For starters, a stamp is a convenient method of paying for postal services. Which means it must contain a certain amount of essential information: the name of its country of origin, its denomination, and some invisible coatings on the surface that can be detected by electronic mail sorting equipment. But a stamp is far more than that. It is also symbolic.
Because they are produced in such vast numbers and are used by almost everybody, postage stamps are little icons and reminders of important national events and achievements.
Stamps have been around for more than 150 years. They’ve become so much a part of our lives that most people take them for granted – after all, about 1000 million are produced every year in Australia alone!
While we might notice a new design when it first appears, we tend to be completely unaware of the complex technologies and exacting skills behind each individual stamp.
The printed stamp is the end result, but between finished art and printing lies a fascinating production process.
Once the design for a new stamp has been approved, the designer prepares finished artwork. This consists of a final version of the illustration with any text (the word ‘Australia’ and the denomination, for example) included on a transparent overlay.
In full colour printing, the first step is for the artwork to be laser-scanned so the design can be separated into the four process colours: blue, red, yellow and black.
The illustration will often involve subtle gradations of tone which cannot be reproduced directly by the printing machine, or press, as it’s called. These are broken up into tiny dots, using a halftone screen.
Most people have seen the effects of a screen – you can see how it works by looking closely at a newspaper photograph. As newspapers are printed quickly and cheaply, they use screens of around seventy-five lines per inch, which means that the dots are visible with the naked eye.
Stamps, however, are printed on very high-quality stock, using an especially fine halftone screen of 250 to 300 lines to the inch. So it is very difficult to detect the screen on a stamp without a magnifying glass.
Every stage of stamp production is subject to stringent quality control. When the colour separation process is completed, for example, an actual-size proof, called a Cromalin proof, is made for Australia Post to check. When this has been approved, an entire proof sheet is made so the layout can be checked before the printing plates are made.
Printing plates are thin sheets of light–sensitized zinc. Transferring an image to the plate is very like reproducing a photograph, with the plates exposed to light through the film. The surface of the plate is subsequently hardened, allowing for the non-printing parts of the image to be etched away.
The paper accounts for more than half of the cost of producing stamps. Gummed stamps are printed on imported paper but for self-adhesive stamps an Australian-made paper has to be laminated onto a backing sheet before printing can begin.
The backing sheet has a layer of silicon on one side and a thin coating of adhesive is applied to this. This is dried on to the silicon using heat. In the process, the moisture content of the paper is reduced, so it is later remoisturised.
The face paper, which has a light water-soluble coating on the back, is then bonded to the self-adhesive backing sheet in a giant roller.
The printing process proceeds in stages. For self-adhesive stamps, the reverse of the backing sheet is generally printed first. Booklets have full-colour images on backing sheet, but rolls bear only the printer’s name. Gummed stamps are not printed on the back at all. The printing process is similar for both types and uses a very large machine capable of printing all the required colours in a single operation. Although the printing machine is very sophisticated, it uses a simple process known as offset lithography, which is a development of a technique invented in the late eighteenth century.
Full sheets of paper are transferred through the printing press by means of grippers, which ensure that the positioning of the colours is exactly right. The colours are printed consecutively, usually the darkest first and the lightest last, although this order may change if the design demands it.
Because the modern offset press works at very high speed, the inks have to dray almost instantly if smudges are to be avoided. Several different drying methods are used. The conventional one is to spray a fine powder on to the printed sheet, which not only speeds the drying process but forms a barrier between the sheets when they are stacked. In addition, catalysts may be added to the inks. In UV printing, an interesting new technique, the inks contain additives which react to UV lights in the press, drying them almost instantly.
Obviously the density of the inks and the strength of the colours have to be very carefully controlled. Also, if the colours are going to print exactly in line with one another, the paper has to be positioned under the cylinders within pinpoint accuracy. Final adjustments are made at a computer console which controls every operation of the offset machine. The skilled operator will regularly pull a sheet from the press and examine it closely, noting any defects, any changes in colour balance or density or any necessary adjustments to the registration.
The printing process, although noisy, is remarkably quick, clean and efficient. Only two people are needed to control the giant offset machine, along with another to load the paper and ensure the free flow of the inks.
Now anyone familiar with basic printing techniques will recognise all the above. It is fairly run-of-the-mill printing technology, albeit at the highest levels of skill and on an impressively large scale. However, some of the processes which follow are not only unique to the production of stamps but many of them were developed here in Australia.
Phosphorescence is the property of some substances to absorb radiant or other types of energy and give off light in response. It differs from fluorescence in that light emission occurs after the radiation that caused it has ceased.
At this point we have a stack of sheets with some printing on the backing sheet (in the case of self-adhesive stamps) and the completed stamp design on the face - or at least the visible part of the design. But there is another part of the stamp you can’t see: an invisible phosphorescent coating which can be detected only by electronic means.
This coating has two important purposes. First, it allows the mail sorting machine to identify the presence and position of the stamp on an envelope. Second it determines that the stamp is genuine. When a miniscule black light in a sorting machine flashes at a stamp, the phosphor flashes back. This tells the sorting machine where the stamp is so it can face all the envelopes the right way and allow the stamps to be cancelled.
The surface coating of the imported paper used for gummed stamps already contains the phosphor. For self-adhesive stamps, a innovative method has been developed in Australia to print a phosphor coating on the printed stamp surface. It’s not as simple as it sounds, for a couple of reasons.
Firstly, when the stamp is cancelled the cancellation ink has to penetrate right into the paper, so that there is no possibility of the stamp being reused. An overall coating of phosphor would prevent this penetration.
Secondly, stamp collectors need to soak stamps off their envelopes. As the adhesive on self-adhesive stamps is not water-soluble, the water has to come in from the face to dissolve the water-soluble coating. The answer is simple: only part of the stamp is coated with phosphor.
If you look at a self-adhesive stamp from a certain angle, you can see slightly matt areas. This is where the phosphor has been applied – usually on lighter-coloured areas which have the thinnest coating of ink.
Once the stamps are printed on the sheet, the perforations are cut. A computerised perforating machine is used for gummed stamps, but the self-adhesive kind present a particular challenge. Because self-adhesive stamps are printed on a double sheet, the stamp must be cut out while leaving the backing sheet intact.
This means that the cutting process has to be very, very precise – within about half the thickness of a human hair, in fact! It can take many hours of patient work to adjust the machine before cutting can begin. Success depends on the accuracy of the die-cutting machine and on the skill of the operator.
Gummed stamps are now guillotined into sheet size, inspected, counted and packed ready for sale.
The sheets of self-adhesive stamps must be processed into either rolls or booklets.
After being inspected, then collated, matched and joined, the sheets go to a stripping machine where the waste part of the face sheet around each stamp is stripped away and discarded.
Next, the sheets are made into so-called jumbo rolls, of which there are two different sizes: one eight stamps wide and 60,000 stamps long, the other eight stamps wide and 80,000 long.
The jumbo rolls are then slit into single rows and cutting them into useable lengths of either 100 or 200 stamps. These rolls are then automatically boxed and ready for sale.
For booklets of ten stamps, sheets are inspected before being processed through a folding and slitting machine. After another thorough inspection, the individual booklets are counted and packed.
So, while the technology for making gummed stamps is widely used throughout the world, the perforated, die-cut self-adhesive stamp incorporating phosphor-tagging is a very sophisticated object indeed. And many of the processes which brought it into being are uniquely Australian developments. Australian stamp technology, along with our stamp design, continues to lead the world!
Information has been obtained from Australia Post Philatelic Center