State of the art At present, classical processes for printing and reproduction are used, in which gelatin layer or the layer of synthetic polymer hardened by chromium salts, mostly dichromates, are used for printing and reproduction. Silver halides are used only in photography.

Common is the method using gelatin layer or synthetically prepared polymer layer impregnated with potassium or ammonium dichromate, or by the combination of both. This layer deposited on base mount is exposed by light passing through a negative or positive, which results in hardening of illuminated parts of gelatin or polymer. In the non-exposed parts no hardening takes place, and for that reason, in a course of consequent rinsing, complete or partial removal of non-illuminated parts from the mount, on which it is deposited, takes place. By this way the printing face (block) is created, which is, after drying, ready for printing.

The method described above is from the environmental point of view strongly inappropriate, because the excretion of chromium salts to sewage water significantly pollutes the environment. Another disadvantages are great energetic demands, and time consumption during the block production.

Another method is using the ability of gelatin to swell proportionally according to the degree of hardening. This is the classical process of collotype according to Jakub Husnik (1868). A gelatin layer is deposited on a support or mount, most often a glass, and than, after the drying, the layer is sensitized by treatment with potassium and/or ammonium dichromate in the chromizing solution. Subsequently, the layer is exposed through the half-tone slide. This results in the proportional hardening of the layer proportionally to the amount of light passed through the slide. Remaining dichromate salts are removed in water bath (print characteristic, i. e. the degree of gelatin wrinkling is determined by the bath temperature) and the layer is dried.

By this, gelatin layer is ready for printing. Before the printing itself, it is necessary to remoisten the layer with water solution of glycerol which results in differential degree of swelling of the layer depending on the layer hardening intensity. The offset i. e. the collotype printing ink is rolled up onto the layer prepared in the way described above during the printing process itself.

The disadvantages of this technique consist of significant time consumption and great demands on air conditioning of the workplace environment, particularly because of high humidity, therefore it is used in small scale production only. The other disadvantages are: use of breakable glass support, significantly high energy consumption, and pollution of environment by the chromium salts.

Another known way of gelatin utilization for printing purposes is the method of lithographic plate preparation described in the US patent No. 5 064 745. Gelatin layer containing silver halides is developed after light exposure, and in the next stage of preparation, the resulting metallic silver is oxidized in the layer in the presence of monocyclic components to which it is bound and forms together lipophilic foundation of the printing surface.

Silver remains in gelatin, and the gelatin itself forms only hydrophilic areas of the printing matrix.

Disadvantage of this method is the time consuming preparation, which takes approximately 14 days, lipophilicity of the matrix surfaces, and the fact, that it can be used only for printing of lithography.

Another method of gelatin utilization is the production of phase hologram using the process according to US patent No. 4 187 106. Thick gelatin layer containing silver bromide is exposed in a special way and developed. Metallic silver is reversed to silver bromide by bleaching. With certain illumination, light refraction appears in the gelatin structure containing reversed silver bromide, which is the principle of hologram. However, this method is not related to printing.

Summary of the invention The disadvantages mentioned above are eliminated by the process devised herewith. The substance of the process is in the principle that for the printing plate production, instead of the layer containing only dichromate, directly the layer containing silver halogenides is used, which is done so that the natural gelatin and/or synthetic polymer or polymers layer containing silver halogenides is fixed onto the solid support, exposed to the light, preferentially through photographic positive or negative, and it is developed and fixed by the process common in photographic practice. Then, the layer is either treated by bleaching liquid until complete or partial bleaching occurs, or it is eluted by washing solution, then fixed, washed with water, possibly containing detergent, and dried. The carrier in the form of natural gelatin and/or synthetic polymer means usual commercially produced photographic layer or even any other layer containing for instance agar, polyvinylchloride, polyvinylacetate etc. Silver halides contained in the layer are particularly silver chloride, bromide, and/or iodide.

Washing solution is the aqueous solution containing cupric dichloride in the concentration 5 to 30 g/1, weak organic acid in the concentration 0.1 to 60 (wt./wt.), and<BR> <BR> <BR> <BR> <BR> hydrogen peroxide in the concentration 0.01 to 10 t<BR> <BR> <BR> <BR> (wt./wt.). Changing the ratios and concentration of this solution changes the speed of the process. The solution described above is optimal for the production of printing plate for dry printing (dry relief offset)-production of autotypic printing block and line block.

For the half-tone printing, the bleaching liquid is used which is the aqueous solution containing cupric chloride in the concentration 4.6 to 9.2 g/1 and sodium or potassium dichromate in the concentration 2 to 4 g per liter of the solution. Changes of printing properties of the printing plate are achieved by changing the components ratios and concentration of the basic bleaching liquid. This basic process is applicable to the production of printing plate for dry printing (dry relief offset)-printing block -as well as for wet printing (collotype). If the printing plate is used for collotype, it is necessary to remoisten the plate before the very use. When needed, the process described above is followed with that modification that the silver halides containing layer can be additionally washed with water (with the addition of detergent when appropriate) before immersing it to the eluting solution or bleaching liquid.

The method devised herewith allows common photographic layer to be used for the production of high quality printing plate. After processing, this layer can be used as effective part of the printing plate, either for direct printing from it, or as an effective part of the printing block, screen or offset plate.

The weak organic acid for the process described above can be preferentially acetic or citric acid. Addition of such weak acid in the amount specified herewith ensures acid conditions which are needed for successful completion of the reaction and which is simultaneously gentle against the gelatin layer. Using other acids, for instance some known strong inorganic acid, would result in unwanted erosion of the gelatin layer.

Bleaching liquid based on cupric chloride can be prepared from various soluble salts so that the needed amounts of chloride and cupric ions are present in the solution. Taking into consideration quality of the achieved result, it is advantageous to prepare this solution as. a water solution containing cupric sulphate (pentahydrate) in concentration 6 to 12 g/1, sodium chloride 4 to 8 g/1, and potassium dichromate in concentration 2 to 4 g/1 of the solution.

For practical use of the method for wet printing, it is required to moisten the plate before the actual use. Optimal quality is reached when moistening with water containing glycerol in concentrations ranging from 16.6 up to 75 vol.

%, possibly with the addition of ammonium hydroxide or ammonium carbonate in concentration up to 10 wt. %. This ensures the swelling of the non-hardened parts of the layer and the plate is ready for printing.

The main advantages of the method described above are: lower time and energy demands in comparison with the methods used at present, environmentally more acceptable operation, lower demands on workplace environment when there is no need for high relative humidity of the workplace. Simultaneously, high accuracy and high quality printing and reproduction is achieved.

The method of printing plate production described here is suitable for production of the plate for direct printing of artist's pieces of graphic art from the plate, as well as for direct industrial printing after the adjustment of printing machines. It facilitates production of autotypic blocks as well as line blocks for printing.

Description of the preferred embodiments Example 1 Concrete example of the method application is the manufacturing of the printing plate for dry printing -block.

Technical film of the hard gradation was exposed through slide with autotypic screen in such a way that the layer was exposed through up to the support. After the routine developing, fixing, and washing, the layer was dipped into the solution of the following optimal composition: Cupric chloride CuCl2.2 H2O.... 8 g acetic acid 98% C H 0.... 40 ml 2 4 2 hydrogen peroxide 10% H202.... 30 ml water up to 1 000 ml In this bath, all the metallic silver together with gelatin was removed from the exposed parts of the layer within 5 minutes at 20°C. Remaining relief surface was usable as autotypic block after 10 minutes of routine washing in water and subsequent drying. Than, for the printing itself, the film was sticked by its base to the mount with the help of double-sided self-adhesive tape.

Example 2 Other example is the preparation of printing plate for wet printing.

Technical reproduction film was exposed through a negative, directly by magnifying apparatus. The exposition can be performed also by other ways, e. g. by contact method from film or tracking paper. Exposed film was immersed in bleaching liquid of the following composition: Cupric sulphate CuSO. 5 H2O.... 12 g sodium chloride NaCl.... 8 g potassium dichromate KCr0.... 4 g water distilled up to 1000 ml In this bath, the bleaching and simultaneous proportional hardening of the gelatin layer according to the amount of metallic silver contained in the layer occurred within 7 minutes at temperature 20-30°C, and with moderate movement. Completion of the reaction was manifested by complete bleaching of the picture in the most intensely covered parts. Bleached film was washed for several minutes in running water, and then it was fixed again in common fixativing bath until complete disappearance of the milky picture. The film was washed again in the running water and then dried.

For the printing itself, the layer was sticked on solid, flexible support from plastic, with the help of 2 mm thick self-adhesive tape. Before the use, the layer was activated by the solution of following composition: 1 vol. part of glycerol 1 vol. part of ammonia-24 (wt./wt.) solution of NH OH in water 4 4 vol. parts of water After 7 minutes of treatment, swelling of the non-hardened parts of the plate was clearly visible. The plate prepared in this way was ready for printing.

Advantageous amount of glycerol ranges from the water to glycerol ratio 1: 4 to 4: 1 vol. %, with addition of weak alkali in the final concentration up to 10%. Observing the ratio ensures optimal qualitative properties of the product.

Example 3 Other example, alternative to example 1 is the following model production of the printing plate for wet printing: Process similar to embodiment 1 was used with that difference that washing solution of the following composition was used: Cupric chloride..... 5 g/1 Acetic acid..... 0.1 (wt./wt.)<BR> Hydrogen peroxide..... 0.01 (wt./wt.) The next steps and the result corresponds to the process described in the embodiment 1. Metallic silver was washed out from the layer after approximately 5 hours of treatment with washing solution at room temperature and with occasional stirring.

Example 4 Other example, alternative to eexample 1 and to the previous example is the following model production of the printing plate for wet printing: Process similar to embodiment 1 was used with that difference that washing solution of the following composition was used: Cupric chloride..... 30 g/1 Citric acid..... 60 % (wt./wt.)<BR> Hydrogen peroxide..... 10 % (wt./wt.) The next steps and the result corresponds to the process described in the embodiment 1. Metallic silver was washed out from the layer after approximately 5 seconds of treatment with washing solution with continuous stirring.

Example 5 Other example, alternative to example 2 is the following model production of the printing plate for wet printing: Technical reproduction film was exposed through a negative, directly by magnifying apparatus. However the exposition can be performed by other ways, e. g. by contact method from film or tracking paper. Exposed film was immersed in bleaching liquid of the following composition: Cupric sulphate CuSO. 5 H2O.... 6 g Sodium chloride NaCl.... 4 g Sodium dichromate K2Cr2O7.... 2 g Water distilled up to 1000 ml In this bath, the bleaching and simultaneous proportional hardening of the gelatin layer according to the amount of metallic silver contained in the layer occurred within 15 minutes at temperature 20-30°C, and with moderate movement. Completion of the reaction was manifested by complete bleaching of the picture in the most covered parts. Bleached film was washed for several minutes in running water, and then it was fixed again in common fixativing bath until complete disappearance of the milky picture. Film was washed again in the running water and then dried.

For the printing itself, the layer was sticked on solid, flexible support from plastic, with the help of self-adhesive tape. The layer was activated before use by the solution of the following composition: 4 vol. parts of glycerol 1 vol. part of water (i. e. approx. 80 % final concentration of glycerol) After 10 minutes of treatment, swelling of non-hardened parts of the plate was clearly visible.