William Willis Patent 1913 #20022
Improvements in or relating to Photographic Printing and Paper therefor
British Patent #20,022 , 4th September 1913, William Willis Jnr
Specification
This invention consists in improvements in or relating to photographic printing and paper therefore, and the expression “paper” in this specification is used to include paper, textile fabrics, wood, and other surfaces suitable for receiving photographic prints.
The principal object of this invention is the production of photographic printing paper having a coating or surface sensitive to light and containing salts of iron, silver and platinum, which paper after exposure to light under a negative and subsequent development shall yield an image composed of silver and platinum, the platinum constituent of such image being variable in quantity in some proportion to the amount of platinum salt used in the sensitive coating of such a paper.
Thus the said paper may be produced either so as to give images composed of silver with very little platinum or images composed of silver with an increased proportion and quantity of platinum. And in practice the quantity of platinum in the image may advantageously be so increased until a point is reached where such image may be considered permanent. This point may be defined as the point where the image in platinum remaining after the silver content of the original silver and platinum image has faded, or has been removed, will of itself satisfactorily represent the gradation and retain all of the detail which existed in the original silver and platinum image before its silver content had faded or had been removed.
A further object of this invention is to secure such permanent residual images in platinum as hereinbefore described by the employment of a quantity of platinum salt in the coating of the said printing paper less than that used in the platinotype process.
And other objects of the invention are that such photographic printing paper shall be easy of manipulation in weakened ordinary daylight ; shall require only moderate exposure to light and shall remain in good condition during storage ; and further that the images produced on such paper shall require no subsequent toning.
This invention is based on the discovery that I have made that the reduction of chloride of silver by a solution of ferrous oxalate in potassium oxalate is greatly facilitated, increased in rapidity and made more complete, by placing in contact with this silver salt a small quantity of potassium chloro platinite before the reducing agent (ferrous oxalate) is applied to the silver salt. And I have found that the only salts of platinum of value in aiding in the reduction of silver chloride by ferrous oxalate are salts of chlorine and of platinum that is to say chloro platinites or chloro platinates, salts which are also described as platino-chlorides and platini-chlorides. And I also find platonic chloride to be of value. The salts of platinum just named are themselves reducible by a solution of ferrous oxalate and when one of these salts is used as above described to aid in the reduction of silver chloride it will be reduced together with the silver salt and both platinum and silver will be liberated. This effect of platinum salts in aiding the reduction of silver chloride can be easily shown by a purely chemical method but perhaps more readily by a practical trial on paper in the manner following: If the paper is first of all coated with silver chloride and then with ferric oxalate, then exposed to light under a negative and then developed in oxalate of potashit yields only a poor image, but if in addition to the silver chloride and ferric oxalate the coating is made to contain a small quantity of potassium chloro platinite, then on exposure and development a stronger image of blackish colour will be obtained ; and it has been proved that the intensity of this image is due mostly to the increased reduction of the silver salt and only slightly to the reduction of the small quantity of platinum salt used in this experiment. One of the methods in which this discovery is applied in this invention to the production of an improved photographic printing paper is as follows :—
The paper first of all receives a coating of chloride of silver which may be applied or attached to the surface as an emulsion in gelatine or other suitable medium or formed on the surface by the well known process of double decomposition. This coating of silver chloride should be free from other salts of silver. The coated surface is then again coated with a solution of ferric oxalate in which is dissolved some potassium chloro platinite (K2PtCl4). After the paper has been dried, it is exposed to light under a negative. The action of light on the negative form on the sensitive coating of the paper a faint image consisting mainly of ferrous oxalate. This exposed paper is now immersed in a solution of oxalate of potash which quickly commences to dissolve the ferrous oxalate forming the image, but this solution of ferrous salt in oxalate of potash is a strongly reducing agent. Now salts of silver and platinum are in contact with the ferrous image on the paper and as this ferrous image is dissolved by the application of oxalate of potash, the resulting solution of ferrous salt reduces these salts of silver and platinum “in situ” forming or yielding an image composed of silver and platinum more or less in the metallic state and coinciding with the position of the original image of ferrous oxalate. This final image of silver and platinum is of a black tone agreeable to the eye and does not require toning. All that now remains is to dissolve out the unaltered salts from the paper in any suitable way.
In the production of this photographic printing paper it is essential to the invention that the salt of silver employed shall be the chloride and not a salt of silver like the nitrate and others which will decompose the platinum salt.
In my previous patent No. 2011 of 1873 the object of which was to obtain photographs or pictures in which the image should consist of platinum, iridium or gold the final specification (Example 2) gave a method somewhat similar to the method described in this invention.
In that method paper was coated with nitrate of silver.
In practice it was found that the developed images on such paper contained very little metallic platinum, that they were very variable in quality and often required subsequent toning to render them of value. The process was unsatisfactory and quickly abandoned. The most important distinction between the method employed in 1873 and that used in this invention lies in the fact that in the 1873 method of coating paper the platinum salts is decomposed by nitrate of silver, whereas in this invention the platinum salt and silver chloride do not suffer material mutual decomposition and remain as such in the finished paper prior to the process of development.
The following is a description of one of the methods adopted for coating paper according to this invention.
I first apply to a sheet of plain paper such as is made for the platinotype process, a coating of silver chloride by the well know method of double decomposition the solutions used for this purpose being potassium chloride and silver nitrate. The potassium chloride must be in excess so as to secure a complete decomposition of the silver nitrate.
The potassium chloride solution is made by dissolving 5 grammes of potassium chloride in 100 cubic centimetres of water and the silver nitrate solution is made by dissolving 5 grammes of silver nitrate in 100 cubic centimetres of water. After the coating of silver chloride has been formed on the paper by the use of these solutions the paper surface is usually washed in water in order to remove the soluble salts.
But instead of applying this coating of silver chloride to the paper by the method of double decomposition I sometimes make use of an emulsion of chloride of silver in gelatine or other suitable medium and apply such emulsion to the paper by one of the usual and well known methods.
The second coating solution now to be applied to the paper I prepare by dissolving a small quantity of potassium chloro platinite in a solution of ferric oxalate, this ferric oxalate being such as is employed in the platinotype process.
Now in order to prepare the second coating solution I dissolve in 500 cubic centimetres of the ferric oxalate solution one gramme or less up to six grammes or sometimes more of potassium chloro platinite. If thje object is to form the image mainly of silver I dissolve say one gramme of potassium chloro platinite in the ferric oxalate solution but if the object be to make an image which will have or contain a permanent residual image of platinumI dissolve say six grammes of this salt in 500 cubic centimetres of the ferric oxalate solution. The second coating solution is applied by brushing the solution over the paper previously coated with the silver chloride or by floating the paper on the solution or by other suitable means.
The coating is finally well dried in a current of warm air or before a fire or in any suitable way.
The paper thus coated is now ready for use. It is first of all exposed to light behind a negative or other suitable screen and then immersed in a developing solution, made by dissolving one pound of potassium oxalate in 64 ozs. of water. In about one minute development is usually complete. The developed print is then moved to a clearing bath made thus
Potassium bisulphate 3 ozs.
Potassium oxalate ½ oz.
Water 100 ozs.
In this bath it should remain from ten to fifteen minutes and then it is well washed in several changes of water. Finally the print is “fixed” in a “hypo” solution made by dissolving one ounce of sodium “hyposulphite” in ten ounces of water. The print is finally washed in several changes of water.
Other methods and salts may be used for developing and clearing or “fixing” the prints but the method above is an excellent one.
In preparing the said photographic printing paper I sometimes vary the operations described above or invert their order, that is to say, I may change the order in which the various salts are applied to the paper. For instance, instead of applying the potassium chloro platinite salt mixed in solution with ferric oxalate I sometimes apply the potassium chloro platinite dissolved in water or in any suitable solvent as a separate coating which I apply usually between the coating with silver chloride and the coating of ferric oxalate.
And I sometimes employ for coating the paper an emulsion of silver chloride in a suitable medium in which is dissolved the requisite quantity of platinum salt and of ferric oxalate.
I sometimes apply the sensitive coating to other surface than paper, for instance to cotton or linen fabrics, to silk fabrics, to wood, &c.
In preparing the said photographic printing paper or other surface I sometimes employ other salts than those named for instance in the coating of the paper I may use the bromide of silver rather than the chloride of silver but I usually prefer the latter salt. And instead of the potassium chloro platinite I sometimes use the sodium or ammonium chloro platinite or other suitable chloro platinites or platonic chloride but of those salts I prefer as the most generally useful the potassium chloro platinite. And instead of a solution of ferric oxalate I sometimes employ a solution or solutions of the double salts of ferric oxalate and potassium sodium or ammonium oxalate or other suitable oxalate, or a solution of ferric citrate or its double salts, or mixtures of solutions of any of these with a solution of ferric oxalate ; these solutions are hereinafter referred to as equivalents of ferric oxalate.
And in the formation of the coating of silver chloride on the paper by means of double decomposition I do not confine myself to the salts named in the above described process, namely potassium chloride and silver nitrate but I may use other salts which by their mutual decomposition will precipitate silver chloride.
I do not restrict myself to the exact proportions or quantities of the chemical salts used in the above described process, as these proportions and quantities may be varied within certain limits according to the results desired to be obtained or to other conditions.
I do not confine myself to the use of aqueous solutions of the chemicals hereinbefore referred to, as these salts may be dissolved and used with any suitable solvent.
I have further found that particularly advantageous results can be obtained in the above process by using paper the surface of which has been parchmentised by treatment with acid or by other well known means. The paper is coated or treated on each side with sulphuric acid sufficiently strong to attack the paper ; the paper is well washed in water to free it from acid and is then dried. The acid is not left on long enough to penetrate the paper, the aim being to get a film of altered paper on each side.
In a further modification of the photographic printing process according to this invention, the platinum salt may be wholly or in part contained in the developing solution, that is to say, the coating of chloro platinite or chloro platinate may be omitted from the paper or may be reduced to a minimum and the paper, after exposure may be treated with a solution of the chloro platinite or chloro platinate in admixture with the developing solution.
Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is :—
1. The hereindescribed process for the production of photographic printing paper in which the paper is coated with silver chloride, with a chloro platinite or chloro platinate or platonic chloride, and with ferric oxalate or its equivalent.
2. Photographic printing paper coated with silver chloride, a chloro platinite or chloro platinate or platonic chloride and ferric oxalate or its equivalent.
3. The modification of the subject matter of Claims Nos. 1 and 2 in which the paper in the first place is partly parchmentised so as to obtain a film of altered paper on both surfaces, for example by treating it on each side with sulphuric acid sufficiently strong to attack the paper after which the paper is washed in water to free it from acid, and dried.
4. The photographic printing process in which paper treated a specified in Claims Nos. 1 2 or 3, is exposed to light behind a negative or other suitable screen, and thereafter developed, cleared and fixed, substantially as described.
5. The modification in which the subject matter of the preceding claims, in which silver chloride is replaced wholly or in part by silver bromide.
6. The modification in the subject-matter of the preceding claims in which the platinum salt is wholly or partly contained in the developing solution.