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.

Materials

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.

Printing

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.