Glossary
modeling clay
A non-hardening substance used for modeling sculptures-- for sketches, models for casting, and by students. It is plastic (in the sense of being workable). It cannot be used for permanent work (unlike ceramic, water-based clays, it is never fired or glazed). Although it becomes less useful as its oil either dries or is absorbed from it (making it brittle) or as it picks up impurities, but it can be reused for many years if kept relatively clean. Even new it can vary greatly in quality. Most common varieties are made of clay mixed with petroleum greases, oils (typically linseed oil), and a pigment. It softens as it is modeled by the hands (because of their warmth), pieces joined to each other by pressing them together and blending with fingertips. Equipment that might be used with modeling clay includes modeling tools and armature. Molds can be made from modeling clay. There are numerous commercially produced types, including the brand named Plasticine. (Hint: before washing it off of hands and other surfaces, scrub with a dry paper towel.) It is sometimes called model clay.
offset printing
The printing process in which an inked image on a metal or paper plate is transferred to a smooth rubber cylinder and then to the paper.
See Also: lithography, planography
oil gilding
Also known as mordant gilding, this is attaching gold leaf to a non-absorbent surface which has been coated with a mordant size (an adhesive)-- either a slow-drying type containing linseed oil, or a quick-drying type sometimes called Japan gold size. The leaf must be applied only when the mordant size is no longer wet, but is somewhat sticky or tacky. If the surface is absorbent, it must first be sized with some kind of shellac, varnish, or paint. Oil gilding is generally easier than water gilding. One advantage is that oil gilding can be applied to wood or stone without first coating them with gesso. However, it cannot be burnished (water gilding can be), and it is not so long-lasting (the oil likely to darken or to show some other decay).
See Also: fire gilding, gilt
oil paint
Slow drying paint made when pigments are mixed with an oil, linseed oil being most traditional. The oil dries with a hard film, and the brilliance of the colors is protected. Oil paints are usually opaque and traditionally used on canvas.
oiling out
The practice of rubbing linseed or drying oil on the surface of a cured oil painting to even out the luster.
one-point perspective
A form of linear perspective in which all lines (describing straight edges that go from points nearer to points farther) appear to meet at a single point on the horizon.
opacity
The quality of being opaque. In painting, the power of a pigment to cover or obscure the surface to which it is applied.
opaque
Something that cannot be seen through; the opposite of transparent, although something through which some light passes would be described as translucent. (pr. oh-pake')
opaque projector
A device using a bright lamp, lens and mirrors to project an enlarged image of an opaque image or object onto a flat surface, usually so that its image may be traced. At the end of the 19th century, it replaced a somewhat similar device called a camera lucida, which had earlier replaced another called camera obscura. There are various other sorts of projectors also in use today.
orders of architecture
A system for classifying Classical architecture by the capital of a column. The three original types of orders are Doric, Ionic, and Corinthian, although the Tuscan, colossal, and Composite orders have also been added.
Example: Bramante's Tempietto, Rome, Italy, 1502, has a Tuscan colonnade.
organic
An irregular shape, or one that might be found in nature, rather than a regular, mechanical shape.
orichalcum
A yellow bronze, an alloy of copper and zinc, resembling gold when new. Its name comes from two Greek words: oros meaning mountain, and chalkos, brass. The Romans made two coins made of orichalcum: the sestertius and the dupondius.
origami
Originating in Japan, the art or process of folding paper into shapes, representing animals, for example. A decorative object made by folding paper. (pr. o-ri-gah'me)
ormolu
Bronze or brass which has been gold leafed and used in decorating certain styles of furniture, clock-cases, chandeliers, and jewelry. Ormolu is cast and chiseled, then finished with gold leaf.
ounce
A unit of dry as well as liquid measurement (in the US). To convert ounces (US dry) into grams, multiply by 28.3495; into pounds, divide by 16. To convert ounces (US fluid) into cubic inches, multiply by 1.80469; into liters, x 0.02957; into pints, x 0.0625; into tablespoons, x 2. Abbreviated oz.
overlay
Something-- perhaps a layer of paint or some other material, such as wood veneer or gold leaf-- that is laid over or covers another surface. It may also be a transparent sheet like an acetate or cell containing images in some areas, which is placed atop another image to be incorporated into it.
See Also: animation, bleeding through
overpainting
A finishing layer of paint applied over another layer of paint, or underlayer, once it has dried.
See Also: fat over lean, glaze, pentimento, scumble, dragging
oxidation firing
A glazing technique in which additional oxygen is introduced into a kiln while firing.
oxide
Any element combined with oxygen. Common rust is iron oxide, one of the most widely used colorants-- making red bricks red for example. Silver's oxide is more often referred to as tarnish. Bronze's is more often referred to as a patina. Pure gold cannot oxidize.
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