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.