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In relief printing, a flong is a temporary negative mould made of a forme of set type, in order to cast a metal stereotype (or "stereo") which can be used in a rotary press, or in letterpress printing after the type has been broken down for re-use. The process is called stereotyping.
Invented in Lyon in 1829 by the French printer Claude Genoux, a flong was a papier-mâché mould made with the aid of heat and pressure of a set forme of type. After placing the flong in a casting box, a thin replica of the original (metal) type (and illustrations) would be cast against it all in one piece. A limited number of duplicate casts could be made from one flong. The back-shaved stereo plate would be attached to a press cylinder or honeycomb base and allow the original undamaged type to be distributed or recycled. The stereotypes could be stored for repeat editions in much less space than standing forms.
A further improvement to the technique was made in 1893, when the dry flong replaced the wet flong. More recently, flongs have been made of phenolic resin boards (perhaps also plastic and rubber) and are still used in places to cast rubber stamp sheets instead of type metal stereos, the last commercial use is certain types of rubber relief printing mats (sheets) for flexo printing. They remained widely in use until the invention of offset lithography in the late 19th century led to rotary presses being mostly replaced by the new technology.
A process called electrotyping produced a similar result using electrodeposition of copper usually to form a plate replica of the printing forme.
The word is derived from the French flan; or the Latin word "flana".
In popular culture
The Flong were the indigenous population of the fictional island of San Serriffe in British newspaper The Guardian's 1977 April fool story. This was only one of many printing and typographical puns in the stories.