As an introduction to traditional printing technologies as in use during the twentieth century How it Works: Printing Processes by David Carey and illustrated by B.H. Robinson, © Ladybird Books Ltd, 1971 is reproduced here by kind permission of the publishers.
The Main Printing Processes
The three most important methods are:
- Letterpress printing which is carried out from a raised surface.
- Lithography, or litho, to use its more common name, involves printing from a flat or very slightly recessed surface.
- Photogravure, normally shortened to gravure, is printing from a recessed surface. This process is also known as intaglio printing. (The word comes from the Italian and means ‘to cut into’.)
Setting & Assembling the Type
Setting the Type
When movable type was first invented in the mid-sixteenth century, all the separate pieces were kept in two cases, one above the other. The upper case always contained the capital letters and the lower case held the small letters. Capitals thus became known as upper case letters and the small type as lower case, and this terminology is used in printing even today.
Type was always set by hand, one letter at a time being placed in a narrow tray, known as a composing stick, which was held in the other hand. By moving a slide along the tray and fixing it in the desired position, the lines of letters could be set to a precise width. Most type-setting today is done by machines which do the job very quickly, but the composing stick is still used when a small amount of setting has to be done, such as display headings, for corrections or when there are a number of different sorts and sizes of type to be set.
The man who sets the type and composes the page is called a compositor. Type letters are made the wrong way round so that they will reproduce the right way when printed onto paper. Hold some printed matter up to a mirror and you will get a compositor’s eye view of the type.
Machines for setting and casting metal type have been in use since the late nineteenth century. There are two main kinds used today: Monotype by which the characters are cast individually, one at a time, and Linotype which casts a complete line in the form of a type slug.
Although the two machines are really quite different in operation they both have keyboards containing all the necessary upper and lower case characters, figures, punctuation marks, spaces and so on. The operator presses the appropriate keys on the keyboard and this sets the various mechanisms in motion.
Apart from the normal type keys, the keyboard also includes what are known as justification keys. Usually on a printed page, the lines of type end evenly, each line being of exactly the same length. In other words, the lines are justified. When the keyboard operator has nearly reached the end of the line he is setting, a justifying indicator tells him what space is left. He must then decide whether he can complete the last word or whether the line has to be justified. If the word is too long, he presses a justification key and this adjusts the spaces between the existing words so that the line is set to the exact measure required.
The Monotype Process
Operating the keyboard of a Monotype machine does not immediately produce letters and words. What it does is to cause a succession of holes to be punched in a roll of paper mounted above the keyboard, each character being represented by a different arrangement of two holes.
Just below the paper is a series of air-operated punches. Every time a key is pressed two jets of compressed-air are released, two valves open and the appropriate two punches are made to perforate the paper. This happens for each character in turn until the keyboard operation is complete. The paper roll is then removed and transferred to the type caster, which is a separate machine and the one that actually makes all the pieces of type. Its functioning is entirely controlled by the perforations in the paper.
A pattern, or matrix, of every piece of type likely to be needed is contained in a matrix case which is put in place on the caster. As the perforated paper is fed though the machine it passes over a series of compressed-air pipes. Where every two perforations occur, jets of air are allowed to pass through. These raise pins which limit the movement of the matrix case, bringing the matrix for that particular character over a small hollow, or mould. Molten metal is injected into the mould; it sets immediately and a cast impression of the type is made.
The Linotype Process
Unlike the Monotype system, which has a separate keyboard and caster, the Linotype method combines both functions in the one machine. It is perhaps the more popular of the two, especially where a large amount of type-setting is needed, as in newspapers for instance.
Instead of a roll of paper above the keyboard, the Linotype machine carries a large matrix magazine (1). When a key is pressed the required matrix is ejected onto a conveyor which takes it to an assembly box where it is quickly followed by other matrices, shown as arrows on the illustration, to make up the words of a complete line (2). At the end of each line the keyboard operator moves a lever and the box of matrices passes in front of a geared wheel (3) carrying a number of moulds. The rotation of the wheel is timed so that as each line of matrices arrives, one mould will fit over it. Molten metal is pumped into the mould, and the line is cast in a slug of type which is ejected onto the galley (4). The next mould on the wheel fits over the next line of matrices and so the process is continued in a succession of rapid, casting operations.
The supply of matrices in the magazine is limited, so there is a device incorporated in the machine which sends each matrix back to the magazine to be used again as soon as the type has been cast.
There is a third method of setting type and that is by means of photographic film. It is a process that has gained in popularity and use over the last few years. There are several makes of machine available, some operated electronically, but the one in most general use at the present time is the Monophoto Filmsetter.
The machine retains some of the features of the Monotype system, described earlier, in that the operation of a keyboard causes holes to be punched in rolls of paper tape. However, instead of the normal matrix case and type caster, the system uses negative images of the type on pieces of film, contained in a film matrix case, which can be selected and reproduced photographically.
The paper tape is fed past air pipes as before, and the arrangement of holes, allowing air to pass through, controls the selection of the pieces of type film in the film matrix case. A beam of light is projected through the selected matrix and a positive image of the type is photographically printed on to a sheet of film material contained in a drum. The photographing is done through an arrangement of lenses and prisms, the adjustment of which enables the type image to be enlarged or reduced as required. As with type casting, film setting is carried out at quite fast speeds. The resulting film can be used for making letterpress printing blocks or lithographic printing plates.
Galleys and Proofs
When each line of type has been produced by a type caster, it is ejected into a long, shallow, metal tray placed at the side of the machine. This tray is known as a galley. An impression, or proof, of the type in the galley is next printed on to paper. The paper is usually in long strips similar to the shape of the galley, and a proof of this kind is called a galley proof.
The galley proof provides a first opportunity for the type-setting to be checked visually, and the person who does this job is the proofreader. He is a very important member of a printing organisation and is skilled in finding any errors which might have been made by the keyboard operator, and any faults in the type itself. He checks the proof against the original text, or copy, from which the type was set and makes sure the setting exactly follows the original, letter by letter and word by word. Any errors he finds are noted with special marks so that they can be corrected by the compositor.
After galley proofing, the long strips of type are split up into page lengths and a further page proof is taken to make sure the original mistakes have been corrected and no additional ones have occurred.
Formes and Furniture
The pages of a book are not printed separately, one at a time, but on larger sheets of paper carrying four, eight, twelve or more pages on each side. The areas of type must be so arranged that when they are printed, and the sheets folded, the pages follow one another in their correct sequence. The arranging of the type in this way is known as imposition.
Imposition for letterpress printing is carried out on a big cast-iron table with a perfectly flat top (a stone), and the various areas of type are positioned within a steel frame known as a chase. Furniture, in the form of pieces of metal or wood, is inserted between the page areas of type and between the type and the chase to keep them the correct distance apart. Steel wedges, or quoins, are then tapped into position between the furniture and the chase, and finally tightened up by means of a key until nothing can move. This complete and now solid assembly of chase, type, furniture and quoins is known as the printing forme. After a last proof has been taken, as a final check, it can be put on a letterpress machine for printing.
Reproducing Illustrations — Letterpress Process
So far we have dealt with the basic facts about making, setting and imposing type, but what if the printed book, newspaper or magazine needs to include illustrations? This requires an entirely different technique which, for letterpress printing, is known as process engraving or blockmaking.
In printing terms there are just two kinds of illustrations, line and half-tone. With line blocks the printing surface produces a solid colour on the paper without any gradations of tone. In other words it is simply one colour or white without any shades of colour between. This kind of block prints well on even the poorest quality paper and is suitable for reproducing type-matter and pen-and-ink drawings.
Half-tone blocks are used to reproduce from subjects such as photographs or wash drawings in which the tones vary throughout the illustration. If you look through a magnifying glass at a photograph in a newspaper or magazine, you will see that it is broken up into a mass of dots. These dots are larger and more closely-packed in the dark areas, and are smaller, more widely-spaced in the light areas. Later in this book we shall learn how this dot formation is achieved. Generally speaking, half-tone blocks reproduce better on good quality paper, although the screen plays an important part in this.
Making a Line Block
The drawing from which a line block is made is mounted on a copy board which is brightly and evenly lit by arc lamps. Facing the copy board is a large process camera which is mounted on rails so that it can be adjusted backward and forward to obtain the correct focus and to enlarge or reduce the image. A negative image (1) is produced and this is photographically printed down onto a metal plate, normally of zinc alloy treated with a special, light-sensitive solution.
Negative and plate are fixed in a vacuum frame (2) from which all air is excluded to ensure perfect contact. Both are then exposed to a bright light for a pre-set time. Where the light penetrates the lighter areas of the negative and reaches the solution, it hardens slightly. The remaining area of the solution remains soft and is afterwards washed away with water (3). On the zinc plate there is then an image matching that of the negative, the light areas of the negative showing as hardened solution on the plate, and the dark areas as bare zinc alloy (4). The hardened solution is then further hardened by heat which forms an acid-resisting enamel over it.
The plate is then placed in an etching machine containing a bath of acid. Motor-driven paddles splash the acid onto the plate, and the light, unprotected areas are etched away, leaving the enamelled metal printing surface raised above the surrounding metal. From this a proof (6) can be taken.
Making Half-tone Blocks
Half-tone blocks are produced in an essentially similar manner to line blocks, the subject being photographed and a negative printed down onto a light-sensitive solution previously applied to a metal plate, which is then etched in acid. In this case, however, the metal is usually copper and the varying tones of the subject have to be reproduced by means of dots.
In fact, the subject is photographed through a glass screen. This consists of a circular piece of optical glass on which are drawn a great number of black lines crossing each other at right angles and forming a fine grid. Because the subject is photographed through this screen, the lines of the screen appear on the negative. They appear dense and heavy in the dark areas and as thin lines in the light areas. The negative is printed down onto the sensitized plate, the soft areas of the solution (matching the dark areas of the negative and the screen lines) later being washed away and leaving the hardened areas. When the plate is etched, the thin lines of the screen lose their regular shape and become the raised dots of the half-tone block.
Screens vary in the number of lines drawn across them A greater number of lines gives a fine screen, less lines gives a coarse screen. Screens of 55, 65, 85, 100, 120 and 133 lines to the inch are common. Poor printing surfaces need coarser screens.
Engraving for Colour
Printing illustrations in their natural colours, instead of black and white, is done by what is called the four-colour process. In other words all the colours of the subject have to be produced by four coloured inks only, namely- yellow, red, blue and black.
Half-tone colour blocks – a separate block for each colour – are made in the same way as a black and white one except that a light filter is placed in front of the camera lens so that only one colour is photographed at a time. A violet filter is used to isolate yellow for the yellow negative, a green filter for the red negative and a red filter for the blue negative. A combination filter is used for black, which is really an extra printing to give depth and contrast to the illustration.
To ensure that the four different coloured dots do not print one on top of the other, the screen is rotated a given number of degrees for each colour exposure and the dots print in a circular pattern. Where necessary the white paper shows through the colours to produce highlights and other bright areas.
When printing by the letterpress four-colour process, yellow is usually the first colour on, followed by red, blue and black, in that order. The illustrations on the opposite page show the yellow, red and blue printed separately, and the effect produced by printing them together with the black.
As with almost every other technological process, electronic equipment is being introduced more and more into blockmaking. We will therefore take a quick look at a machine called the Vario-Klischograph which makes blocks automatically without negatives and without photographic printing onto metal.
The subject, a drawing or photograph, is held by vacuum onto a glass-topped table and the plate to be engraved is similarly secured to another table. An electronic scanner ‘reads’ the subject by sweeping a spot of light backward and forward across it. The light is reflected back into the scanner from where it is transmitted onto photo-electric cells which turn the light spots into tiny electric impulses. Greater reflection is provided by the pale areas of the subject, giving a larger impulse. Less reflection is received from the dark areas which produce a correspondingly reduced impulse.
The electric impulses are used to operate an engraving head mounted over the plate. This engraves dots into the plate, the size of the dots varying according to the light and dark areas of the subject as read by the scanner. In this way the engraving on the plate becomes a faithful reproduction of the subject picture. For colour engraving, the light spots are passed through appropriately coloured filters and the plates are rotated for each colour as with the glass screen.
Stereos and Electros
It is sometimes more convenient to print type matter from a block rather than from the type itself, and it may also be necessary to make duplicate copies of these blocks. Duplicates of both type blocks and line blocks are known as stereos and the process of obtaining them is stereotyping. The method is quite simple. The type forme or block is placed in an electrically heated moulding press and a sheet of plastic is laid over it. The press is operated at a pre-set temperature and timing, and an impression of the type or block is taken by the plastic sheet. This now becomes a mould, or matrix, into which molten metal is poured and the stereo is made. Papier Mâché is sometimes used for matrices and, quite often, stereos are made from plastic or rubber.
Copies of half-tone blocks can also be made. These are produced by a process of electrotyping and the copy is known as an electro. An impression of the half-tone block is taken in a moulding press. The moulding material can be lead, but plastic Vinylite is more common. The mould is then sprayed with an electricity-conducting solution and put into a bath of copper sulphate in which is suspended a copper plate. A low-voltage current is passed through the bath and, by a process of electrolysis, copper is deposited from the plate onto the mould. The mould is removed and the hollow back of the electro shell is then filled with molten metal to give it strength.
There are two basic kinds of letterpress printing machine, although there are many makes and variations of each kind. There is the flat-bed machine which prints from a forme or some other horizontal mounting of type and blocks, and the rotary machine in which the printing surface is made in the form of a wrap-round plate fitted to a rotating cylinder. Very large versions of the rotary printing press are used in the production of newspapers.
On flat-bed machines the printing forme is fixed to a reciprocating forme bed which travels backward and forward. At one end of its travel it runs under inking rollers which spread the ink evenly over the printing surface. At the other end it comes into contact with the paper onto which the inked impression is transferred.
Paper is stacked in sheets at the feeding end of the machine and is picked up by suction cups, one sheet at a time. It gravitates down a feeder board and on to a rotating cylinder. As the leading edge of the sheet reaches the underside of the cylinder it is pressed into contact with the inked forme and then carries on round the cylinder before detaching itself and moving down a delivery board on to the stack of printed sheets.
The process outlined on the previous page is simplified in order to give the essential details rather than a description of a particular machine. One point not mentioned is that immediately after a sheet of paper is printed a fine powder is sprayed on the printed surface to prevent set-off This happens if wet ink on one sheet is allowed to mark the underside of the following sheet in the stack.
Colour printing can be done one colour at a time as described, the other colours being added by sending the paper through the same or another machine again for each of the remaining colours. Two-colour and four-colour machines are often used; these print two or four colours in quick succession as the paper is fed through a series of cylinders and over the appropriate number of printing plates. Of course, when the paper has been printed on one side it must go through the machines again so that the reverse side can be printed.
On rotary newspaper presses, there are two cylinders carrying the curved printing plates and two controlling the paper. The cylinders rotate one against the other and the continuous web of paper runs between. Each pair of cylinders prints one side of the paper which is thus printed on both sides at once.
The Lithographic Process
Litho printing is done from a very thin metal plate, usually made of zinc and aluminum, which can be bent to fit round a printing cylinder. Because this form of printing uses a flat, or very slightly recessed surface, a means has to be found of confining the ink to the image areas of the plate and keeping it away from the non-printing portions. Without some such method the rollers would simply cover the whole plate with ink and the result would be a terrible mess.
The system depends on the actions of two natural enemies — grease and water. A greasy substance is applied to the areas to be printed, and the non-printing areas (which have a very finely-grained surface) are dampened with water. A very fine film of water is retained by this grained surface. The greasy printing ink adheres to the greasy image but is rejected by the water on the dampened part of the plate which therefore remains clean and does not mark the paper.
Photolitho and Offset-litho Printing
Litho printing plates are produced photographically by making a negative image of the subject; it might be typematter or illustrations or both. The illustration negative is then photographed through a screen and a positive screened image is produced on film. Alternatively, the positive film can be produced by the Vario-Klischograph described (see Electronic Engraving) and other similar machines. This film is then exposed onto a light-sensitive plate. The printing image thus produced may be left flat or lightly etched into the metal with acid. Unlike a letterpress block, the image on a litho plate is the right way round. This is because the plate does not print directly on the paper as in letterpress. The impression is first transferred, or offset, onto a cylinder carrying a rubber blanket, and from there onto the paper. This process is known as offset-litho and is the one now universally adopted. It gives a softer appearance to the printed picture than letterpress, it can be used on less-expensive, non-coated papers and is suitable for long printing runs without the plates wearing out.
Litho colour printing, which can be in line or halftone, requires a separate plate for each colour. Multicolour presses are frequently used. One model, called the Web Offset machine, prints four colours in quick succession and on both sides at once of a continuous web of paper fed from reels.
Printing by photogravure is exactly the reverse of printing by letterpress. Instead of the ink being applied to raised type or dots, it is contained in tiny hollows or cells, recessed below the surface of a copper-plated, steel cylinder.
The type matter and illustrations are photographed, and transparent positives are made. These are laid down in their correct positions on a glass screen. The assembled positives are then transferred photographically onto a paper coated with a light sensitive gelatine and known as a carbon tissue. The carbon tissue has already been exposed to light through a screen made up of a cross hatch of fine, transparent lines which form a pattern of tiny, light proof squares. The light has the effect of hardening the gelatine in the cross line formation whilst leaving the square dots unaffected. When the positives are photographically exposed to this prepared surface the light begins to harden the dot areas too, but this hardening process varies according to the lightness or darkness of the details on the positives.
The carbon tissue is placed around a copper-plated cylinder and the backing paper is removed with water. The water also has the effect of dissolving away the gelatine, but the light-hardened areas tend to resist this and the resulting gelatine layer therefore varies in thickness.
Finally, the cylinder is etched in a bath of acid. The thinner parts of the gelatine etch away before the thicker parts and, eventually, the cylinder has cells of varying depths; the darker the tones the deeper the cells.
When it is quite certain that no alterations or corrections have to be made, the surface is given a hard chromium facing in order to make it hard wearing.
All gravure printing machines work on the rotating principle, as with litho, but no water is used and there is no process of offsetting onto a rubber blanket. The printing area of the cylinder comes into direct contact with the paper. Machines can be of the web-fed variety in which the paper is run continuously from reels, or sheet-fed in which the paper is cut into sheets before printing.
The lower portion of the etched cylinder is constantly immersed in a reservoir of ink which flows thickly over the whole cylinder as it rotates. The cylinder also runs in contact with a thin steel blade known as a doctor blade. When the ink-covered area emerges from the reservoir the surface is scraped clean by the blade, leaving only the ink held in the cells which prints on the paper in the familiar dot formation.
You can always distinguish gravure printing from the other two main processes by examining the typematter under a magnifying glass. You will find that all the letters, as well as the illustrations, are reproduced by dots because the type and illustrations are photographed together through the same screen. It will be seen, also, that the dots in gravure printing vary in density as well as size.
Multi-colour gravure machines are common. A separate cylinder is used for each colour and the paper is passed between rollers from one to the other.
The system uses a screen made of a substance such as silk, nylon or fine wire mesh fitted within a frame. A stencil of the lettering or design to be printed is firmly attached to the underside of the screen, which is then fitted down onto the material that is to take the impression. It may be paper, glass, metal, plastic, fabric— almost anything. Ink is then forced through the screen by means of a squeegee and the inked image of the stencil is transferred to the material. The ink coverage by this process is so good that light-coloured inks can be printed on dark backgrounds; even white on black.
Screen printing is never employed for the normal range of printed matter. It is comparatively slow compared with the other processes, and although it can now be used for half-tone and colour work the result tends to appear coarse. It is ideal for bold displays, for printing directly on awkward shapes like bottles or plastic containers, and it is extensively used for printed electrical circuits.
Finishing, paper and ink
How a book is made – Making Up and Finishing
When the printing operation is complete, the sheets are held in stacks. They have been printed on both sides with the various areas of type and illustrations imposed so that when the sheets are folded they form the pages of the magazine, book or whatever it is. The individual operations differ according to the size and kind of book concerned and the type of machinery used.
The stacked printed sheets are taken from the machine room to the warehouse from where they can later be drawn in convenient quantities and transferred to the bindery. In the bindery they are cut into sections suitable for folding. The folding is done by machine. The machine shown uses both the knife folding and the buckle folding devices illustrated in the diagrams. In each case the paper is pushed between two revolving steel rollers which complete the fold. The paper moves on through the machine and the action is repeated until the right number of folds have been made in each direction.
After folding, a pile of sections is put into a bundling machine which compresses them, gets rid of the air from between the pages and makes them into a compact bundle. Next, sections are placed in their correct order in a gathering machine which collects them together in sets, each set of sections now representing one book.
The sections are then sewn. This is done by opening each section in the middle and placing it on the saddle of a sewing machine. The saddle swings round and locates the section under a row of threaded needles, which stitch together the layers of paper in the fold. The machine works automatically, dealing with the sections rapidly – one after the other. The sets of sections making up each book then go for back-stripping where a strip of linen is glued down the sewn edges to hold them together. Finally, a three-knife trimmer is used to first cut the top and bottom edges of the book and then the front. This trims the paper to the right size and cuts off the outside folds so that the individual pages can be opened.
While the inside pages are being dealt with, the cover has been made in a case-making machine. The two parts are glued together in a casing-in machine and, after pressing, the book is complete.
Paper and Ink
Paper is largely made from wood-pulp, although esparto grass from Spain and North Africa is sometimes used for particularly good quality papers. Wet pulp is contained in a tank at one end of a paper-making machine. It enters the machine and flows over a vibrating wire-mesh conveyor which allows most of the water to drain through. The remaining fibres cling loosely to one another and are then passed between rollers which compress them together. After final drying between heated rollers, the paper is wound onto large reels. Different kinds of paper vary in the amount and kind of pulp used and in the pressures applied during rolling. Some kinds are coated with a china clay or plastic solution to give them a fine, smooth printing surface.
Printing inks are not like the inks we use for writing but are of a greasy, paint-like substance which adheres to the paper as a thin film without blotting or running. They can be of any colour for printing separately, one at a time, but for the four-colour printing process the primary colours of yellow, red and blue are specially prepared to a standard colour specification. This enables them to be printed one with the other, and it is the combined effect of these special tones, plus black, that gives a coloured print its natural appearance.