-based media than when painted in the traditional tempera paints.

Many Renaissance sources credit northern European painters of the 15th century with the "invention" of painting with oil media on wood panel — Jan van Eyck is often mentioned as the "inventor". The popularity of oil grew in 16th century Venice, where a water-durable medium was essential.

Recent advances in chemistry have produced modern water miscible oil paints that can be used with, and cleaned up in, water. These are still "real" oil-paints in every sense of the meaning. Small alterations in the molecular structure of the oil creates this water miscible property.

A still-newer type of paint, heat-set oils, remain liquid until heated to 265–280 °F (130–138 °C) for about 15 minutes. Since the paint never dries otherwise, cleanup is not needed (except when one wants to use a different color and the same brush). Although not technically true oils (the medium is an unidentified "non-drying synthetic oily liquid, imbedded with a heat sensitive curing agent"), the paintings resemble oil paintings and are usually shown as oil paintings.

Process of oil painting

The process of oil painting varies from artist to artist, but often includes certain steps. First, the artist prepares the surface. Although surfaces like linoleum, wooden panel, linen, and cardboard have been used the most common surface is canvas. Canvas is a cotton fabric with thick fibers. The artist first prepares a wooden frame called a “stretcher" or a "strainer." The canvas is then pulled across the wooden frame and tacked or stapled tightly to it. The next step is for the artist to apply a ground to isolate the canvas from the acidic qualities of the paint. Traditionally, the canvas was coated with a layer of rabbit skin glue and primed with subsequent layers of finely ground chalk and rabbit skin glue. Later the process was changed to a priming of rabbit skin glue with subsquent layers of whiting (gypsum, chalk, barium dioxide, titianium dioxide) mixed with linseed oil. Modern gessos are made of titianium dioxide with an acrylic binder. The artist might apply several layers of gesso, sanding each smooth after it has dried.

Next the artist might sketch an outline of his intended figure, called a “cartoon.” Then he begins to apply the pigment to the surface. “Pigment” may be any number of natural substances with color, such as sulfur for yellow or cobalt for blue. The pigment is mixed with oil, usually linseed oil but other oils may be used as well. The various oils dry differently creating assorted effects. Traditionally, an artist mixed his or her own paints for each project, but in the late 1800’s paint in tubes became available. Artists then could mix standard colors easily to create subtle variations of hue.

The artist most often uses a brush to apply the paint. Brushes are made from a variety of fibers to create different effects. For example, brushes made with hog’s bristle might be used for bolder strokes. Brushes made from miniver, which is squirrel fur, might be used for finer details. Sizes of brushes also create different effects. For example, a "round" is a pointed brush used for detail work. "Bright" brushes are used to apply broad swaths of color. The artist might also apply paint with a palette knife, which is a flat, metal blade. A palette knife may also be used to remove paint from the canvas when necessary. Some artists even paint with their fingers.

Most artists paint in layers. The first coat or "underpainting" is laid down first, painted normally with turpentine thinned paint. This layer helps to "tone" the canvas, and cover the white of the gesso. Many artists use this layer to sketch out the composition. After this layer dries, the artist will then begin to paint a "mozaic" of color swatches, working from darkest to lightest. The borders of the colors are blended together when the "mozaic" is completed. This layer is then left to dry before applying details. After it is dry, the artist will apply "glazes" to the painting, sometimes using a process of "Fat over Lean" which means more oil/paint ratio than the previous layer. A classical work might take weeks or even months to layer the paint properly. Artists in later periods such as the impressionist era often blended the wet paint on the canvas without following this layering method. This method is called "Alla Prima." When the image is finished and dried for up to a year, an artist would seal the work with a layer of varnish typically made from damar crystals dissolved in turpentine.