Another advantage of the letterpress process is that the act of impression allows numbering machines to be used: the printing surface is changed each time the paper hit it. Litho, by contrast, must use the same, static plate for each impression.
Capitalising on this, specialist manufacturers designed numbering machines known as ‘numbering blocks’ or ‘clocks’ by different printers. The principle is that when the impression is made this pushes down a plunger which forces numbered wheels to rotate. These numbers are inked and so leave a different number with each impression.
Numbering is needed on many documents: invoices, receipts and draw or raffle tickets being the best examples.
Numbering machines are very clever pieces of equipment as they have to jointly meet the specifications of a numbering machine, but also a piece of the letterpress forme. As such they demand a lot of care and attention: they should be thoroughly cleaned after use with a specialist fluid, then oiled with a high-quality light machine oil and finally stored in a sealed box.
In terms of using them to print there are some special precautions to take —
- In order to positively depress the plunger, you might have to use a very hard packing perhaps using a brass or lead rule to take the force of the plunger. You should cut an outline to receive the brass or lead from the packing to assure that the end result is roughly the same depth as the original
- If you’re using a cylinder press (a proofing press, Wharfedale or a Vertical Miele-type press) then you need to make sure that the plunger is depressed first as the cylinder travels over the numbering box: this will make sure the numbers are in the correct position to receive the ink and impression
- Think carefully about printing first and then numbering in a second run so that re-numbering does not have to be done for the sake of a single spoilt original
Common problems, and their solutions are —
Numbering machines are tested to 16,000 impressions per hour which is around twice the usual speed of the fastest letterpress machines. Problems with numbering machines are rarely from too high a speed; but there is some anecdotal evidence that too low a speed can cause problems
- Numbering block does not advance a number
When locking the numbering machine up, care should be taken to make sure that the lockup is not too tight for this will prevent the machine from working correctly
- Nearby characters are not printing
If prefixes are needed for the number (for example, to number books sequentially as Book No. 1 / Invoice No. nnn) then the type in the forme should be at least two inches away from the numbering block. There is a tendency for rollers to bounce on the higher-than-type-high plunger and so not deliver ink on these prefixes. A rather extreme alternative is to file a piece of type to fit in place of the plunger
- Rollers are causing the numbering block to advance
As well as raising the height of the rollers, it might be possible to cut away the portion of one roller that will pass over the plunger
Numbering by hand is still an option for shorter runs, and might be preferable where time is not in short supply. One tip is to avoid any registration marks — for example, the pre-printed ‘No.’ — so that the manual numbering does not show as being out of register.
Over time many printers developed timesaving schemes in printing numbered work. Two are described below.
For those printing ten-up (that is, ten numbers to each page) it should be possible to pre-print the last digit in the forme (numbers 0 to 9) and then use ten numbering blocks in front of those numbers which will advance on each impression. So, once the numbering blocks are at 87 that will print 870, 871, 872 to 879. across the page. On the next pass the final digits remain the same but the numbering block moves on by one, so printing 880,881, 882 etc.
If no numbering blocks are available, or larger types are needed (runners numbers in a race, for example) then it’s possible to get through twenty forme changes for 99 numbers rather than 99 forme changes.
- Set and print ten of each of the second character. Place them in ten piles of 0 to 9
- Change the forme to now have a number 0 in the first position. Take a sheet from each of the ten piles and print these. It will generate a 00, 0 (from the second pass) and another 0 (from the first). Pass 9 will create 09
- Now change the forme to have a number 1 in the first position. Using the sheets from the piles, your first print here will create a 10, 1 (from the second pass) and another 0 (from the first pass)
- Continue changing the first number until you reach 99!