While most small jobbing printers worked in single- or two-colour work, the public was keen to see full colour. And although letterpress was not the only process that could deliver high-quality colour work, it was within the reach of the medium- and larger-scale printers. The process is the same as modern-day colour printing: the original is divided into a number of colours and each colour is printed with a different ink. Just as your modern desktop computer printer uses cyan, magenta, yellow and black (CKMY) to create any given colour; the printer had to use this approach to get full-colour letterpress work. It’s the practicalities of doing this, though, which makes full-colour letterpress a difficult undertaking: everything must be precise and of the best quality, for a single error in one colour can spoil the whole set of printed sheets. The advice below is summarised from The Art of Letterpress Machining by Jack Deller and gives a picture of the state of the art in the late 1950s.
Colour Blocks – the Engraver's Art
The full colour original must be made in to a series of blocks to print each colour sequentially. This was a task delegated to the colour engraver, a very specialist trade supplying the printing industry. The original would be photographed through a series of coloured filters on to a sensitised surface and this would be used as the basis for producing a plate with the relevant high and low surfaces for letterpress printing. The screen would define the ‘resolution’ of the finished image and was originally just that: a screen through which the image would be photographed. The finest letterpress screens had something like 175 lines to the inch, but this challenged even the best printers. Fine work was typically run on specialist paper at 150 lines to the inch. Each colour plate was produced using a screen at a different angle so that the dot of ink sat next to each other rather than on top of one another. The engraver would supply the blocks along with progressive proofs, showing how the colour image would be built up colour by colour. The colour of each block was typically marked on the flange of the metal plate. If you have full colour blocks to print but no markings then these tips might help: yellow will be the densest of the blocks; black will be the lightest; blue will look like the nearly finished image and red will be the remaining block.
Preparation: Makeready and Printing Sequence
Makeready is the process of eliminating inequalities in the forme: this is important so that the whole forme prints with even pressure and so produces the best result. It’s an important step in all letterpress printing, but becomes crucial when using halftones and even more so when working in colour. As a general principle the least possible impression should be used and the thinnest film of ink delivered to the plate for printing. To achieve this you should use an interlay— a series of thin paper sheets between the plate itself and the mounting. Use interlay for heavy areas but make sure that this does not bring any light areas so high that they will print. To avoid the plate moving around during printing it should be tacked in place with the pins facing towards the centre of the plate. Looking at the lockup, avoid using wooden furniture (which can warp and bend) and lock the block using two quoins on each side (eight in total). This will allow for minute adjustments to be made to the position of the block without unlocking the whole forme.
The very best papers and ink should be used. Paper should be kept in the press room for a period before printing so that the paper is unlikely to stretch during the printing process which would distort the subsequent colours. Paper should be trimmed on two edges to the finished size so that register is accurate for each sheet. On inks, it is wise to take the manufacturer’s advice based on the paper and screen used. No driers should be used because this would reduce the ability of subsequent layers of ink to ‘stick’ to lower layers. The machine and inks should be fairly warm (closer to the room temperature) so that it does not cause pick— the tack of the ink pulling the surface of the paper away.
The traditional sequence for printing in colour was: yellow, red, blue and black. Experiments were conducted in the 1950s to work on a better sequence and it was found that the best approach was: black, red, blue and yellow. The reasons were: (i) the first colour is darker and so can show that the position of the block is correct, (ii) red shows better that the film of ink is of the correct thickness, and (iii) the key colours are printed earlier. Progressive proofs should be examined under daylight, rather than fluorescent or other artificial light. Rollers need to be of the best quality. Finally the different colours should be printed in quick succession so that subsequent colours can ‘take’ on top of the colours underneath. A gap of three hours is probably a good guide. Longer gaps may lead to crystallisation where the vehicle or varnish of the ink is absorbed to the paper, but the pigment remains on the surface of the paper and can be rubbed off.