Full Colour Letterpress Printing

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.