Flong

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". 
Types of flong.
Strictly speaking only moulds of the so-called papier-mâché wet process are flongs, but the term was applied more widely to moulds for stereos. The following have been used for moulding type to create stereos:
The process for making moulds for electrotypes was similar, except that these were made with soft materials such as beeswax or the naturally occurring mineral wax ozokerite. The thin electrotype shells had to be backed with type metal to a depth of 8mm make them robust enough for use.
The papier-mâché flong.
Partridge describes the papier-mâché process thus: "A few sheets of thin paper are soaked in water until soft and then pasted together to form a flong. This flong is beaten into a page of type and dried, thus forming a matrix to receive the molten metal, which, when cooled, becomes an exact duplicate of the type page. A large number of duplicate casts may be made from the same matrix, either in flat form as required for flat-bed presses, or curved to fit the cylinders of rotary presses." 
The flong was constructed by pasting together two sheets of wetted soft but tough matrix paper and four sheets of strong tissue paper. A rice-straw based tissue paper was used for the side of the flong facing the type. 
After making up the flong matrix, it can be kept for several days if kept suitably moist by wrapping in a wet blanket for example.
The flong slightly larger than the forme was laid over it and then carefully beaten into the forme of type using a brush with stiff bristles. Many gentle blows were better to fewer strong ones. Any hollows in the back of the flong after it was beaten in were filled, either with strawboard or pieces of flong or with a packing compound. The flong was then covered with a sheet of backing paper and moved, still sitting on the forme, to a steam drying table. Here it was covered with four to eight pieces of soft blanket and pressed down to ensure that the flong stayed in contact with the forme while it dried. Drying took six to seven minutes typically, but this depended on the steam pressure.
The golden rule for stereotyping was to have cool metal and a hot box to avoid problems with shrinkage cavities on the face of the plate or sinks, where the face of the plate shrank away from the front. Sometimes a casting board was used to slow the cooling at the back of the casting, as this could help to avoid problems due to the flong being a poor conductor. Before casting, the casting box was heated. This could be done by ladling hot type metal into it as many as three times and removing the resulting plate. Alternatively, the mould could be gas-heated.
The dry flong was then trimmed, leaving just enough of a margin to go under the gauges in the casting box. These gauges were the pieces of metal, typically an "L" shaped piece and a straight piece to border the sides and bottom of the flong in the casting box. The flong was then place in the casting box and the gauges placed at its sides. The box was closed up, with scrap paper used to form an apron to help funnel the molten type metal into the box.
The type metal mixture used for stereotype plates had from five to ten percent of tin and fifteen percent of antimony, with the balance in lead. The percentage of tin varies with the type of mould as tin makes the cast sharper. Five percent was fine for text letterpress, but ten percent was needed for half-tone blocks.
Illustrations of the papier-mâché process.
The following illustrations from "Stereotyping and electrotyping etc." (1880) by Frederick J. Wilson show some of the steps in the process of making and using a flong.
Multiple casts.
Wheedon stated that a limited number of duplicate casts could be made from one flong. However, Partridge states that a large number of duplicate casts may be made from the same matrix. Dalgin states that to his knowledge as many as thirty, and maybe more, plates have been cast from a single flong.
Fleishman states that while flongs could make multiple casts, they typically could not be removed and reused. This would seem to be contradicted by Kubler who stated that in 1941, the United States Government Publishing Office in Washington stored over a quarter of a million flongs, and some of those in use were over thirty years old.
Etymology of the word flong.
Wilson notes that the word "flong" is an English phonetic form of the French word "flan", which is pronounced in almost exactly the same way. The word is attributed both to Claude Genoux who used the word "flan" in his original patent to describe the papier-mâché matrix, and to James Dellagana, a Swiss stereotyper in London. Apparently, when living in Paris, Dellagana was keen on a pastry, called a "flan" which was built up of different layers. . This is a fairly good description of the way in which the flong is built up, with layers of paper interspersed with paste.
Kubler states that the outside of France and England the general term for a papier-mâché mat was not a flong but a "wet mat". However, several technical manuals from the United States use the terms including Kubler himself and Partridge, as well as the United States Bureau of Labor Statistics 1929 study of productivity in Newspaper Printing.
Further reading.
Fleishman provides a thorough and well illustrated explanation of the process in his blog.. Dalgin provides a good overview of the mechanics of newspaper production in the middle of the 20th century, including different methods of reproduction. There are also books on the whole stereotype process such as those by Wilson, Partridge, Hatch and Stewart, and Salade.
In popular culture.
On 1 April 1977 "The Guardian", a UK newspaper published a seven-page special report on "San Serriffe" an imaginary island to the North-east of the Seychelles The hoax was full of typographical and printing puns, with town named after different fonts. The indigenous inhabitants were said to be the "Flong" and their language was ki-flong. The hoax is well described, along with images of the pages in the seven page special report which perpetrated the hoax on the Museum of Hoaxes website. The Guardian followed up in 1978 with parodies of twelve UK and Irish newspapers across ten pages: The SS Guardian, The SS Financial Times, The SS Times, The SS Morning Star, The SS Mirror (half page), The SSun (half page), The SS Daily Express (half page), The SS Daily Mail (half page), The SS Irish Times, The SS Telegraph, The SS Sunday Times, and the News of the SS World. However, this was considered to be less successful than the original. Most of the parody newspapers make some reference to the flong. The San Serriffe hoax is ranked fifth in the top one hundred April Fool's Hoaxes by the Museum of Hoaxes.