US2312874A - Color photography - Google Patents

Description

Patented Mar. 2, 1943 cocoa rno'rocaarnr Albert L. Bunting, Detroit, Mich, assignor to Union Research Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Application April 27, 1940, 1 Serial No. 332,067

8 Claims.

The present invention relates to new and useful improvements in color photography, relating more particularly to a photographic process in which an initial photographic image is reproduced in the terms of the color blue, as well as relating to a colored photographic print resulting from the process.

It is well known in the art of color photography that a. plurality of colored images each of which is reproduced in a primary color, such as blue, yellow or red, may be assembled, superimposed in registry, or otherwise combined to produce a completed colored'image which covers the full color range of the visible spectrum. These initial colored images must be carefully selected as to .color value and intensity so that when combined a balanced colored print is produced.

The blue photographic print of the present invention may be used in any desired method of assembly or combining of the separate color images utilized to produce such a finished colored print. The other initial colored images also used for this purpose, may be produced by any known process which provides colors suitable for use with the blue color prints herein disclosed, and may also be produced according to the disclosures of my co-pending application Serial No. 332,069 and according to the disclosures of thecopending applications of myself and Raymond W. Thomas, Serial Nos. 332,068, 332,071 and 332,070.

It is one object of the present invention to agent and a solvent for the compounds which form no part of the blue colored compounds of the blue image thereby preventing such compounds from hampering or preventing the formationof the blue image; and to providesuch a process in which a. bleached initial black-and-white image may be. oxidized prior to subjecting it to the combined action of the blue color producing agent and solvent.

A still further object of the invention is to provide a blue photographic print of the nature provide an improved blue photographic print and a simple, effective and convenient method for the production thereof, the method being readily controlled to produce a blue color of predetermined color values and light transmitting properties.

It is another object of theinvention to provide a process for producing a blue photographic print which includes a conversion of an initial conventional photographic black-and-white image to a blue image which faithfully reproduces the graduations and tones of the initial image and which is light-fast and capable of repeated satis factory exhibitions or uses particularly photo-.

graphic projections.

Another object of the invention is to provide a process for producing a blue photographic print n which a bleached initial black-and-white image bythe combined action of a blue color producing 55 referred toabove which is relatively inexpensive to manufacture and which is adaptable for use in the production of all kinds of color photographs such,'for example, as paper prints and transparencies.

Other objects and advantages of the invention will become apparent during the course of the a following description and of the appended claims forming a part hereof.

GENERAL DISCUSSION or PRESENT Pnocrss cyanide radical. This intermediate image is reactedlwith ferrous or ferric ions to produce a substantially water insoluble blue colored image believed to be ferriferrocyanide, ferroferricyanide,

- or'mixtures thereof as explained in detail hereinafter. The initial image may consist of any compound capable of being. reacted with a reagent suitable to form a substantially water insoluble complex iron cyanide.

Plionucrron or rim INITIAL IMAGE One satisfactory method of producing such an initial image is to produce a conventional developed ,black-and-white image. This image is preferably a developed silver image. In carrying out the present invention, the initial image is converted to an image consisting chiefly of a 'substantially water insoluble ferrocyanide or ferricyanide. This conversion is often referred to in the art as bleaching. Among other bleaching. agents which may be employed are chromic acid, chromates and halide acids. However, images. bleached by agents other than those having acomplex iron cyanide as a part of their composition should be treated to form a complex iron cyanide in a separate step. Whether this complex iron cyanide be formed directly from-the developed black-and-white image or by suitably treating a bleached image, it is then treated with a solution containing ferrous or ferric ions for the production of a blue colored image.

Because a blue positive image made in accordance with the present invention is a faithful reproduction of the initial black-and-white image, it will be understood that the intensity or density of the initial image determines the density or intensity in the blue one derived therefrom. Therefore, the control of the density of the initial image by anyconventional method suitable for such a purpose controls the color intensityof the image in the blue positive print. Intensity control ofthe initial image may be accomplished by conventional photographic procedure as, for example. by control of exposure times. mode of development, choice of emulsions, or by combinations of these factors and in other ways well known in the art.

In preparing the initial black-and-white posi-. tive image any suitable film or plate may'be used. Motion picture positive stock which possesses but a single coating of emulsion has been used successfully and is recommended for this purpose, particularly the film known as Eastman positive nitrate film No. K1301. Also, reversible film stock gives satisfactory results. The lightsensitive material contained in the emulsions of these films is preferably a silver compound or mixtures thereof although any suitable lightsensitive material may be utilized.

- Drzvstorusn'r or Im'rnr. hues For purposes of developing the initial image referred to. any standard or conventional developing solution may be employed, but I have found that the developers known as D-ll and DK-20 of the Eastman Kodak Company produced, particularly good results. However, the so-called M-Q developer is also satisfactory. This development may be accomplished in trays.

image, according to the present invention the initial image is converted into a water insoluble complex iron cyanide image by immersion in a solution containing iron cyanide ions. Assuming that the initial image is a developed silver image and the bath consisted of an aqueous solution of potassium ferricyanide the converted image would be silver ferrocyanide. The strength of the converting solution is not critical, as the reaction goes to completion. Although I do not fully understand the exact nature of this con- This equation is given as typical of the general reaction which other complex iron cyanides undergo with silver and is intended to be illustrative in nature and not limiting. Moreover, other converting agents, some of which have been mentioned hereinbefore, may be used, provided the image is then treated with a solution suited to produce "a complex iron cyanide image of the in a tank, or by any suitable procedure. In the of fourteen minutes was employed also, at about 18 C. The above times app y to the D-ll developer. However. suitable development times for any given developer employed as .well as suitable technique and procedure therefor may be readily worked out by one skilled in the art. Also a developer may be selecetd to produce a desired efiect such as a fine grain, desired intensity and contrast values and the like, since thecharacteristics of developers are known in the art.

After development, the initial image is washed, placed in a conventional short stop bath, such as a weak aqueous solution of acetic acid, and is again washed, according to standard photographic practice.

If colored images other than a blue one produced in accordance with the present invention are to be produced in the emulsion layer containing thisblue image. the film should not be fixed at this stage of my process. Otherwise. the film may be fixed at this stage by immersion in a suitable hypo bath. Fixing at this point is recommended if it can be done, since the rebetter blue color print results.

Common or me 1mm. Inca general type specified above.

It'is desirable that the iron cyanide'hleaching bath be buifered to produce a basic solution because the iron cyanide solution is more stable if. basic. If it be neutral or acid it tends to break down, for instance by reacting with the carbon dioxide of the atmosphere. Also. the iron cyanide bath has been found to bleach the initial black-and-white image better if basic. In this bleaching reaction the metal forming the blackand-white image is oxidized to form a complex metal iron cyanide. Therefore the bleaching reagent is itself reduced and is an oxidizing agent. An iron cyanide, used herein as the bleaching agent, is a better oxidizing agent if in a basic solution. Although any bufiering agent suitable to produce a basic solution may be used, alkali carbonates, bicarbonates or hydroxides are recommended.

The complex iron cyanide image produced by the above step is grayish-white in color but is otherwise a faithful reproduction of the initial silver image. Substantially all of the silver of the initial image is converted to silver iron cyanide complexes and the print is left in the solution until the reaction goes to completion.

The converted film is next washed in order to remove all surplus ferricyanide, or a surplus of other converting materials or soluble reaction products which may remain within or upon the film. However, a complete removal thereof by an extended washing is not essential at this point provided that any excess ferricyanide or other soluble complex compound or excess reagent which may remain isremoved prior to the blue color producing bath to prevent staining.

' The image is next further converted by treatment with an oxidizing agent, preferably an aqueous solution of nitric acid or chromic acid. The concentration of this solution is important only at the upper limit which should not exceed approximately 50% (by volume) of pure nitric This reaction goes to completion and the time For thepreparaticn of the blue-and-white to of the immersion therefore is not critical provided that the film is not-left in the solution so long that the gelatin, emulsion, or carrier is attacked or stripped. I believe this oxidation reaction produces a different complex cyanide radical from that of the bleached image, probably a radical in which the valence of one of the elements has been increased. Moreover, the dual-character of the former complex AgzK [Fe (CN) B] the oxidized bleached image is allowed to stand the reaction which causes this change in color of the image.

AgK[Fe(CN)e] range The initial metallic black-and-white image having been bleached with ferricyanide or bleached with some other suitable bleaching agent and treated with a ferricyanide to form an image consisting of a complex metal cyanide,

may be oxidized to producethe oxidized orange image by subjecting the latter to the action of any oxidizing agent capable of oxidizing the complex cyanide; For instance, a bleached image consisting of silver ferrocyanide may be oxidized to an image consisting of silver ferricyanide. In.

that one skilled in the art may select one ,of

these or other oxidizing agents suitable for oxidizing the gray-white bleached image to the oxidized orange image.

The print is next given a wash to remove the excess of nitric acid or other oxidizing agent as well as any soluble reaction products from the bleaching reaction which may remain in or on the film. In order to determine whether or not the film is sufficiently washed at this point,

drippings from the washed film may be added to 'a solution containing ferrous ions. If the solution turns blue or bluish the wash is incomplete. After this wash the film is placed in an acidified aqueous solution containing ions of ferrous-or ferric iron, the concentration of which is not critical, although the rate of converting the orange-silver ferricyanide image into the desired blue image is increased if the ionic concentration of the ferrous or ferric iron is relatively high. This reaction is permitted to go to completion which may be determined by the appearance of the blue colored image. Thereafter, the blue image is washed to remove the soluble iron salts and is placed in a conventional hypo bath until it becomes clear and transparent and thereafter is washed and dried; The resulting print is a transparent print in which the image appears in a blue scale exactly corresponding to the gray scale of th initial black-and white print.

It is recommended that ferrous ions be used. however. If. oxidation of the bleachedimase is complete at the oxidizing operation, ferric ions tend to produce an inferior image. I have found, however, that if oxidation is not complete, or if too long before being immersed in the blue produclng bath, that ferric ions may be employed fro-provide a fair blue image. Apparently, the oxidized bleached image tends to reduce to a certain extent on standing. Complete oxidation and the use of ferrous ions is preferable inasmuch as more constant and more easily controlled blue color values are produced by this procedure. Ihave found that the intensity of a blue image produced by using ferrous ions may be intensified somewhat by immersing the film, after the production of the blue image,- in an acid bath or by utilizing a long wash after the hypo operation. It is also pointed out that a conventional hypo solution tends to-reduce the blue color forming compound which does not destroy the blue color, but does make control of color values more diillcult. Therefore, the use of an acid hypo bath is recommended.

EXPLANATION or BL E COLOR The reaction producing the blue precipitate making up the blue image is represented, so far as now. known, by either of the following equations, depending upon the condition of the reaction.

Aga[Fe (CN)5]+FeS0 b FB:4[F6(CN)012 (Turnbuil's Blue") Aga[Fe(CN) ]+Fe:(S0i): -o Fe4[Fe(CN)s]z ("Prussian Blue") Thus my blue color producing compound is believedto be a mixture of Prussian blue and Tumbull's blue as it can be produced by either ferrous or ferric ions. Inasmuch as ferric ions plus a ferricyanide gives a brown color, a reduction to ferriferrocyanide (Prussian blue) appears to take place where ferric ions are used. However, if oxidation of the bleached image is complete and ferrous ions are employed in the blue forming bath, I believe that the blue color roducing compound is substantially completely Turnbulls blue. While the concentration of the ferrous or ferric ions in the blue color forming 1 bath mentioned affects the rate of the reaction, that concentration does not appear to affect the character of the resultant .blue image. This concentratlon may be varied within relatively wide limits without materially ailecting the said conversion.

It should be pointed, out, however, that the following reaction in addition to those mentioned probably occurs:

(Potassium ferriierrocyanide) The compound produced by this reaction is also blue and my blue color producing compound probably contains a relatively small amount of it. This reaction appears to occur since potassium ferrocyanide is capable of oxidation.

The present process, as will be readily understood'after a consideration oi the foregoing disclosure, particularly .utilizes the reaction of any ferrous or ferric ion with a complex iron cyanide. Therefore, any suitable iron salt producing ferrolls 'or ferric ions dissolving, plus any suitable image having a complex iron cyanide, prefer'ably aife'rri or i'errocyanide, as a'radical inits image producing compound maybe employed.

For instance, an image composed of the reaction instance.

ALTERNATE PROCESS FOR PRODUCING THE BLUE IMAGE Another satisfactory method of carrying out the present invention is as follows: An initial black-and-white" positive print is made in the way explained above. This image is treated with potassium ferricyanide or some other bleaching agent as above disclosed and is preferably oxidized by means of a treatment with nitric acid or other suitable oxidizing agent to produce the orange colored silver ferricyanide image previously described. However, the unoxidized bleached image may be used, although this procedure is not on the whole as satisfactory. The image, preferably the oxidized or orange image, is next directly. converted into the desired blueand-white image in a one step process, without the final treatment in a hypo solution. The conversion mentioned takes place as a result of immersing the said orange image in a bath containing ions of a so-called hypo such as sodium thiosulfate. and also ferrous ions preferably in a ratio of about 2 to 1.5 respectively. The solution is acidified with the conventional acid mixtures used in the art for forming an acid hypo s'olution. The presence of acid in the hypo solution serves the double purpose of preventing the precipitation of sulfur and preventing hydrolysis and its resultant precipitation of basic iron complexes. means of suitable mixtures of acids adapted to produce an acidified hypo solution, such as an acidified solution of sodium thiosulfate. Acid mixtures suitable for forming an acid hypo solution, are also suitable for present purposes and are well known in the art. In particular, the acid hypo formula known as F-5a of the Eastman Kodak Company is recommended. While variations within the scope of the foregoing explanation are permissible, the ratio of hypo to ferrous ions should be controlled.

It is believed that the blue color creating compound of the present modification differs from that of the form of the invention previously described. In the first place, since ferrous ions are used, I believe that relatively pure Tumbulls blue, that is, ferrous ferricyanide is produced, The blue compounds believed to be intermixed with this product in the modification of the invention formerly described, namely, Prussian blue (ferriferrocyanidel and potassium ferriferrocyanide evidently are not present in this The former requires the presence of ferric ions, while the comparatively strongly acidified solution of the present modification pre-, vents the formation of the latter which is formed in a basic or in a relatively weakly acidified solution. Since but one color creating compound is produced, the process embodying the present modification is easily controlled and produces images having constant color values and intensities.

A further advantage of this modification of i the invention resides in the fact that the hypo (sodium thiosulfate. usually) dissolves the reaction products formed by the blue color forming reaction, other than the blue image forming compound and thus clears the blue image at the Such precipitation may be prevented by i -Tumbulls blue (ferrous ferricyanide).

compound. is unreacted and later lost by being dissolved oil.

SUMMARY OETHE PROCESS ANn A RECOMMENDED Annrrrormr. OPERATION fIHEREm Briefly speaking, two principal procedures embodying the present invention, have been pointed out. Both utilize a black-and-white image and the bleaching thereof to form an image consisting of a complex silver and iron cyanide. One procedure utilizes the oxidation of this complex cyanide apparently to form silver ferricyanide and treats this oxidized image in a single fixing and blue forming bath containing hypo and ferrous ions to form an image consisting chiefly of However, the fixing or clearing operation may be separately performed in a separate hypo bath after reacting the oxidized image with ferrous ions to form an unfixed blue image. The other procedure does not utilize the oxidizing step and treats the unoxidized bleached image with ferrous or ferric ions, preferably ferric ions, to form an image thought to consist of a mixture of Prussian blue (ferroferricyanide) and Tumbulls blue (ferriferrocyanide) v However, apparently this image is composed chiefly of Prussian blue. In

this procedure also, hypo may be added to the blue forming bath containing ferrous or ferric ions, or the print may be fixed or cleared in a separate hypo bath.

If the oxidizing step is used, it is recommended that a silver halide be added to the blue forming bath containing ferrous ions. This halide may be added to the combined blue forming and fixing bath if it is used, that is, it may be added to a bath containing hypo in addition to ferrous ions. Any silver halide may be employed but silver fiuoride does not operate satis-' its use is thereare insoluble. Therefore it is thought that the image produced is entirely Tumbulls blue (ferriferrocyanide) without mixtures of Prussian I .blue (ferroferricyanide) if the silver halide is added to the blue forming bath. The ionic concentration of the silver in the blue forming bath is kept depressed by this means until the silver complexes are removed by the fixing agent, such as hypo, to leave a cleared blue image.

While the addition of a silver halide is one effective way of depressing the'ionic concentration of silver in the blue forming bath. it will water (about 18 C.).

readily be understood that other silver compounds having the same eifect may be utilized. Such compounds are known to skilled chemists. Having thus described my process or method in detail and having given the chemicalexplanation thereof insofar as such an explanation is known to me, the following examples of specific embodiments serve to reveal typical ways in which the invention may be practiced. These examples are given by way of illustration and not of limitation.

Each of the following examples presupposes the prior production of a silver black-and white" image and utilizes such a print as its starting point. It is to be understood, however,

that variations may be required in the examples given hereinto produce the desired blue colored image.

Example I Example I! (1) Follow steps 1 and 2 of Example 1.. (2) Place the converted and washed print in an aqueous solution containing approximately 17 to 35% by weight of nitric acid (100% HNOa).

' that if the initial image is of a different type r v the solution,.washed. from about one to five min- (l) Immerse a photographic print containing a developed silver image, which may or may not have been dried after development in an aqueous solution containing approximately. by weight of potassium ferricyanide buffered with about gms. of sodium carbonate for each 250 cc. of solution. Allow the print to remain in this solution until the reaction has gone to completion which may be readily noted visibly inasmuch as the black image is completely converted into a grayish-white opaque one as viewed through the back of the print. These observations will reveal when all of the silver has been converted. The, print may be removed, from the solution and subjected to the following operations.

(2) Wash the converted print in cool water I (approximately 18 C.) for about five minutes.

Gently running water is recommended, and a somewhat shorter washing period' has been found to produce no disadvantages.

(3) Place the converted and washed image in a bath prepared as follows:

Ferrous sulfate (C. P.) approx. gms '10 Sulfuric acid, dilute (about 10%). approx. cc 10 Water approx. cc 100 Allow the print to remain in this solution until its image is completely converted to an opaque blue one. However, the point at which complete conversion of the image from white to blue has taken place may be readily noted by visual ob-v servation.

(4) The converted image is then rinsed in cool A rinse of one-half to one minute is recommended.

(5) The rinsed image is placed in a hypo bath,

the formula of which may be as follows:

Water at about 125 F cc 95.6 Sodium thiosulfate gms 36.25 Sodium sulflte "gms 2.25 Acetic acid (28%) cc 2 Boric acid (crystals) gms 1.1 Potassium alum"-.. .gms 2.25 Cold water to make cc aLthough a 10% aqueous solution (10 gms. of sodium thiosulfate per 100 cc. of water) produces good results.

The now completed blue print is washed in running water at about 18 C. for approximately ten minutes and is dried. I

The print is allowed to remain in this solution until the reaction has gone to completion. This change may be visibly noted inasmuch as the grayish-white appearance of the print gradually assumes an orange color- The reaction hasv reached completion when the entire image has changed in color, which may be observed and normally takes place in about one minute. The print may be removed from the solution for purposes of such observations. When the change has become completed the print is removed from utes in'cool water to remove soluble oxidation products and an excess of the oxidizing agent and is then subjected to the following operations (3) Place the converted and washed image in a bath prepared as follows:

Ferrous ammonium sulfate approx. gms 16 Sulfuric acid, dilute (about 10%) approx. cc 10 Water approx. liter 1 Allow the print to remain in this solution until its image has been completely converted to an opaque blueone.

(4) Follow steps 4, 5 and 6 .of Example 1. Example III (1) Follow steps 1 and 2 of Example (2) Place the converted and washed image in a bath prepared as follows:

Silveriodide approx. gms 2 Sodium thiosulfate n, do; 2, Acid mixture approx. cc 20 Ferrous sulfate solution do 30 to make Water (at approximately 18 C.)

- approx. cc 150 The ferrous sulfate solution referred to is made up as follows: Sulfuric acid (concentrated). Approx. 1 cc. (1.8 gms.) Ferrous ammonium sulfate Approx. 5 gms.

Water to make Approx. cc.

The acid mixture referred to is made up as follows: Water (about 52 C.) approx. liters Acetic acid (28% glacial acetic acid Y approx. cc- 940 Sodium suliite v(desiccated) approx. gms 300 Boric acid (crystals) i do Potassium alum. -do 300 Cold water to make approx. liters 4 The acid mixture should be added before. the ferrous sulfate mixture in order to prevent undesired precipitation of sulfur or basic iron complexes.

' Allow the print to remain in this solution until its image is completely converted into a transparent blue-and-white one. The actual time required for this reaction will vary. somewhat depending upon the type of film employed. However, the point at which complete conversion has taken place may be readily noted visibly. It is preferable to leave the film in this solution for about two minutes after the image has turned to a cleared, substantially transparent blue one. (3) The now completed blue print is washed in running water at about 18 C. for approximately two minutes and is allowed to dry.

IN Grimm.

In all of the above examples the following additional steps of procedure and technique have been found to be helpful although not indispensable to my process itself. The gelatin of the film may be hardened by conventional hardening baths either after development of the black-and-white positive and before fixing thereof. after fixing thereof, after development only if no fixing step is utilized, or it may be used after development and also after fixing. A bath of chrom alum or one of formaldehyde has been found to care for the extreme cases of gelatin stripping and softening which otherwise might be adanger. in using the present process with certain films. If a single hardening bath is considered preferable, the following one used after development of the black-and-white. positive may be employed. The solution is made as follows:

Grams Water 200 Formalin (40%) 2 Sodium carbonate l The positive is immersed in this bath for approximately five minutes and is then washed from three to five minutes before the various succeeding steps already described are taken. This hardening step is particularly recommended where the washing baths exceed the temperature control limits given above and in particular where such baths exceed 70 F., although it is to be distinctly understood that such increased temperatures are to be avoided and may one embodiment of the invention the initial image is bleached and is prepared for the production of a blue color therein during a subsequent oxidation resulting from the treatment of the image with a suitable oxidizing agent. The

process also appears to be novel in that the oxidized image and print containing the same may be converted to a blue colored print and at the same time cleared in a solution which contains a solvent forthe compounds which form no part of the blue color-producing compounds. As pointed out, the blue color forming and clearing steps may be combined with beneficial results by ing 'an oxidizing agent adapted to form a silver ferricyanide image, and thereafter immersing the oxidized bleached image in a solution containing sodium thiosulfate and a ferrous iron compound in a molecular ratio of approximately 2 to 1.5 respectively.

2. A method of producing a blue photographicprint from an initial photographic silver image comprising reacting said image under conditions suitable to produce an image composed chiefly of silver ferrocyanide, oxidizing said reacted image by immersing the print in a bath containing an oxidizing agent selected from the group consisting of nitric acid, potassium permanganate, perchloric acid, and chromic acid, thereafter immersing said oxidized image in a bath containing a water soluble iron compound and also a silver halide to form a substantially water insoluble blue colored compound of iron in chemical combination with an iron cyanide complex radical, said blue colored compound being thereby directly substituted for the initial photographic, image.

3. A method of producing a blue photographic image from an initial developed silver image.

- comprising reacting said image with a solution of prises bleaching said silver image to produce an insoluble silver complex iron cyanide image, and

thereafter treating said bleached image with a solution containing a thlosulfate and an iron compound to form ablue colored compound of iron and an iron cyanide complex radical, the

concentration of said thlosulfate to said iron using a bath which contains a hypo solution in which free iron ions are provided.

The blue color of the completed print has been separated and tentatively identified and has been found to consist chiefly of the compound Turnbulls blue, Fea[Fe(CN) 612, which also probably contains a relatively small amount of Prussian blue, Fe4iFe(CN)la, and also a relatively small amount of potassium ferriferrocyanide, KFe'iFetCNh], which I believe to be due to some reduction of the iron in the iron cyanide complex probably by the action of organic reducers in the film such for instance as the cellulosic carrier.

I claim:

l. The method of converting a developed photographic silver print to a blue color print, comprising bleaching the silver print to form a silver ferrocyanide image followed by oxidizing the compound in said solution being so controlled that the molecular ratio of the former to the latter is approximately 2 to 1.5.

5. A method of producing a blue photographic image from an initial silver photographic image developed in a gelatin layer, which comprises reacting said silver image under conditions suitable to produce a bleachedimage composed substantially entirely of silver ferrocyanide, thereafter oxidizing said bleached image to produce an image composed substantially entirely of silver ferricyanide, and thereafter treating the resulting silver ferricyanide image with a solution containing ferrous ions to form a blue image consisting substantially entirely of ferroferricyanide.

6. A method of producing'a blue photographic 7. In a method of producing a blue photographic print from a, silver image developed in a gelatin layer and which utilizes a precipitated compound of iron and a, complex iron cyanide as a color-producing means, the steps of first bleaching the silver image to an image consisting substantially entirely of silver ferrocyanide, and thereafter treating said bleached image with an oxidizing agent before the aforesaid color producing compound is formed to convert the bleached image to an image consisting substantially entirely of silver terricyanide.

8. A methodof producing a blue photographic image from-an initial silver photographic image,

whichoomprises reacting said silver image under conditions suitable to produce a bleached image composed substantially entirely of silver ferrocyanide, thereafter treating said bleached image with a, solution containing nitric acid to produce an image composed substantially en tirely of silver ferricyanide, and thereafter reacting said silver ferricyanide image with'a water soluble iron compound to produce a water insoluble blue image formed of iron in chemicalcombination with an iron cyanide complex radical.

ALBERT Ll BUN'I'ING.