Hot metal typesetting (also called hot lead typesetting or simply hot metal) is a term used to encompass a range of different 19th century technologies to create or compose text for use in the letterpress method of printing.
Generally speaking, this method injects molten type metal into a mould that has the shape of one or more letters, which are later used to press ink onto paper.
Two quite different approaches to mechanising typesetting were independently developed in the late 19th century. One produced characters on individual type bodies, known as the Monotype system; the other, Linotype, created slugs, usually comprising a whole line of text.
Both systems met with success in different fields: the Monotype caster was more popular for bookwork and the Linotype system found success in newspaper production. A manual linecasting solution known as the Ludlow Typograph also met with success because it was able to cast display body sizes that other mechanical composition systems were unable to produce.
The key feature of the Linotype was a series of special word spacers which were long sliding wedges. The type itself was a series of brass negative molds (mats) that came down from a magazine at the top of the machine. Several of these magazines could be loaded to allow the operator access to several type sizes and styles.
As a key was pressed a mold would be released from the magazine and fall down to assemble into a "stick" like holder. Each time the space bar was pressed one of the special wedge shaped spacers was inserted.
An indicator told the operator when a line was within a capture range, that is contained enough type. Too little type and the white space would be too much for the wedge spacers to do their job and lock up. Too much type and the spacers would not have room to work. If the makeup of a line was too far out of specs a "squirt" would happen when the lead was injected into a line mold which failed to seal properly, encasing the front of that area of the machine in lead. This took some time to clean up and interrupted the flow of typesetting. It was considered very bad form to generate a squirt.
When the line was assembled it would be rotated into place in front of the injector and a bar would force all the spacers into the stick until they all jammed the type molds tightly from side to side (by increasing the spacing between each word) and sealed the stick. An upshot of this brilliant design was automatic justification of each line by equally adjusting the white space between each word. Since the type used was proportional and not fixed in width, solving this justification problem mechanically was very important. Additional white space could be introduced with special molds containing no type high symbols. The machine then rotated to cooling and then to extaction.
The line of brass molds were lifted by a "cherry picker" which elevated them back to the top of the machine held by a series of ledges on the "V" shaped notch on the top of each individual brass mold. As the machine slid the line of type across a special "V" shaped bar at the very top, patterns of notches in this bar would allow the brass molds to release at the right moment to fall back into the correct slot in the magazine. This automatic replacement kept the magazine filled. The entire action of a Linotype kept in constant motion by a well trained operator was quite a mechanical ballet.
Lead was supplied in "pigs" which were long ingots of lead weighing about 22 pounds each. They had an eye in one end from which they were suspended by a hook and chain above the melting pot. As the level in the pot went down, the pig would be lowered a bit by the chain to keep the level of molten lead constant. The eyes had a gap in them. When the pig went all they way into the lead and the eye melted at the bottom the two sides would fall into the pot and the chain would rapidly zip up to the top on a counterweight letting the operator know it was time to hang another pig.
The typeset line slugs were recovered after printing and tossed into a "Hell Box" for recycling. At intervals the lead would be remelted and a bit of "plus metal" to repalce the type alloy metals that evaporated or oxidized during the remelt process. This plus metal came in small one pound ingots. The lead was then poured into pig molds with particular care given to the eye of the pig. If defective it would break when the typesetter hung the pig and molten lead would splash around, and often on the operator.
A manual linecasting solution known as the Ludlow Typograph also met with success because it was able to cast display type sizes that other mechanical composition systems were unable to produce.
The Ludlow consisted of a very heavy table with a flat top about waist high and a depressed slot into which a "stick" was inserted. Underneath was a pot of molten lead and a plunger. The stick was used to hand compose the lines of type, typically headlines in 18 point or larger with 72 point commonly being available. This was from brass molds stored in cases on either side of the Ludlow. The cases were not the traditional "California Cases" used to set body type, but simpler alphabetically arranged wooden cases, each one containing a given font in a specific size and style such as bold face, italic or condensed. The range of available fonts was typically much larger for a Ludlow than for a Linotype.
After a line of type was assembled into the stick a special blocking slug was inserted to seal the end. Then the stick was placed mold side down into the slot on the table, a clamp locked down to securely hold the stick and the Ludlow activated. The molten lead was injected by a plunger up against the mold with considerable force. If any mistake had been made in assembling the stick such as forgetting the special terminating block, a dreaded "squirt" would result, often encasing the operator's toes in molten lead and leaving a mess that needed to be peeled off the Ludlow surfaces. Operators were encouraged to wear heavy boots with steel toes and be quick at removing one. The Ludlow used a melting pot and chain feed system very similar to the Linotype and loaded the same lead "pigs".
Towards the end of its life as a common backshop type setter, the Ludlow was often joined by the "Super Surfacer" a specially designed surface plane that would smooth the surface of the freshly cast type and ensure it was exactly type high. A Ludlow slug was just the letters overhanging a central spine about 12 points wide (T shaped viewed from the end). It needed to be bolstered by Elrod slugs on either side for support. The number of slugs above and below the central spine could adjust the white space above and below the type making it a very flexible system for large type.
The Elrod was a machine used to cast slugs, non-type high lead strips of a specific width, which were used extensively in page justification, that is adjusting the white space between paragraphs and any other area when small bits of white space were needed. Large areas of white space were created by wooden blocks called "furniture".
Towards the end of its life hot metal composition in newspapers was kept alive by the proof press. As each page was set and locked up, it was moved on a turtle (a rolling steel table) to the manual proof press where it was hand inked and a single very high quality proof was pulled. This proof could then be photographed and converted to a negative.
Black paper was inserted before the proof was photographed for each of the photos on the final page to create clear windows in the negative. The separately made halftones would be taped into these clear windows on the negative. This negative could then be used to burn the photosensitized printing plate for an offset press. In this way the heavy investment in hot metal typesetting could be adapted to the newer offset technology during a transition period.
The nature of text printed via the hot-metal method is notably different from that produced by the phototypesetting processes that followed it. As the lead type used to print (letterpress) a page had been directly formed from the type matrix a good fidelity to the original was achieved. Phototypesetting suffered (at least in its early days) from many problems relating to optical distortion and misalignment. These disappointing results were a thorn in the sides of many authors and readers (especially of complex or mathematical texts that had many small sub and superscripts). A desire to recreate the aesthetic qualities of hot-lead spurred Donald Knuth to create one of the first general purpose digital typesetting programs, TEX.
Although strictly speaking not typesetting, stereotyping (electrotype or nickeltype) could be used to cast a reproduction of an entire typeset page (or pages imposed in a forme) using a mould made with an impression using flong (similar to papier-mâché). The ensuing casting could be made curved for use on a rotary press or flat for the slower flat bed presses. This technique was often used in newspaper production.