Constant Plate Varieties 101: The Intaglio Printing Process
The earliest types of printing from primitive woodcuts to the first Gutenberg printing presses used a process known as relief printing. In this type of printing, the areas of the print surface (the plate) that are to be printed are at the surface of the plate whereas the non-printing areas (the white space) are cut below the surface. This effectively gives the effect of a raised print area on the plate much like you would see on a rubber date stamp. Relief printing is one of the least complicated printing processes. Ink is applied to the raised portion of the plate and the plate is stamped onto a piece of paper. Interestingly this is the type of process that postal employees traditionally use when they manually cancel a stamp. The hammer that is often used to cancel postage stamps has an impression of the postmark in relief on its surface.
Canada's engraved postage stamps are printed using a different process that is called intaglio. In this process, it is the non-printing areas that are at the surface of the printing plate. The areas of the design that will print are etched into the plate below its surface. Ink is applied to the plate and then wiped away from the surface so that only the engraved grooves continue to hold ink. The paper is then pressed against the plate with great pressure so that it makes contact with the ink in the grooves. Once removed and dried, intaglio printing generally leaves a very accurate printed design with the ink slightly raised above the surface of the paper. On the gum side, the design can appear depressed into the paper. This gives many of Canada's stamps a nice textured feel, especially if they are mint or in pristine fresh condition.
This process sounds simple. And it is would be relatively simple if postage stamps were each printed one at a time from an engraved plate. However, in order to be able to print an entire sheet of stamps at the same time. The original engraving needs to be reproduced many times on the printing plate. It is this process that creates many of the varieties that are seen on engraved stamps.
The process explained and illustrated below is a generalised description of how Canada's classic engraved stamps were created. There are actually many different variations on intaglio printing (wet/dry paper, flat/curved plates, gravure/engraved plates) and the exact methodology changes over time as better technology, more advanced printing presses, and availability of raw materials have progressively improved the printing process.
Step 1: Stamp Design
The process begins with the development of a design for the stamp. In addition to looking attractive and providing key information such as any text or the value of the stamp in a clear manner, the stamp design needs to take into account the intaglio process. Since the ink is absorbed by the paper from the grooves in the plate, large areas of solid colour do not print very accurately since the ink tends to pool unevenly and be absorbed in a haphazard manner by the paper. This is why print areas that appear solid when you look at an engraved stamp will usually appear as a series of lines or grill patterns when viewed under high magnification. The final design for the stamp needs to take into account both the aesthetics and functional aspects of printing a tiny work of art.
Step 2: Preparation of the Master Die
Once a design for the stamp has been approved, it is provided to the engraver to produce a master die. The engraver uses various cutting tools (usually burins) to engrave a tiny version of the design into a piece of soft steel. This is an exacting manual process, whereby the engraver must accurately render the design as a mirror image. This may take weeks of work and the engraver will usually create progressive proofs of his work by inking the die and testing how it looks when printed on paper. The final die will show the printable area of the design in the engraved ridges below the surface of the plate. The surface area of the die will not be printed. Once the final die has been completed and approved, it is then hardened using a special process of heating and cooling the steel.
Step 3: Preparation of a Transfer Roll
The hardened master die is then used to transfer the engraved design to a transfer roll. The transfer roll consists of soft steel that is wrapped around a mandrel so that it can act like a rolling pin. The transfer roll is machine-pressed under very high pressure against the master die and then rocked back and forth until the design of the die is transferred to the transfer roll. More than one impression is usually made on the transfer roll and these can sometimes come from different dies. The new impression on the transfer roll is now a positive version of the design in relief. This means that the raised portions of the surface are now the printing areas of the design. Like the master die, the transfer roll is hardened once all the necessary impressions have been transferred to it. Although this process is relatively uncomplicated, it required great care and it was still not uncommon for separate impressions on the transfer role to have minute differences even if they came from the same die.
Step 4: Preparation of a Printing Plate
The printing plate is prepared using a process that is the reverse of the transfer roll preparation. In this case the hardened transfer roll is rocked under great pressure against a plate of soft steel to transfer the impression for each stamp position on the sheet. This process is called siderography and the craftsmen who perform the operation are called siderographers. Before modern machines were available to automate this process or at least to measure the transfer roll position with great accuracy, it was up to the siderographer to perform this process by visually ensuring that each impression of the stamp design was spaced and lined up correctly. To aid in this process, various dots and lines were often engraved on the plate to ensure visual alignment. If an impression was entered incorrectly, it could be burnished out and then re-entered. The stamp impressions would be lined up in a matrix to form the various panes that make up a complete printer's sheet. Once this was complete, the guidelines and dots were sometimes burnished out and text imprints would be added to the plate in the outside selvedge areas with identification information including the name of the printer and the plate number. Sometimes additional marks and information such as the printing order number, transferrer (siderographer) number/initials, or cutting guidelines would also be added in the selvedge.
When the plate was complete, proofs would be printed from a proofing press. If problems appeared on the proofs, the plate could be reworked either by re-entering specific subjects from the transfer role or by manually engraving minor retouches directly to the plate itself.
Step 5: Printing
Canada's earliest stamps were printed from flat, unhardened plates. Each plate contained one or more panes of stamps and these would be printed, gummed, perforated, and trimmed as required to make the final panes of stamps that were sold from post offices across the nation. Intaglio printing is done by first inking the plate using an ink roller or spray system. The plate is then wiped manually or automatically by a mechanised blade (called the doctor blade) so that ink only remains in the recessed grooves of the engraved plate. The plate is then applied to the paper under pressure so that it absorbs the ink from the grooves. This creates a positive printed image from the reversed impression on the plate.
As the unhardened plates begun to show signs of wear on the press, periodic reworking of the plates could be performed through re-entries and retouches as described above. So it is possible for each plate to have different states that may show up on the final printed panes of stamps even if they come from the same plate number. For specific denominations or issues that would require large print quantities, it was also common to produce multiple plates (sometimes more than 100) for use on the press over the lifespan of the issue.
The practice of hardening the print plates was first begun for Canadian stamps around 1905. This made the plates more durable and long-lasting but made it much more difficult to perform minor reworking of the plates. By 1927, a new innovation of chroming the plates became popular. In this process, a thin (from 1 to 3/10,000th of an inch) coating of chrome is applied to the plates via electrolysis. This made them even more durable.
Several of Canada's definitive stamp issues from the first half of the twentieth century were printed by both flat and rotary presses. In the case of rotary press printing, the plates needed to be bent or curved in order to fit the cylinder of the rotary press.
Until the end of 1922, all Canadian stamps were printed by the wet printing process whereby dampened paper was used on the presses. The gum was only applied to the paper after it had been printed and dried. Late in 1922, a new process was introduced in which the stamps were printed from dry paper that had been pre-gummed before going to the press. For some stamps, such as the Admiral definitives, the use of wet and dry printing processes can be detected due to the smaller size of the design on wet printings where the printed stamps have shrunk slightly while drying.
The technology and processes for manufacturing plates and printing stamps changed quite radically during the first century of Canada's stamp production. The frequency of new designs and the quantities of stamps required also changed dramatically over that time. The availability of high quality steel was also effected by events such as World War I and II. All of these processes and changes are responsible for the many varieties that exist on Canadian stamps.