ONCE FIRED — A pot that has undergone a single glaze firing. The glaze is applied directly on to the dry or leather hard pot thus avoiding the bisque firing. This approach, although offering certain economic and aesthetic advantages, can create technical problems for the potter.
OPAL GLASS — A term used for glass that is solid in white in color and is opaque. Opal glass will have little or no translucence and will not transmit light. Said of any glass into which a material has been introduced at the raw materials stage (usually fluorine or phosphorus) which causes a degree of crystallization to occur, and creates opacity in the glass. Reflected light is colorless, therefore white. The degree of opacity (and "whiteness") is variable depending upon composition and temperatures used in the manufacturing process. Commonly then, white glass is called "opal."
Solid Color Opalescent Glass: Glass which is both colored and crystallized, creating a single color sheet, more opaque than a cathedral. Sometimes called "opaque" glass.
Mixed Opalescent Glass: White glass (opal) mixed with one or more other colors to create a variegated, multi-colored sheet. Light transmission varies with composition. Also called "variegated opalescent," sometimes "streaky."
OPALESCENT — meaning semi-translucent, refers to white colored glass that shows some orange or “fire” translucence when held up to a direct light source, usually around the edges of the piece, flowing into a different color of glass.
OPEN — To make a clay more open or porous in structure by adding fillers or grog.
ORMULU — Refers to decorative metal added to an object made of glass.
OVERSHOT — Achieved by rolling a gather over a steel plate on which small glass particles are placed, reheating the object to melt the sharp edges of these fragments, and then blowing the object to its final size and form, giving the object a rough texture and nicely speckled
OXIDE — Any element combined with oxygen.
OXIDATION — A firing where there is either no combustion occurring (electric kiln) or where there is sufficient oxygen in the kiln to allow the fuel to burn cleanly. The atmosphere of the kiln (oxidation, or reduction) dramatically affects the resulting clay and glaze colors, for example; copper in oxidation is green (as is copper oxide) in reduction it becomes red (more like copper metal).