US2312875A - Color photography - Google Patents

Description

Patented Mar. 2, 1943 2,312,875 COLOR PHOTOGRAPHY Albert L. Bunting and Raymond W. Thomas, Detroit, Mich, assignors to Union Research Corporation, Detroit, Mich., a corporation of Michican.

No Drawing. Application April 27. 1940,

' Serial No. 332,071

8 Claims.

The present invention relates to new and useful improvements in color photography, relating more particularly to an improved method for converting an initial metallic image developed in an emulsion layer to an image having a desired color reproductive of the object photographed, and to an improved colored print resulting therefrom.

It is well known in the art relating to color photography that colored positive images may be produced in primary colors and may be matched as to color value and intensity to form, when combined or assembled in registry, a colored print which covers the full color range of the object photographed.

These colored positives maybe initially produced in separate films and assembled or combined, or they may be produced successively in a single film. One general method of producing each cblor photographic image .thus assembled or combined depends upon the bleaching of an initially developed metallic black-and-white image and immediately thereafter either reacting said image in situ to form a precipitated color producing compound or to dye it by selective adsorption of a dye by the image. The former procedure is usually referred to in the art as "chemical toning and the latter as dye toning. Neither of these methods of coloring a bleached image has been satisfactory however. While chemical toning processes usually produce a color image reproductive ofthe initial blackhave not been wholly satisfactory and'dye tonedcolored images have been poor in gradation and definition.

forming the initial black-and-white image and and-white image in gradation intensity and( definition and therefore a satisfactory image in these respects, no wholly satisfactory-group of bleaching agents and color forming agents reactive therewith have been produced to date which provide suitable colors of the desired shades and of the required stability to be satisfactory for photographic purposes especially for proJection. In particular, the compounds produced to date by chemical toning processes havebeen very limited in number and the colors which may be so produced have also been limited.

I Dye toning of a bleached black-and-white image has been unsatisfactory chiefly because nobleached image has been disclosed to date which is very effective as a selective adsorbing means for the dye. Consequently, the dye is but partially adsorbed producing a color image of insufficient intensity. Moreover, relatively few dyes can be selectively adsorbed by the bleached image thus making the colors produceable by dye toning means limited in number. In addition, the shades of the dyes which can be so adsorbed also normally constituting an element in the bleached image is replaced by another and chemically more active metal; and to provide an im proved method for producing such a converted image. Our improved converted image coacts with color forming agents in an improved manner to produce a satisfactory color image whether treated by dye toning means or'chemical toning means.

A furtlrer object of the invention is to provide a process for producing an improved color print by assembling in registry initial color images produced in the foregoing manner, in which each image thus assembled may be colored by reacting 'our improved converted bleached image with chemical toners or coloring agents andthereafter removing from the image carrying medium the substance initially forming the converted image to leave a cleared colored image. and to provide'as a product a converted image colored by said chemical toners or coloring agents and suited to be cleared in this way.

Another object of the invention is to provide a process or method for producing a colored photographic print which includes converting an initial metallic black-andwhite conventional photographic image to a stable substantially water insoluble colored print by improved, novel chemical toning or coloring means, the final color print corresponding to the initial blackand-white image in intensity, definition, gradation and detail. In addition, the colors of the print are light-fast and the print itself is relatively resistant to wear and abrasion.

It is another object of the present invention to provide an improved photographic color print formed by assembling in registry selected color images of the foregoing character having desired color values; and to provide a positive comparatively simple, effective and convenient process for the production of -such a color print, the method being readily controlled.

Other objects and advantages of this invention will appear in the following description and i appended claims. Before explaining in detail In making a color print, the present invention utilizes some system of photographic negatives in which the colors of the object photographed are recorded separately. Such negatives may be a balanced set of color separation negatives or may be of the monopack type such as are produced by the Eastman Kodachrome films. These negatives as well as methods of making them are well brown in the art. The filters used in making the color negatives are of such a'character and are so selected that the entire visible spectrum is covered by the set with substantially no gaps and also no appreciable duplication or overlapping at given wave lengths.

Thus, if the positive colored print made from each color negative corresponds in color value to the color recorded by the negative and a balanced set of such colored prints be assembled in registry, a complete. full color print will be produced.

For use with the present process, a satisfactory set of color separation negatives were made'using as filters the standard Wratten A, B and C5 filters which are well known in the art. While these filters are particularly recommended. it is to be expressly understood that any suitable matched set of filters may be employed and the color process of the present invention may be variedas desired to permit. the use of any such other filter systems.

One of these color representing negatives is selected and used to print a black-and-white positive image. It is to be understood that this negative'is selected so that the positive rint pr duced therefrom is to be given the color which the said ne ative represents. In preparing the black-andwhite positive prints it has been found that films rather than plates produce more satisfactory results, and in particular motion picture positive stock which possesses but a single coating of emulsion is recommended. Films such as manufacturers prepare for use in reversal processes norable film may also be used. It is also to be understood that while, normally, 'suchfilm possesses light-sensitive silver salts such as silver bromide as its light-sensitive material, that other lightsensitive'material may be utilized. The blackand-white positive print is developed in suitable developing solutions, the technique and procedure for so doing being well known to skilled photographers. I

Because a colored image made in accordance with the present invention is a faithful reproduction of the initial black-and-white positive image,

it will be understood that the intensity of the initial black-and-white image determines the intensity 0f the ultimate colored image derived therefrom. Therefore, the control of the intensity of the initial image controls the color intensity of the ultimate colored image. This intensity control of the initial image may be accomplished by conventional photographic procedure. It has been found that a relatively intense negative of high contrast is best suited for the production of a color print according to the present invention. A gamma value of about one is recommended. 1 x

In preparing the initial black-and-white positive images it has been found that the exposure time is especially important, for such exposure time constitutes the principal means of controlling the density of the silver deposit making up the initial black-and-white image and utilized herein according to the methods set forth to produce the color print of the present invention. These prints are preferably so exposed and developed that they are given a gamma value of approximately 1 /4.

After development the film is washed for approximately one mlnute in cool gentle flowing water and may, if desired, be hardened in an appropriate hardening bath many of which are well known in the art. For instance, Eastman hardener SH-l may be used which is made upas follows:

Formaldehyde (40% aqueous solution) cc 10 NazCOa gms. 6 Water to make liter 1 The film is immersed for about five minutes in this hardening bath. It is'to be understood that the hardening step may be dispensed with in cases I where gelatin softening or stripping from the carrier are not problems. If the hardening step be used, the dim is thereafter again washed in gently flowing water for about one minute.

For the preparation of the colored images herein disclosed the initial black-and-white positive image above described is converted into an intermediate reaction product image. This step is commonly referred to in the art as bleaching."

and-white image which is a reduced metallic image, usually a reduced silver image, reacts with the bleaching agent to produce a reaction product image, but which is commonly called a bleached results.

image. 'I'hereforaany suitable reagent canbe employed which is reactive with the black-andwhite image to form a substantially water insoluble reaction product. The use of a solution of a ferricyanide, particularly potassium ferricyanide, is especially recommended as a bleaching agent, since it produces especially satisfactory In particular a 10% -(10 gms. to cc.) aqueous solution of potassium ferricyanide is recommended. This solution may be rendered basic by any suitable means such as the introduction of a small amount of sodium hydroxide. Preferably the bleaching solution is buffered as with sodium carbonate, bicarbonate or the like. Also it may be rendered acidic if the acid is also an oxidizing agent, such as nitric acid. The film is left in this bath for about five minutes, and

will usually be substantially completely reacted t or bleached in this period. Longer immersions,

however, produce no apparent deleterious effects.

the treatment continued until the image assumes a grayish-white appearance.

In connection with the bleaching of the initial v black-and-white image, it is thought that the way in which the particles making up the blackand-white image are deposited or precipitated afifects the ease and also the extent of the conver-v sion. Apparently, if the black-and-white image is' over-developed or over-exposed the resultant deposit is dimcult to bleach or convert. It is, therefore, recommended that the image be devel-- oped no longer than the normal developing time for the developing solution employed and that over-exposures be carefully avoided. However, the acid converting baths referred to above, especially a bath containing nitric acid, has been found more efiective than other converting baths in cases where conversion is relatively difiicult. In addition, for extr me cases as where the condition of the blac '-and-white image is such that conversion is very diilicult, an oxidizing agent may he added to the conversion bath. For instance, when the black and-white image is made up of metallic silver particles and the converting agent is a ferricyanide, a relatively strong concentration 'of nitric acid has been found very effective. The following bath, for example, may be employed:

Potassium ferricyanide gms 25 Nitric acid (concentrated) cc 55 Water to make cc 100 It should be understood, however, that any suitable concentration of nitric acid may be used, but solutions stronger than 50% are not recommended inasmuch as the emulsion tends to be stripped from the carrier in stronger solutions. Furthermore, the concentration of the ferricyanide may be varied within relatively wide limits and may be used up to saturation which would be approximately 38 gms. in the above solution. In addition, other suitable oxidizing agents may be employed with the ferricyanide converting agent or duced by normal converting or bleaching baths.

This oxidized converted image may be used if desired in succeeding steps of our process. However, it a reduced intermediate converted image is considered more desirable, this image can be produced by subjecting the oxidized image to a suitable reducing bath. For example, oxalic acid-citric acid, tartaric acid, or the like, may.

be employed as well as other well known reducaluminum, cadmium, tin and nickel. These ing agents such as amidol, hydroquinone and hyor supplemental converting bath. This bath consists of a solution of a metal compound, the metallic element of which is capable of replacing the metal initially forming the black-and-white image, but after the bleaching operation, forming an element of the bleached image. The replacing metal must be above the replaced metal in the electromotive series, in order to replace the latter. Usually the metallic element of the bleached or initially converted image is silver. In this event a metal above silver in the electromotive series may be employed. For example, a solution of a zinc salt may be used, such as zinc bromide. The film is left in this converting bath until the replacement reaction goes to completion which-takes place relatively quickly. Normally a five minute immersion is ample.

Our improved converted image is produced after the bleached image has reacted in a solution of a metallic compound, the metal of which is capable of replacing the metal initially forming the black-and-white image. converted image consists of or contains a reaction product having the replacing metal as one constituent thereof and as another constituent the radical or element of the bleaching agent which was formerly in combination with the metal forming the initial image. For instance, if theinitial black-and-white image be a silver image and if a ferricyanide was employed as the bleaching agent and zinc as the replacing metal, one such converted image may consist of or contain zinc ferrocyanide.

Our improved converted image produces superior color images when treated with suitable color forming agents whether it be dye toned or treated by chemical toning operations in the manner set forth hereinafter. that such superior color images may be produced by practicing the present invention because the metal forming an element in 'our converted image and which replaced the silver or other metal initially forming the black-and-white image, is a chemically active metal. More particularly, we believe that this replacing metal possesses unsatisfied partial valence bonds; In addition to zinc referred to above, the replacing metal may be selected from the group consisting of copper,

metals are particularly-recommended if the initial black-and-white image was silver, but may be successfully used with other initial metallic images provided that the replacing metal is capable of replacing the metal of such initial image after the bleaching operation. Suitable compounds of these metals may be employed in reacting the bleachedimage to produce our converted image. This reaction in which the metal of the bleached image initially. forming the 1 black-and-white image is replaced by another metal has been referred to as a replacement reaction. It may also be accurately referred to as double decomposition also called metathesis.

In addition, it is recommended that a halide of the replacing metal be employed, particularly the bromide. Wher these-halides, and especially the bromides, are used, it will be readily underwhich may be described aptly as an additional stood that the replaced metal of the bleached image will then form a metallic halide in the conversion operation. These halides such as bromides, particularly silver bromide, are usually insoluble and will precipitate and distribute themselves throughout the emulsion and thereby regenerate the light-sensitive material or at least form an additional light-sensitive deposit in the .This improved It is our belief I emulsion. For many processes, this production of a regenerated light-sensitive material is important, particularly where it is desired to reexpose the emulsion and develop a second image therein. However, it will be appreciated that this feature is but an additional feature and is not essential forthe production of any one colored image in accordance with the present invention. Other compounds may be used, 'for instance, a nitrate, a chloride, a sulfate, and so forth.

Furthermore, a fixing agent such as hypo, normally sodium thiosulfate so prepared that acids will not precipitate free sulfur from the solution, may be a'dded'to the converting bath. Inthis case, the print will be cleared while our novel converted image is being formed and need not be separately fixed or treated to remove the silver salts. It is necessary, however, to employ a bath having a pH value of twelve or more to remove or dissolve out the image formed by the zinc complex, in the manner more fully described hereinafter, if a transparent colored image is desired.

The converted image, that is, the bleached metallic image in which the metal forming the initial black-and-white image has been replaced by another metal, may be colored in several suitable ways which form the subject matter of our co-pending application Serial No. 332,068, filed April 27, 1940, reference to which is hereby made. Accordingto the present invention this converted image is colored as set forth below. As the first operation in this coloring procedure, the film is placed in a solution of a diazonium compound. Any diazonium compound may be emplayedv which will react with our improved converted image or will at least form an addition product therewith. While it is impossible to predict whether or not every conceivable diazonium compound will have this characteristic,

' we have tried a representative number ofthem, some of which are expressly disclosed hereinafter, and all of them appear to reactin this manner. These diazonium compounds may be created by methods well known to a chemist the principal method being to subject a selected aromatic compound having an NH: group to. the action of nitrous acid. As is well known, this reaction shouldtake place in a relatively strongly acidified solution. Furthermore the customary way of producing the nitrous acid is to subject anitrite to hydrochloric or a like relatively strong acid, thus producing this desirable relatively strongly acidified solution.- This reaction should/be carried out in thef-cold and should ever exceed 5 C. in order to avoid possible ex plosions. However, other methods well known in the chemical art may be employed. Moreover, it is well known, that diazonium compounds are converted to phenols when allowed to warm excessively. This phenol formation should be avoided as.it destroys the diazonium compound and renders it ineffective for present purposes. The temperature control necessary to prevent this phenolformati'on varies with different diwithin the scope of these coupling agents. they az'onium compounds but usually should not ex- The concentration of the diazonium compound in the diazonium solution will vary somewhat with the. particular reagent but in general a 1 molar solution has been found to be satisfac-'.

itially relatively strong because of the relatively strong acidity of the solution forming the diazonium compounds as explained above. Sodium acetate is particularly recommended, because the reaction between the diazonium solution and the bleached image has been found to take place much more satisfactorily in a weak acid such as acetic acid, which acid is formed by reaction of the sodium acetate and the relatively strong acid initially present in the diazonium solution. Other buffering agents may be employed unless they form undesirable complexes with the converted image. Citrates and tartrates, for example, may be employed. An excess of buffering agent is recommended. About 10 gms. of sodium acetate per 250 c. c. of solution for a molar solution of the diazonium compound has been found to operate effectively.

The film is ,left in this diazonium compound solution until the reaction is completed which is visually noticeable. This time interval is usually about two to three minutes, but it is preferable that the film remain in the solution for about five minutes. Moreover, longerintervals do not appear to produce an deleterious efiects. The particular diazonium compound selected will depend upon the color of the image to be produced.

The film is then washed for approximately five minutes in water, the temperature of which should not exceed 10 C. because the diazonium compound tends to break down at higher temperatures. The upper limit which can safely be used varies somewhat with the diazonium compound employed. For example, diazotized p nitraniline can be washed in water in excess of. 25 C. In the usual case 10 C. is a safe upper limit, however, and it is recommended that this wash water be kept at or below such temperature. 1

The film containing the bleached and converted metallic image reacted with a diazonium compound is immersed in a solution of a suitable coupling agent. These coupling agents are compounds reactive with the diazonium compounds described above, to produce a water insoluble colored reaction product. In particular, compounds sometimes referred to in the art as azo-dyestufi coupling components are recommended. In this case, the coupled reaction product is known as an azo-dyestuff. We have found that aromatic compounds possessing NH: groups (aromatic amines. or substituted aromatic amines) or OH groups are particularly satisfactory although compounds having other groups in addition to NH: and/or OH groups, such as COOH, may be employed.- Although a wide variety of chemical. families are included are known in the art relating to azo-dyestufi's and methods of producing them. These cou- 'pling agents may be dissolved in a neutral aqueous solution, although some of them are but slightly soluble. In this event a slightly alcoholic solution may be employed, but the use of alcohol is not recommended as a solvent because it tends in a saturated aqueous solution. This, however,

depends on the compound used. A concentration of 2 to 5 gms. per cc. is recommended as satisfactory. The coupling agents are preferably dissolved in a basic or acidic solution inreduce 'th acidity of the solution which is in- 75. stead of a neutral solution, for they form water" soluble-salts. Care should be taken to select an acid or base which will form a water soluble salt of the particular coupling agent used. For instance, if benzidine is the coupler, sulfuric acid cannot be used as benzidine sulfate is insoluble. However, benzidrine acetate is soluble so acetic acid is satisfactory. A basic sodium salt is suitable for alpha or beta naphthol as a coupler as the sodium salts of both are soluble. v

In case a coupling agent possesses an OH group it is preferable that the coupling take place to the diazonium compound in the ortho position inasmuch as the colored reaction product compounds are less soluble when thus coupled. The

'coupling reaction is preferably carried out in a basic solution although various coupling agents are reactive in neutral and even in acid solutions. Therefore, the pH control of'the coupling solution will depend on the particular agent employed. It also should-be pointed out that an excess of nitrous acid in the diazonium solution should be avoided inasmuch as it might be carried over in the gelatin and cause diazotization of the coupler and thereby produce a double coupling which as a rule is not desirable. The film is left in the coupling solution until the reaction goes substantially to completion which may bezonium compounds. Moreover, the same chemical reagent ma be diazotized and used as the diazonium compound and used in the undiazotized condition as the coupling agent. The posl- I tion in which the coupling agent couples with the diazonium compoundalso aflects the color. For instance, where the coupling takes place in the ortho position of the diazonium compound, the color may differ from a coupling with the same compounds but in the meta or para position. This positidn may be controlled by the pH of the coupling solution, by blocking ofcertain positions, and in other ways well known to-a chemist. I

It is to be expressly understood that any suitable diazonium compound and coupling agent may be employed and that many such reagents are contemplated. While these diazonium compounds and coupling agents are not listed in detail'herein they are set forth in detail in our c0- twelve or more and which preferably also con-' tains a salt in order to prevent any possible dissolving of the colored image. For example, a 2% solution of sodium hydroxide saturated with k the present disclosure and without departing from r sodium chloride has been found to work satisfactorily. The film is left in the clearing solution until it is cleared. Usually .three to five minutes suifice. It will be appreciated that in some instances the coupling reaction itself is carried out in a basic solution. In this case the image may be cleared at the same time as it is coupled thereby rendering a separate later clearing step unnecessary.

Afterimmersion in the clearing bath the film is passed into a 5% acetic acid solution (5 cc. of glacial acetic acid to 100 cc. of water) or other equivalent acids to counteract or neutralize any basic solution carried in the emulsion, and is then washed Three to five minutes in the acid bath is satisfactory. The washing is carried on until all excesses of reagents are removed from the emulsion. ,This usually takes about five minutes in gently running water. In addition, in the event that the coupling solution is basic, the acid treatment creates or regenerates the acid form of the colored compound, which is often of a more desirable color and also more stable. For example, the acid form of p nitrophenyl azo alpha naphtholis yellow and is created in the acid bath from the deep purple basic form such as the sodium salt of p nitrophenyl azo alpha naphthol, which was created in the basic coupling solution, If the basic form is stable and of a suitable color, the acid treatment may be dispensed with but the print should be washed to remove an excess of the coupling solution. It

is desirable to use the acid treatmentwhen possible, however.

If desired, the print may then be rendered transparent by treatment in a conventional fixing or hypo solution such as a solution of sodium thiosulfate and may be suitably washed. and dried. It is to be understood that this hypo step may be left out, especially if other or succeeding images are to be produced in the same emulsion layer productive of the principal colored image.

The temperature of all of the aforesaid baths,

including the washes is preferably kept within 15 to 18 C. except whereotherwiseexpressly specified. These limits, however, are not critical as fair results may be attained otherwise. It will be appreciated, however, that the upper limit must be kept below the stripping point of the .emulsion which in the usual case is below F.

Having described our invention fully, some specific embodiments thereof are disclosed in the following examples. It is to be expressly understood that these embodiments are given by way of illustration and not of limitation and that other suitable embodiments and variations may be devised by one skilled in the art in view of the scope of the invention as set forth herein.

Each of the following examples utilizes a set of color separation negatives produced in the manner fully explained above.

Example I (1) Positive film (such as. Eastman nitrate film bearing emulsion-No. K-1301) i exposed through the negative produced by the green appearing or B Wratten 8: Wainwright filter for of a second with a 15 watt printing light placed six inches from the film.

(2) The film is developed in Eastman D-ll developer for about five minutes to produce a print of high intensity and high contrast.

(3) The film is washed for one minute in gently flowing cool water (not above 70" F.) and is placed in a conventional short stop solution such as acetic acid and is again washed, if desired.

(4) The film is immersed for five minutes in the following hardening bath (Eastman formula SH-l):

Formaldehyde (40% aqueous solution) cc 10 NazCos gms 6 Water to make liter 1 The film is washed for one minute as outlined in step 3. r

(6,) The film is immersed in a bath consisting of gms. potassium ferricyanide to 100 cc. of water plus about of a gram of solid sodium hydroxide. The film is left in this solution until the developed black-and-white image is substantially completely-converted which may be visibly noted by temporarily removing the film and viewing the same umier a red safe light, preferably from the back. The image will change in appearance and seem practically invisible under a red light. Actually it changes from a black image to a grayish-white one. The time of conversion is about five minutes with practical limits of three to fifteen minutes although longer immersions do not appear to produce deleterious effects.

(7) The film is washed for five minutes in'the manner outlined in step 3.

(8) The film is next immersed in an approximately 10% (10 gms. to 100 cc.) aqueous solution of ZnBrz and is washed. Acetic acid may be cording to step 3. The temperature of this bath is preferably below 20 C.

(11): The film is then placed in a 1' to 2% (1 to 2 gms. to 100 .cc.) solution of alpha naphthol. The. solution is rendered alkaline by adding /2 to 1 gm.-of.NazCoa to form the sodium salt of alpha naphthol. The film is left in this solution until the coupling action is completed which is readily noticeable visually. The time of immersion should be'about five minutes.

.(12) The film is rinsed in water and immersed in a 2% solution (2 gms. to 100 cc.) of sodium hydroxide saturated with sodium chloride. The film is, left in this solution until it is rendered clear and transparent. This takes place in about five minutes.

(13) The film is immersed in a 5% solution of acetic acid (5 cc. of glacial acetic acid to 100' cc. of water) for five minutes.

(14) J The film is then washed in gently running cool water. for approximately five minutes and is suitably dried. It may, if desired, be treated with a conventional fixing'or hypo solution to render it transparent just prior to this drying step.

The color of-the resultant image is yellow and corresponds in color approximately to-the primary color of which the C5 Wratten 8r Wainwright is the complement.

Example]! I The steps outlined in Example I are followed except for the changes expressly indicated below:

In step 1 the film is exposed through the color separation negative exposed through the green appearing or B Wratten & Wainwright filter.

In step 9 the film is placed in a bath consisting I may make up the diazo solutions mentioned in step 9 of each of the foregoing examples and also the coupling solutions mentioned in step 11 thereof.

However, in order that the present invention may be still more clearly set forth, these solutions may be made by the following specific procedure.

It should be expressly understood that this prov cedure is given by way of illustration only.

The diazo solution mentioned in step 9 of Example I maybe made up by dissolving of a mole of p riitraniline in 10 cc. of concentrated sulfuric acid to which approximately 100 cc. of

. water have been added. The solution is cooled to make 250 cc.

' azotized dianisidine.

. by suitable means such as chipped ice to approximately 5 C. An excess of s of a mole of sodium nitrite dissolved in water is then slowly added with stirring until N02 gas isliberated,

and the solution is diluted with water to make about 200 cc. It is repeated that during this diazotization the solution must be cold, that is, held below 5 C. After diazotization has been completed the solution may be buffered with sodium acetate. Sufilcient sodium acetate is added to free the solution of excess mineral acid."

The diazo solution mentioned in step 9 of Example 11 may be made in the way just pointed out except that alpha naphthylamine is employed'instead of p nitraniline. Also it is desirable in this instance to dilute the solution I with a weakly acidic solution instead of water.

This diluting solution may be made by utilizing 25 cc. of glacial acetic acid for each cc. of

water. I

The coupling solution mentioned in step 11 of Example I may be made up as follows: 5 gms. of alpha naphthol are dissolved in 10 to 20 cc. of

a 10% aqueous sodium carbonate solution, that is, 10 gms. of sodium carbonate for each 100 cc'.

of beta naphthol are dissolved in v100 cc. of water.

The solution is rendered basic by the addition of approximately 3 to 4 gms. of solid sodium hydroxide. The solution is then diluted withwater Example III The steps outlined in Example I are followed except for the changes expressly indicated below;

In step 1 the film is'exposed through the color separation negative exposed through the purple appearing or C5 Wratten 8r Wainwright filter.

In sept, 9 the film is placed in a bath consisting of approximately a 1% molar solution of di- In step 11 the film is immersed in a 1% Na-zCOa solution of 1 amino 8 naphthol 4 sulfonic acid (4 gms. per liter).

a blue colored image by 1 The 'color tr the image is blue. According to the present invention, a completed color print may be produced by forming a plurality of initial colored images made accord ing to the foregoing disclosure which balance as to color value, intensity, gradation and detail and assembling them in registry. Usually three of these initial colored images are utilized and their color values would be red, yellow and blue although matched variations may be used such as magenta, blue-green, and an orange-ye w. For instance, an initial yellow image may be produced in the manner set forth in Example I above, a magenta one as set forth in Example II,

and a blue-green one set forth in Example III.

Normally, each of these initial colored images is produced in a separate print having an image carrying medium such as a layer of photographic gelatin carried on a suitable base. These prints may be superimposed in registry by known means and secured together to form the completed color print and may be so assembled in any desired order. However, the image carrying medium of each initial print may be stripped from its carrier and placed in registry on a common carrier to form the completed color print or formed in superimposed layers of emulsion or image carryit in a' solutionhaving an approximate pH of ing mediums initially disposed on a common carrier.

A superior improved completed color print may be made by carrying out the present invention because each image is reproductive of an initial black-and-white image and therefore has a controlled and satisfactory intensity, gradation and detail characteristics. In particular, the plurality of initial images may be readily and satisfactorily balanced. In addition, the color values of the initial images of the present invention may be readily and satisfactorily controlled to produce a natural appearing completed color print, since a wide variety or assortment of color values and shades may be produced by selection of suitable diazonium and coupling compounds. plurality of initial images having correct color values to blend or cooperate with each other may be formed to provide our improved completed color print.

While each of the colored images assembled in registry to produce our completed color print may be formed from an initial bleached and convertmade to the co-pending application of Albert L.

Bunting, Serial No. 332,067, filed April 27, 1940, for procedural details and reagents for forming this 'means.

We claim: 1. A method of producing in a deposit of photographic gelatin carried on a suitable base a colored photographic image from an initial silver black-and-white image developed in the gelatin, which comprises bleaching the silver image with a solution of a'ferricyanide to form a silver ferrocyanide image, reacting the silver ferrocyanide image with a zinc compound to form a zinc fer- Thus a.

rocyanide image, coloring the zinc ferrocyanide more than 11 and thereby dissolving the zinc ferrocyanide to leave the colored reaction product of the diazonium compound and the azo-dyestufi coupling agentas the color image forming substance.

2. A method of producing in a deposit of photographic gelatin carried on a suitable base a photographic image recorded in terms of yellowand-white which comprises exposing and developing a metallic silver image in the gelatin, bleaching said metallic image with a ferricyanide to form a silver ferrocyanide image, reacting the silver ferrocyanide image with a solution of a zinc compound to form azinc ferrocyanide image, reacting the zinc ferrocyanide image with a solution containing p nitro diazonium sulfate, to form an image consisting of zinc ferrocyanide and p nitro diazonium sulfate in chemical combination, reacting the last said image with a solution containing alpha naphthol to form a yellow-and-white reaction product image, and clearing said yellow-and-white image by immersing it in a solution containing sodium hydroxide.

3. A method of producing in a deposit of photographic gelatin carried on a suitable base a photographic image recorded in terms of redand-white which comprises exposing and developing a metallic silver image in the gelatin, bleaching said metallic image with a ferricyanide to form a silver ferrocyanide image, reacting the silver ferrocyanide image with a solution of a zinc compound to form a zinc ferrocyanide image, reacting the zinc ferrocyanide image with -a solution containing alpha naphthylamine diazonium sulfate to form an image consisting of zinc ferrocyanide and alpha naphtlwlamine diazonium sulfate in chemical combination, reacting the last said image with a solution containing beta naphthol to form a red-and-white reaction product image, and clearing said red-andwhite image by immersing it in a' solution containing sodium hydroxide. V

4. A method of producing in a deposit a photographic gelatin carried on a suitable base a photographic image recorded in terms of blueand-white which comprises exposing and developing a metallic silver image in the gelatin, bleaching said metallic image with a ferricyanide to form a silver ferrocyanide image, reacting the silver ferrocyanide image with a solution of a zinc compound to form a zinc ferrocyanide image, reacting the zinc ferrocyanide image with a solution containing diazotized dianisidine to form an image consisting of zinc ferrocyanide and diazotized dianisidine in chemical combination, reacting the last said image with a solution containing 1 amino 8 naphthol 4 sulfonic acid to form a blue-and-white reaction product image, and clearing said blue-and-white image by immersing it in a solution containing sodium hydroxide.

5. A method of producing a colored photographic image from an initial silver photographic image, which comprises bleaching the silver image to form a silver ferrocyanide image, reacting the bleached image with a zinc compound to form a zinc ferrocyanide image, reacting said zinc ferrocyanide image with a diazonium compound to form an image comprising zinc terrocyanide chemically-united with the diazonium compound, reacting the last said image with an azo-dyestufi coupling agent to form a colored image, and treating said colored image with a base for dissolving zinc ferrocyanide to produce a colored image comprising the reaction product of the diazonium compound and the coupling agent.

6. A method of producing a colored photographic image from an initial silver photographic image, which comprises bleaching the silver image with a ferricyanide to produce a sil- V ver ferrocyanide image, reacting the silver ferrocyanide image with a zinc compound to produce a zinc ferrocyanide image, reacting the zinc ferrocyanide image with a diazonium compound to produce a reaction product image comprising zinc ferrocyanide chemically united with the diazonium compound, reacting said reaction prodcut image with an azo-dyestufl. coupling agent to produce an image comprising the reaction product chemically united with said coupling agent, and treating the last said image with a base for separating zinc ferrocyanide from the chemical compound comprising the last said image and for dissolving said zinc Ierrocyanide to produce a colored image comprising the reaction product of said diazonium compound and said coupling agent.

'7. In a method of producing a colored photographic image from an initial silver photographic image, the steps of bleaching the silver image to form a silver Ierrocyanide image, reacting the bleached image with a zinc compound to form a zinc ierrocyanide image, reacting said zinc ferrocyanide image with a diazonium compound to form an image comprising the chemical reaction product of zinc ferrocyanide and the di azonium compound, and reacting the last said image with an azo-dyestufl' coupling agent to form a colored image comprising said reaction product image chemically united with said coupling agent.

8. In a method of producing a colored photographic image from an initial silver photographic image, the steps of forming from the silver image a silver ferrocyanide image, reacting the silver ferrocyanide image with a zinc compound to produce a zinc terrocyanide image, reacting the zinc ferrocyanide image with a diazonium compound to form an-image comprising zinc terrocyanide chemically united with the diazonium compound, and reacting the last said image with a coupling agent to form a colored image.

ALBERT L. BUN'I'ING. RAYMOND W. THOMAS.

- CERTIFICATE OF comm-omen. v Patent No. 2,512,875. 1 March 2, 19 .3.

ALBERT L. emme, ET AL.

It 15- hereby certified that error appears in the nrinted specification of the above numbered patent requiring correction as follows: Page 2, eec' and column, line 25, for "gentle" read -.gently,' page 14., first column; line 56, for "ever" read -never--; page 5, fir h- 11119 6, for "benzidrine' read --be neidine-; page 6, second. column, line 70, for sept read --step-; and that the s aid Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and eeeled this 11th day of ma A. n. 1915.

- Henry Van Aredule (Seal) Acting Commissioner 0:! Patents.