US2160242A - Photomechanical printing surface - Google Patents

Description

Patented May 30, 1939 i STATES FATEv T OFFEE South India, assignor of one-half to Robert Laing Bruce Gall, Ootacamund, Nilgiris, South India No Drawing. No. 28,572.

7 Claims.

This invention concerns improvements relating to the production of printing surfaces, including blocks, plates and the like, and has for its principal object to provide a process for producing these surfaces which is simpler or more economical than the processes hitherto known, and which is less affected or is unaffected, by temperature, humidity and similar factors.

In the process according to the invention, a

photographic originating element having the image defined in a halide of silver, or silver and mercury, is applied in contact with a smooth metal surface, preferably a polished copper plate, in the presence of a deposition agent with which the halide does not react and in which it is only slightly soluble and in the presence of which the halide is reduced by the metal of the smooth surface. In this manner a metallic silver or mercury-silver deposit corresponding to the image is obtained upon the said surface which may then be employed or treated as hereinafter explained, depending upon the nature of the actual printing to be performed. By a deposition agent is to be understood one of the reagents hereinafter referred to as such or an obvious chemical equivalent thereof.

The nature of the originating element employed, for instance whether positive or negative, will usually depend upon circumstances and the nature of the actual printing. In general the originating element may be similar to that which would have been employed hitherto for printing of the same nature. For example, for the purposes of half-tone printing, the originating element may be a positive prepared with the assistance of the usual screen.

In cases wherea developed positive is employed to give the originating element, the metal in which the image is defined is first converted into the chloride, bromide or iodide thereof by a bleaching agent, for example copper chloride, capable of effecting the conversion. As the strength of the bleaching solution does not affect the quality of the image, a strong solution may be used to save time, 10-30 secs. only being then required. After a brief rinse, the element may at once be placed in contact with the metal surface.

In cases where the positive employed has been obtained by a known reversal process, the usual final development step [is unnecessary as the image is then already defined in chloride, bromide or iodide.

Among the deposition agents which may be used for the treatment of the photographic originating element are hydrochloric acid and certain Application June 26, 1935, Serial In Great Britain July 5, 1934 chlorides such as sodium or potassium chloride. An example of how the process may be carried into effect with hydrochloric acid will now be given. In this example, the originating element having the image defined in silver chloride, bromide or iodide is pressed in contact with a clean highly polished copper plate (advantageously chemically clean and free from grease) which has been first wetted with hydrochloric acid diluted with water in, say, the ratio of two to five. A pressure sufficient to ensure true contact between the element and copper plate is all that is necessary. The effect of this contact, which is maintained for about one to three minutes, is to deposit silver upon the copper plate in a polished form and in precise correspondence with the image originally defined in the silver halide. A stronger acid solution or a longer contact period may be employed to give a heavy brilliant deposit, but these are liable to lead to loss of polish upon the bare copper surface of the plate, which is undesirable. Should the polish be so lost, however, it may be restored without damaging the silver deposit by polishing the plate with a soft cloth. Also, in order to obtain a heavier deposit,

the originating element itself may be immersed for a short time, say thirty seconds, in hydrochloric acid of the strength referred to before being placed in contact with the plate.

Among other deposition agents which may be employed for the treatment of the originating element are: soluble sulphites, such as, for example, sodium sulphite, preferably used in aqueous solutions of about 30% strength.

As already stated, a deposit comprising mercury may be employed. This deposit is apparently mercury with an admixture of silver in a small proportion. The originating element for this modification may have the image defined in a halide of silver and mercury, for example silver mercury-chloride. Such an image may be obtained, for example, by bleaching a photographic negative or positive in a mercuric halide solution as is done in the common intensifying process employing mercuric chloride. sulphites, as referred to above, are preferably used as the deposition agent for producing the metal deposit.

For some purposes, the copper plate upon which the deposit is made may be replaced without disadvantage by a zinc plate electrolytically coated with copper or by a brass plate. Moreover, similar results can be obtainedwith, for example, a nickel-coated plate, a plate with an amalgamated surface or a zinc plate.

Further treatment of the surface bearing the metal deposit in order to adapt it for the actual printing will now be described:

In accordance with one such treatment, a copper plate with the silver deposit is dipped into a solution obtained by adding a 12.5% solution of copper sulphate, containing 5% of concentrated sulphuric acid, to a 40% solution of hydrochloric acid (40 parts of hydrochloric acid of specific gravity 1.16 to 100 parts of water by volume) in proportions of 16:1. A compound, which is presumably cuprous chloride, is formed on the exposed copper, whilst the silver is unaffected. The plate is dried and one of two alternative courses followed: According to one course, the entire surface of the plate is covered with printing ink applied by a roller and the plate is immersed in a liquid in which cuprous chloride is soluble, for instance, a strong solution of sodium thiosulphate. The ink that covered the cuprous chloride becomes loosened and can be removed by, for example, passing a roller there over, While that on the silver image remains adhering. Etching may then be performed in the usual manner. According to the other course, the silver is amalgamated by rubbing over the surface of the plate with metallic mercury. The cuprous chloride is then dissolved with a strong solution of sodium thiosulphate and the plate inked up. The ink adheres to the copper surface but not to the amalgamated silver image. Etching may follow.

According to an alternative treatment, the plate with the silver deposit is subjected to a graining step with the object of destroying the polish upon the copper surface whilst leaving that upon the silver surface, so that subsequently the copper surface can be wetted Whereas the silver will take the ink employed. This step may comprise treating the plate the plate, which should be free from grease, with a reagent which attacks copper, such as a dilute solution of ferric chloride. For example, a solution of from 1-2% strength will require about 6030 seconds to produce the necessary graining effect. It is possible to check when the effect has proceeded far enough, for the silver deposit then appears black against a pinkish background. The graining should not be carried too far or the silver deposit may be impaired. The plate is next inked up in the known manner for lithographic printing. It is to be recommended that the inking up should be performed in a lithographic machine and that the whole surface of the plate should be kept moist during the operation.

If after the first inking up, it appears that the graining is insufilcient, it can still be accentuated by applying a stronger reagent. For certain inks, the above described graining step and the first inking up are in any case advantageously followed by the application to the plate of mercury or a solution of a mercuric salt. This has the effect of amalgamating or coating the bare copper parts whilst leaving the silver, which is covered by ink, unaffected. The plate is then ready for lithographic printing and in this case the usual wetting is unnecessary.

When the plate is required for half-tone or line printing, the etching may be performed thereon, after the inking, in the usual manner. If a line negative originating element is employed, an engraved plate can be obtained such as is commonly employed for printing bank notes.

If it is required to obtain a positive printing surface from a negative originating element, the above described graining and inking up may be followed by the deposition of chromium upon the bare copper. The removal of the ink then leaves a positive image in chromium surrounded by the original deposit of silver. Finally an etching agent, such as chromic acid, is applied to remove the silver and etch the copper to the depth required for printing.

The above-described graining step may be replaced by a step of which the object is to deposit upon the bare copper of the plate an amount of mercury which is sufficient to repel printing ink but which does not materially affect the silver deposit. As a very thin film of mercury is sufficient if the copper surface is highly polished and the mercury deposits in a brilliant form, care should be taken to maintain or restore the polish in the copper. The mercury may be deposited by dipping the plate or immersing it for a short time, for instance seconds, in a dilute solution of mercuric chloride, for example an aqueous 0.25% solution containing some free hydrochloric acid (for example 8% measured by volume, of concentrated hydrochloric acid-sp. gr. 1.16). The latter component is believed to assist in producing a mirror-brilliant deposit of mercury.

The plate is next dipped in water for a second or so and dried with an absorbent material, care being taken not to rub the surface.

The surface, which at this stage consists of the silver image surrounded by mercury, is then inked rapidly with a printing roller charged with a very thin layer of ink, whereupon the silver attracts ink whilst the mercury repels it. The amount of ink attracted to the silver in this operation may not be heavy enough to resist the action of an etching bath. To enable the ink deposit to be strengthened, the mercury deposit must be augmented. This may be done by immersing the plate for a short time in a stronger solution of mercuric chloride containing some free hydrochloric acid. Alternatively it may be done more rapidly by passing over the plate a copper or brass cylinder of which the surface has previously been amalgamated. This augments the existing mercury deposit but leaves the lightly inked parts unaffected. In either case, inking up may then be performed with a roller charged with a normal amount of printing ink. The inking operations may be performed by hand or, preferably, by machine. etched and printed from in the ordinary manner.

By a modified process, it is possible to obtain a printing surface which is of the opposite nature to the originating element, for example a positive printing surface directly from a negative originating element or vice versa. Consequently, by adopting the appropriate process, it is possible to produce a required printing surface from any given or convenient originating element. Briefly the modified process resides in treating the plate in such manner as to cause mercury to adhere to the silver deposit but so as ultimately to leave the copper unaffected. In this condition, the amalgamated silver surface will repel ink whereas the copper surface will take it.

A specific example of the production of a positive printing surface from a negative originating element by this modification will now be described: A silver deposit upon a copper plate is produced from the originating element as previously explained. The whole surface of the plate is amalgamated either directly with metallic mercury or by immersion in a dilute solution of mercuric chloride containing some free hydro- Ihe plate is finally t chloric acid, the precise strength of such solution being of no great importance. In either case the surface is gently rubbed, for example with a cotton-wool pad, to facilitate thorough amalgamation of the silver. The amalgamation may require from 30-60 seconds. After treatment under these conditions,the mercury does not adhere as firmly to the copper surface as it does to the silver deposit and this fact is utilised in order to remove the mercury from said copper surface. To this end, the whole surface of the plate is given an intimate coating of an adhesive medium, preferably printing ink, which can be applied by rolling slowly and with pressure. The surface is then treated with a solvent as for example petrol or benzene, to remove or wash off the adhering ink or other adhesive medium. The less strongly adhering mercury on the copper is removed at the same time, leaving the said copper bare, whereas the greater part of the more strongly adhering mercury amalgamated with the silver remains. It is to be assumed that this effect is due to the mercury particles adhering less strongly to the smooth copper surface, and more strongly to the silver deposit, than to a medium such as printing ink. The plate may finally be inked up, the link being attracted to the copper and repelled by the amalgamated silver, and may then be etched.

To assist the etcher, wet ink resists produced in the above described methods of operation are preferably dried in known manner by dusting them over with bitumen and heating.

According to a modification, the ink image obtained upon the original copper plate is not used directly for lithographic printing, but is transferred to an appropriately grained zinc or aluminium plate by the off-set method and the latter plate used for the printing.

An intaglio plate for rotary photogravure printing may be obtained in the following manner: A film is exposed under a normal rotary photogravure screen and a negative of the original picture is then made upon it. This developed negative is bleached and used as the originating element for depositing the silver image on the copper plate. The plate is then treated with a somewhat stronger solution of mercuric chloride (1 2 containing some free hydrochloric acid (16%) by volume of concentrated hydrochloric acid, (sp. gr. 1.16). The mercuric chloride dissolves the copper, depositing on it at the same time an amount of free mercury equivalent to the weight of copper dissolved. This solvent action of the mercuric chloride on the copper is retarded, where the silver image occurs, according to the thickness of the silver deposit which in turn corresponds, from point to point, to the tone in the originating negative. This retarding effect of the silver deposit is due to the fact that the underlying copper can only be dissolved after the silver above it has been amalgamated. The action of the mercuric chloride is continued until the thickest parts of the silver deposit begin to amalgamate (4-10 min). The deposited mercury is then evaporated by heating and the resulting plate is of an intaglio nature suitable for rotary photogravure printing,

Alternatively the plate with the silver positive deposit and a plain copper plate, connected respectively to the positive and negative terminals of a battery, are placed in a copper salt solution, preferably a copper sulphate solution containing a small quantity of sulphuric acid. Upon the passage of a small current between the plates, it is found that the copper under the deposited silver is everywhere dissolved to a depth varying with the thickness of the silver deposit from point to point. This gives a plate of intaglio nature which may be utilised for rotary photogravure printing.

According to a further alternative, a copper plate'with a silver deposit obtained from a positive originating element printed under the appropriate screen is immersed in a weak iodine solution and then in a dilute solution of sodium thiosulphate, ammonium or potassium cyanide. The surface then consists of silver iodide, into which the silver has been converted, surrounded by copper. The plate is next made the cathode of an electrolytic cell whereof the anode is nickel and the electrolyte a solution of a nickel salt. Upon the passage of a small current through the cell, a black deposit, which is presumably nickel, is formed on the silver iodide in accordance with the thickness of the latter which in turn corresponds from point to point with the tones of the originating element. This deposition may require a few minutes. Finally the plate is dried and an electrotype or cast is made from it to provide an intaglio plate which may be printed from in the normal manner.

As will be readily understood, the printing surfaces obtained according to the invention may also be employed in known manner for various purposes indirectly associated with printing. For example, if the surface ultimately obtained is in cylindrical form, it can be employed or adapted for use for the transmission of the image by telegraph or by wireless.

I claim:

1. Process for the production af a printing surface from a photographic originating element having the image defined in a halide of silver, comprising applying the said element in contact with a smooth metal surface in the presence of a deposition agent with which the halide does not react and in which it is only slightly soluble and in the presence of which the halide is reduced by the metal of the smooth surface, and thereby producing upon the said surface a metallic silver deposit corresponding to the image.

2. Process for the production of a printing surface from a photographic originating element having the image defined in a halide of silver and mercury, comprising applying the said element in contact with a smooth metal surface in the presence of a deposition agent with which the halide does not react and in which it is only slightly soluble and in the presence of which the halide is reduced by the metal of the smooth surface, and thereby producing upon the said surface a metallic mercury-silver deposit corresponding to the image.

3. Process for the production of a printing surface from a photographic originating element having the image defined in a halide of silver comprising applying the said element in contact with a polished copper plate in the presence of a partial solvent for the halide which solvent is chemically inactive towards the said halide, to reduce the halide and produce upon the said plate a metallic silver deposit corresponding to the image.

4. Process for the production of a printing surface from a developed photographic positive having the image defined in silver, comprising treating the said positive with a bleaching agent to convert the silver into silver halide and then applying it in contact with a smooth metal surface in the presence of a deposition agent with which the halide does not react and in which it is only slightly soluble and in the presence of which the halide is reduced by the metal of the smooth surface, to produce upon the said surface a metallic silver deposit corresponding to the image in the positive.

5. Process for the production of a printing element from a photographic originating element having the image defined in a halide of silver, comprising applying the said originating element in contact with a smooth metal surface in the presence of a halide solution with which the silver halide does not react and in which it is only slightly soluble and in the presence of which it is reduced by the metal of the smooth surface,

and thereby producing upon the said surface a free metallic silver deposit corresponding to the image.

6. Process for the production of a printing element from a photographic originating element having the image defined in a chloride of silver, comprising applying the said originating element in contact with a smooth copper surface in the presence of a chloride with which the silver chloride does not react and in which it is only slightly soluble and in which it is capable of being reduced by the copper, and thereby producing upon the said surface a metallic silver deposit corresponding to the image.

7. Process for the production of a printing element from a photographic originating element having the image defined in a halide of silver, comprising applying the said originating element in contact with a smooth metal surface in the presence of a solution of a chloride of an alkali metal and thereby producing upon the said surface a metallic silver deposit corresponding to the image.

KEVIN WILLIAM CATON W'EBB.