DEFINITIONS OF OIL PAINTING TERMS

A BRIEF HISTORY

From the time of the Greeks the chemistry of art and the chemistry of medicine were closely related and the recipes used for both were frequently written in the same books. These recipes were kept throughout the early centuries of Christianity by monks until their broader use outside of the monasteries in the middle ages. The use of drying oils is recorded among these recipes, listing walnut oil, poppy oil, hempseed oil, castor oil, and linseed oil as varnishes to seal pictures and protect them from water. Adequately thickened, they became resinous in and of themselves and therefore worked as varnishes quite well. Later on, yellow pigments were added to the oil and it was spread over tin foil to mimic the look of gold leaf, but at less cost. And as early as the thirteenth century oil was used for painting details over tempera pictures. Cennini describes the preparation and use of oils in painting on all surfaces.
Oils were purified and bleached in the sun, and drying time was decreased by the addition of metallic oxides such as Litharge or White Lead. Other methods of preparing oil by boiling and mixing with various substances is recorded throughout the middle ages, into the Renaissance, and beyond.
The procedures involved in making a usefully fluid medium with which to paint entire pictures in great detail were perfected by the brothers Van Eyck in the first half of the fifteenth century. From these Flemish artists and their students it is rumored that the new methods were spread to Italy by Antonella da Messina where, "once adapted to Italian taste, subjects, and dimensions, (the new way of painting) was received with enthusiasm."

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DEFINITIONS

Alla Prima Painting	Painting, usually from life, in a direct manner: Completing a painting in a single session or while the paint is still wet. In past eras used primarily as a means of sketching, but which became a means of producing finished works of art by the impressionists.
Chiaroscuro	A method of painting that represents boldly contrasting lighting, usually drawing highlights out of a dark scene. Also an element of this effect in any picture.
Chroma	The degree of brilliance of color away from neutral value. [color saturation]
Double Ground	Two superimposed paint layers of distinct color covering a sized panel or canvas as a surface upon which to paint.
Fat	Containing a large amount of oil
Fat-Over-Lean	The rule of painting in layers in which each successive layer of paint should have more oil than the preceeding layer. By increasing the oil content, top layers have increasing degrees of flexibility, reducing the risk of cracking or flaking.
Gesso	Gypsum (calcium sulphate) mixed with animal glue and applied as a ground to a wood substrate. Used in Southern Europe (primarily Spain and Italy). Northern Europeans used a similar ground of chalk (calcium carbonate) in a glue binder. A first, coarse layer was known as gesso grosso. While a smooth top layer which could be polished to a fine tooth was called gesso sottile. Some later artists applied only one layer of gesso sottile.
Glaze	A film of transparent color laid over a dried underpainting.
Grisaille	Monochromatic painting usually in various tones of gray. Traditionally the underpainting of a work, where local color is applied over the grisaille as opaque, semi-opaque or transparent color. Often shadows are colored with transparent colors and highlights are built up with increasing thickness of opaque paint.
Ground	The primary surface on which color is applied. Usually refers to an opaque coating rather than the support. Traditionally opaque white oil priming on canvas and chalk or gypsum mixed with animal glue (gesso) on wood panel. White acrylic polymer can be used on either surface. If a colored isolation layer (imprimatura) is used as the primary surface, it can be considered the ground. (see also "Toned Ground")
Highlight	The lightest tone in a painting.
Hue	The simple color of a substance, for example: red, bluish red, or yellow-red.
Impasto	Painting thickly with a bristle brush or palette knife in order to create surface texture.
Imprimatura	An isolation layer consisting of pigments bound in an oil medium and applied over chalk or gesso grounds to prevent the medium in the subsequent paint layers from being absorbed by the ground. Could be bright to dark, transparent or non-transparent. Color provides a middle tone from which one can quickly move between lights and darks to produce a full value painting. Should be mid-tone or lighter -- extremely dark underpainting can show through top layers as the work ages, especially when using lead white. In modern usage on oil primed canvas, "imprimatura" is often used to describe a transparent stain of oil color that is applied to the entire surface to create a unifying midtone. Most common colors: brown, earth-red, grey, or grey blue.
Lightfast	Resistant to fading when exposed to sunlight. Absolute measurement in artists' pigments; relative measurement when applied to industrial coatings applications. Example: a ten-year house paint would be considered lightfast if it resisted fading for ten years. Artists' pigments are judged in terms of centuries.
Local Color	The true or actual color of an object (as compared to the color effect it produces when viewed as part of a whole composition or when influenced by light or atmospheric conditions in nature or by the technique and intentions of the painter in a work of art.)
Medium	A liquid additive used to control the application properties of paint, its drying time, and the elasticity of paint film when dry. In oil painting this usually contains combinations of drying oils, varnishes, balsams, essential oils or solvents, and driers.
Modeling	Indicating the three-dimensional form of an object by the appropriate distribution of different tones. Creating the illusion of volume by painting the effects of light and shadow on form.
Monochromatic Underpainting	A preliminary painting in tones of one color. Overpainted with transparent, semi-opaque, and / or opaque color. See Grisaille above.
Palette	The implement upon which a painter holds or mixes his colors. Or a selected assortment of colors chosen for use in a painting technique.
Pentimento	The visibility of line or color through the increasingly transparent overpainting which was originally used to conceal it. Ghost image. A characteristic of linseed oil since its refractive index increases with age.
Polymerization	An internal molecular realignment brought about by external force which changes the properties of a substance and increases its molecular weight without the addition of any new ingredients. Example: the external force of oxygen upon a drying oil.
Prime / Primer	To cover a surface with a preparatory coat of color. A first coat or layer of paint, size, etc., given to any surface as a base, sealer, or the like. Often used to describe a pigment and oil (paint) ground applied to cloth such as canvas or linen. (see "Ground" above) In 15th century Europe, the guilds of St. Luke recognized the profession of the panel maker as an independent craft within the guild. Artists could purchase panels "primed and prepared" with an imprimatura from such workshops, eliminating this slow and dirty job from their studios.
Scumbling	Scraping or scrubbing or dragging a thin layer of lighter opaque or semi-opaque color over a dark underpainting with a bristle brush, allowing the underpainting to show through.
Tone	The degree of lightness or darkness of a color.
Toned Ground	Where color is mixed with white as a primer to provide a uniform opaque ground.
Underpainting	Preliminary painting, over which successive layers of color are added. Can be monochrome or colored.
Value	Degree of lightness and darkness.
Varnish	Protective surface film imparting a glossy or matt surface appearance to a painting.
Vehicle	The liquid into which a pigment is ground in order to turn the dry powdered pigment into a liquid paint. The carrier of pigment.
Verdaccio	Greenish underpainting