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Having worked as a printer for many years, I find this the most misunderstood aspect of stamp collecting. Why should it matter how a stamp or banknote is printed? Pick up a banknote or Commonwealth GVI stamp, and feel with your fingers the ink raised on the paper. That's the characteristics of "INTAGLIO"- or recess/gravure printing, this simple test could save you being passed a fake banknote. Try reproducing this feel with a colour photocopy! That's 50 you have just saved! How does this affect stamp collectors? Some stamp designs are printed by two different printers using two different printing processes. As a stamp collector (sorry philatelist) you need to identify these different types of printings. The catalogues do list these differences.

I will try to explain with simple diagrams all the main printing processes. I hope you find this information enlightening. 

NOTE: Confusion!  Some of the descriptions given for different printing processes by stamp catalogues and collectors, are not the same descriptions as used in the printing industry.

Most modern printing presses use circular cylinders for printing. some diagrams are illustrated flat to help you understand the processes involved.

RECESS/ENGRAVED/GRAVURE PRINTING

Well where to begin with this one! The printers name for this process is GRAVURE OR INTAGLIO. Stamp collectors and catalogues sometimes refer to this process as "reccess printing" or "engraved". Many of the First stamps where printed using this method using hand engraved FLAT printing plates.  The gravure process is illustrated here with circular cylinders just as used on the modern printing press. The old method was engraving the image into a flat plate. The excess ink being scraped from the surface with a doctor blade leaving the ink in the recessed portions. Then paper was then pressed onto the plate to transfer the ink from the recess to the paper.

The different printing characteristics found in Gravure are due to the way the image/cells are cut/etched into the printing cylinder at the outset.

Gravure & Photogravure is also explained in more detail, just the follow link below to find out more about this process.

WANT TO KNOW MORE GRAVURE? AND HOW TO IDENTIFY STAMPS PRINTED THIS WAY CLICK HERE.

 

LETTERPRESS/SURFACE/TYPOGRAPHY PRINTING

Letterpress is self explanatory. The first books were printed using this printing method. TYPOGRAPHY refers to printing by what printers call the LETTERPRESS process. Using individual or cast lines of metal letters (as pages of text in a book), or an etched illustration, as stamps. The image is raised from the surface.

Stamp collectors unfortunately describe this process as SURFACE PRINTED or TYPO.

The Image/printing plate could be a hand cut woodblock or cast metal type, or in the case of illustrations, cut or etched out of metal.

NOTE: Most  postmarks and cancels are letterpress. The rubber stamp and ink pad is an example of LETTERPRESS.

Because the printing plate is pressing into the paper to transfer the image you can tell letterpress by the following methods. 

Sometimes, the reverse of the stamps show indentations where the printing plate has pressed into the paper. SOME POSTMARKS GO RIGHT THROUGH!

The edges of the image do get slightly to much ink, which tends to show on the front of the stamp as a slight darkening or lightning at the edges of letters or lines. Also solids are very seldom smooth looking all over.

LITHOGRAPHY/LITHO PRINTING

Lithography/Litho printing is as old as letterpress printing. Lithography was used in the early years for producing superb colour illustrations, called lithographs. But with the introduction of  PHOTO LITHOGRAPHY this process has  now become the most popular large scale printing process in use today.  Some early stamps where printed by lithography from designs drawn on a STONE.
Lithography  uses the principle that water and oil /grease do not mix normally. The printing image is greasy and the background attracts water (hydrophilic). The printing plate/stone (which has a flat surface) is first slightly damped, then greasy ink is applied, this ink only sticking to the greasy image and is repelled by the damp background. This inked image is then transferred to the paper. In OFFSET LITHO the image is transferred to a rubber blanket before being applied to the paper, hence the name OFFSET LITHO 
The finished print is flat, with perfect solid blocks of colours being produced. 

Some early stamps printed in litho. They normally are very crude, but some are very good indeed - (early Malaya state of SARAWAK).

 

The difference! The left stamp is printed by OFFSET LITHOGRAPHY, notice the straight clean lines to the number.

The right stamp shows clearly the dots/cells edges as PHOTOGRAVURE produces.

The cut out left side of the stamp at left is a new security perforation.

PRINTING FAULTS EXPLAINED

DOCTOR BLADE FLAW: When a piece of foreign material gets caught by the DOCTOR BLADE, or if the DOCTOR BLADE is damaged, the ink gets under the doctor blade and leaves lines of ink on the surface of the cylinder. These then get passed onto the paper. These inked lines are called DOCTOR BLADE FLAWS by philatelists 
DRY PRINT: This can be caused by two events on the printing press. The ink trough runs out of ink, and streaks first start appearing, as there is not enough ink to fill the recesses/cells. Or the press has been stopped for a time. The solvent dries out in the recesses/cells, clogging them up. When the press starts again, it takes time for the solvent in the trough to clear/dissolve the ink in the cells. Most of these start up sheets are destroyed, but some slip through the checkers.

The most common dry print happens to the PHOSPHOR/FLUORESCENT printing unit, as the printer can only see what he has printed with a UV light. The ink runs out. A stamp variety is created! 

OFFSET: (Print appearing on the gummed side of stamps). Imagine, with reference to the gravure diagram above, if there is no paper between the cylinders. The ink/image is then transferred to the impression cylinder. Next time paper passes through this is transferred from the impression cylinder onto the reverse side of the paper, in mirror image/reversed, gradually fading away with each revolution. THIS IS NOT SETOFF.
SETOFF: This is where the finished sheets coming off the printing press, are still wet. When stacked some of this wet ink smudges onto to reverse of the sheet above. Printers sometimes spray a fine white powder on to the sheets as they come off the press. This is to try and prevent this setoff happening. That's why some printing presses have the appearance of being snow covered. Gravure presses do not suffer with this problem very often, the solvents used  make the ink dry very fast. Modern presses have UV/INFRA RED driers to help dry the ink faster, avoiding this problem..
DOUBLE IMAGE: In gravure or letterpress this can occur if the doubling is on the printing plate to start with! A RE-ENTRY in gravure is due to the image getting slightly doubled during the manufacture of printing cylinders. NOT DURING PRINTING.

IN OFFSET LITHOGRAPHY DOUBLE IMAGING IS VERY COMMON. Remember the image is transferred to a cylinder covered with a rubber blanket first (OFFSET LITHO) before being applied to the paper. Printers move these  printing cylinders and the rubber blanket cylinder in relation to each other OFTEN. This is how they control registration of the images. Every time this occurs it leaves a ghost image on the rubber covered cylinder, this image prints along side  the new image position until it fades away. Slight adjustments are often made by printers, while the press is printing! You have been warned!!!

NOTE: If a sheet goes through the printing press twice, in any of the processes, you will get two images. This is often found when stamps are overprinted.

COMMON PRINTING TERMS

 4 COLOUR PROCESS 

This process is used by all the types of printing processes. If you look at modern colour pictures, as printed in your glossy magazine with a magnifying glass, you will see it is made up of very many dots of various sizes. You will also notice that these dots are only to be found in MAGENTA (RED)/CYAN (BLUE)/YELLOW & BLACK. Printers call these "process colours". Only 4 colours are used to produce all the colours you see in the printed picture. By having dots of different sizes, the eye sees the sum of these dots as a shade of colour. Place a yellow dot next to a blue dot and you will see this as green. The bigger the blue dot in relation to the yellow dot will create an impression of a darker green.

Think. Every dot has to be placed on the paper in exactly the right place to produce this effect. This is called registration. 

 

REGISTRATION: After reading about 4 colour process above, imagine wanting to print a green line by using 4 colour process printing. You would have to print a yellow and a blue line exactly on top of each other, to get a green line. When this does not happen due to bad registration, you will see a green line with a blue line down one side, and a yellow line down the other side, because the lines are not sitting exactly on top of each other. Sometimes you get colour halos around pictures due to bad registration. This mis-registration can be caused in various ways. Sheets moving while being printed, Sheets not being fed into printing press correctly, printing press not set properly by the operator etc. Many stamps have minor colour shifts like this. Some major colour shifts are listed as varieties. NOTE: Small shifts are very common.

 

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