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Polish photogravure printing has been evolved rather quickly. Around 1980 most Polish stamps had been printed in photogravure and hardly any stamp was left printed in offset-litho. The two GOEBEL-pressses [1970 5-colours photogravure and 1979 1 recess-intaglio + 2 photogravure] made it possible to produce an enormous amout of postage stamps in photogravure. But only 10 years later this had changed again. Since the early 1990-ies photogravure is rare - apart from the definitives stamps. The definitives show an uncanny variety of screens - mainly differing slightly in the angle the screen makes. A 45 degrees angle for monocoloured cylinder would be normal, but on a closer look several definitives turn out to have 2 cylinders used for the same colours [3.5, 5, 10 zł Wawel Heads]. You will notice angles of 45, 40, 50, 55 degrees etc. for apparently no reason, but for a likely inaccurate superpositioning of the [glass] screen while preparing the carbon tissue [the thin gelatine sheet that has to be wrapped around the copper-surfaced cylinder before one can start etching] . But the screen angle of the definitives is a chapter on its own... |
Around 1992 the conventional way of preparing photogravure cylinders using glass or film screen-plates to superimpose on the carbon tissue paper was about to be abondoned. Just like in preparing plates of films for offset-litho, the screen will already be present in the film. A computer-programme will 'extract the colours' and assign standard screen angles as follows:
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Traditional photogravure will have depth-variable screen-dots, that is darker spots will need deeper dots, brighter more shallow. Each dot will have the same surface measurements. But along with the computerized colour-extraction there is no longer a need for depth-variable screen-dots only. The screen-dots may vary in surface as well. This is called a autotypical screen. In the period 1992-1996 several Polish stamps were produced using autotypically etched cylinders. When asked Stefan Jakucewicz suggested a shallowy way of etching ['płytko'] instead of autotypia. I doubt it. |
Not only there is a change from a rather messy choice of screen-angles to a systematical use of the 0, 15, 45, 75 degrees for the 4 basic colours of modern [computer supported] printing, but also you can see [enough magnification of course] that the screen dots aren't of the same size but vary indeed. The 1993 "Polska '93" stamps have two traditional [1500, 3500 złoty] and two autotypical [2000, 2500 złoty]. The traditional etchings in this period usually have screen resolution 90 lines/cm, whereas the autotypical screen has R80 or R90. The screen-angle for red [magenta] is 45 for autotypical but the traditional screens will show quite a variety of angles. |
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