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This is just a quick outline of stamp printing by gravure 

Engraving/Gravure printing is where the ink is held in a recess etched or cut into the surface of a printing plate.  The engraver cutting by hand the image which holds the ink. LINE ENGRAVING. Today the gravure process is  mostly reserved  for security/banknote and long run printing jobs (colour catalogues). The modern cylinders needed for this process are expensive to manufacture compared with lithographic plates.

NOTE: Illustrations in early books show the skilled craft of hand engraving at it's very best. 

In the gravure printing process, the printing plate has recesses/cells on it's surface, this fills with ink as it passes through the ink trough, a DOCTOR BLADE then scraps the excess ink off the surface, leaving the ink in the recesses/cells, which is then transferred to the paper.

Every colour will have its own printing unit like this one, the web/paper passing from one unit to the other. This has to be done in perfect registration, so each colour prints in the right place on the paper.

ENGRAVED STAMP PRINTING 

This is where the image is cut/engraved/ impressed into the surface of the printing cylinder. (ENGRAVING) 

LINE ENGRAVED. The image nearly always consists of lines.  Take a close look at some early stamps, you will see it is made up of these lines. 

Engraved print illustrations found in early books are a good example of this craft.

A gravure printing cylinder image can hold lot of ink, which is transferred to the paper during printing. This leaves a lot of ink on the surface of the stamp, which you can feel sometimes. NOTE: You can feel the ink on banknotes printed using this process. Try it.

PHOTOGRAVURE STAMP PRINTING

Photogravure is where the image is etched using a photographic process onto the cylinder. This is process involves screening the image. This produces rows of cells, which make up the image. You always have a CELL, so straight lines are made up of a row of cells (dots). Because the ink involved with the gravure process, is more a liquid, the ink tends to fill in solid areas (These solid areas are really made up of large cells, close together). The edges of designs using this process are most distinct.

HAVE A LOOK AT THE TEXT OF YOUR LARGE COLOUR CATALOGUES, ARGOS/LITTLEWOODS in the UK. YOU WILL SEE THAT EVERYTHING IS MADE UP WITH CELLS/DOTS, including the text. The same applies to stamps printed by photogravure. Look closely at the illustration. You will see these dots/cells along the edges of  lines on the stamp. YOU CAN NOT HAVE A PERFECT STRAIGHT LINE IN PHOTOGRAVURE. The stamps below illustrate the difference at the edges on modern stamps. Some modern screens are very fine, making this screening very difficult to see.

NOTE: Electronic engraving is now used on many modern stamps, so these stamps do not show some of these characteristics. 

 

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

The right stamp shows the clearly the dots/cells edges found in  PHOTOGRAVURE.

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

PRINTING FAULTS FOUND IN GRAVURE

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! 

Further reading: The new observer's book of stamp collecting. Anthony New. ISBN 0-7232-1692-4

 

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