US3870522A - Fogged, direct-positive emulsion containing heterodisperse and irregular composite silver halide grains - Google Patents

Abstract

A silver salt emulsion for the preparation of direct-positive images by imagewise exposure and development of developable fog light-sensitive material wherein the developable fog is destroyed on exposure to light and in which at least one fogged silver salt emulsion contains heterodispersed and irregular grains containing ripening nuclei and at least 10 percent of the grains have a size which deviates by at least 40 percent from the average grain diameter.

Description

United States Patent Moisar et a1.

1 1 FOGGED, DIRECT-POSITIVE EMULSION CONTAINING HETERODISPERSE AND IRREGULAR COMPOSITE SILVER HALIDE GRAINS [75] Inventors: Erik Moisar; Sieghart Klotzer, both of Cologne. Germany [73] Assignee: Agfa-Gevaert Aktiengesellschaft,

Leverkusen, Germany 22 Filed: May 16,1973

21 App1.No.:360,845

[30] Foreign Application Priority Data Mar. 11,1975

3,317,322 5/1967 Porter et a1. 96/108 3,367,778 2/1968 Berriman i 96/108 3,501,305 3/1970 lllingsworth 96/108 3,717,466 2/1973 Florens et al. 96/108 Primary Examiner-Norman G. Torchin Assistant Examiner-Won H. Louie. Jr. Attorney, Agent, or FirmConnolly and Hutz [57] ABSTRACT A silver salt emulsion for the preparation of directpositive images by imagewise exposure and development of developable fog light-sensitive material wherein the developable fog is destroyed on exposure to light and in which at least one fogged silver salt emulsion contains heterodispersed and irregular grains containing ripening nuclei and at least 10 percent of the grains have a size which deviates by at least 40 percent from the average grain diameter.

5 Claims, N0 Drawings FOGGED, DIRECT-POSITIVE EMULSION CONTAINING I-IETERODISPERSE AND IRREGULAR COMPOSITE SILVER HALIDE GRAINS This invention relates to a photographic material and to a process for the production of direct-positive photographic images by the imagewise exposure of a photographic material which contains at least one fogged silver halide emulsion layer, the developable fog being eliminated in the areas corresponding to the image and a direct-positive image being subsequently obtained by photographic development.

Silver halide emulsions which have been fogged by exposure to light or by chemical treatment are generally used for the production of direct-positive images. If certain conditions are observed, the imagewise exposure to light destroys the developable fog but the fog remains in the unexposed areas. A direct-positive image is then obtained by development of the exposed layer. Destruction of the developable fog by imagewise exposure is mainly achieved by making use of the Herschel effect or the solarization effect. When the I-Ierschel effect is used, exposure is carried out with longwave light from the absorption range of silver so that the silver nuclei are destroyed in the exposed areas. If the solarization effect is used, exposure is carried out with shortwave light from the absorption range of the silver halide and again the developable fog is destroyed. These processes are in practice of minor importance because the usual photographic emulsions are relatively insensitive.

Improvement in the sensitivity to light of the emulsion can be obtained by optimization of the fogging methods and by the addition of desensitizers which act as electron traps. Such emulsions have been described in British Pat. No. 723,019. In the process disclosed in the said US. Patent Specification, fogging is carried out by means of reducing agents in the presence of compounds of the noble metals which are more electropositive than silver. According to U.S. Pat. No. 3,501,305, the sensitivity of direct-positive emulsions is further enhanced by using monodisperse silver halide emulsions which are reduction fogged and gold fogged on the surface. These monodisperse silver halide emulsions are characterised by having a narrow grain size distribution, at least about 95 percent by weight of the silver halide in the emulsion having a particle size which deviates by not more than 40 percent from the average size. Such emulsions are prepared by the socalled double jet process, i.e., simultaneous addition of silver salts and alkali metal halides during precipitation.

Processes of this kind have been described in U.I(. Patent No. 1,027,146.

A new type of direct-positive emulsion was first described by E. MOISAR and S. WAGNER in Berichte der Bunsengesellschaft fuer plysikalische Chemie" 67 (I963), 356 359. In these direct-positive emulsions, the sensitivity to light is increased by incorporating ripening nuclei. i.e., electron traps, into the interior of the silver halide grain so that the photoelectrons formed in the primary process, which prevent the destruction of the developable fog nuclei on the grain surface of such direct-positive emulsions, are trapped in the interior of the grain. The controlled double jet process described by KLEIN MOISAR in Berichte der Bunsengesellschaft fuer physikalische Chemie 67 (1963), page 349,

and in British Patent No. 1,186,718 is employed for producing such emulsions with a composite grain structure because, according to the present state of the art, controlled incorporation of internal nuclei into the silver halide crystals has only been possible by such precipitation methods. Emulsions with a narrow grain size distribution are always obtained by this method.

Monodisperse cubical or octahedric emulsions, on the other hand, have considerably disadvantages which arise from their method of preparation since the double jet process and the requirement to observe definite pAg values require expensive and complicated apparatus. Another disadvantage lies in the photographic properties of such monodisperse direct-positive fogged silver halide emulsions in that the images obtained have a relatively steep gradation. The practical application of the process was restricted to cases in which a steep grada-- tion was desired or, at least, not undesirable. The said process was however of limited utility for the production of continuous tone images owing to the steep gradation obtained. It has been proposed to flatten the gradation by mixing several monodisperse direct-positive silver halide emulsions which have been fogged to varying degrees. When this method is employed, stepshaped positive gradation curves are obtained. By mixing a sufficiently large number of such monodisperse emulsions, the steps in the gradation curve are kept small and direct-positive emulsions with a flatter gradation curve are in fact obtained but even this method is in practice of limited interest since the production of munerous monodisperse direct-positive emulsions which have each fogged to different extents is relatively complicated and, above all, difficult to reproduce.

it is among the objects of the present invention to provide a process for producing direct-positive silver salt emulsions by a technically simple process by which direct-positive images with a flat gradation such as are required for continuous tone images can be obtained.

We now have found a photographic material containing a direct-positive silver salt emulsion layer in which the silver salt grains have been fogged on the surface so that they can be developed and contain internal electron traps, the direct-positive silver halide emulsion being heterodisperse and irregular with a grain size distribution such that at least 10 percent and preferably at least 20 percent of the number of grains have a grain size which deviates by at least 40 percent from the average particle diameter.

The direct-positive silver salt emulsion used for the photographic material according to the invention are prepared by methods known per se. The simplest method consists of adding an aqueous silver salt solution, preferably a silver nitrate solution, to a gelatincontaining solution of the other precipitation components. The precipitation components used are preferably alkali metal halides, in particular alkali metal bromide or iodobromide solutions. The desired average grain size and grain size distribution can be modified as desired in known manner by using an excess of halide and by suitably selecting the conditions under which physical ripening is carried out, in particular the temperature and time. i

The resulting silver salt grain cores for the subsequent direct-positive emulsion with a composite grain structure are now subjected to a treatment by which electron traps are produced on them. This is achieved, for example, by the formation of ripening nuclei, e.g.,

by chemical sensitization with noble metal compounds, in particular gold or iridium salts, or with sulfur compounds such as thiosulfate or a combined treatment with noble metal salts and sulfur compounds. Suitable compounds for this purpose are e.g., the alkali metal salts of the following noble metal ions:

Ripening of the cores of the emulsion grains can also be achieved in known manner with reducing agents such as hydrazine, formamidine sulfinic acid or tin(ll) chloride. Reduction ripening may also be carried out in the presence of noble metal compounds.

For the incorporation of electron traps, the cores of the emulsion grains may also be treated with aqueous solutions of polyvalent metal salts, e.g., of trivalent bismuth.

The compounds required for producing electron traps, e.g., by chemical ripening as described above, may have, if desired, been added at the precipitation state, i.e., during the production of the cores of the grains for the subsequent silver salt emulsion. In this variation of the process, the electron traps or ripening nuclei are produced in statistical distribution within the core of the silver salt grain. In the first variation of the process described above, these electron traps or ripening nuclei are preferably formed on the surface of the core of the silver salt particle.

The composite silver salt grains are now produced by precipitation of an outer shell of a silver salt on the resulting heterodisperse and irregular silver salt cores which contain ripening nuclei. These shells may consist of the same silver salt as the core or some other silver salt, in particular a silver halide. Precipitation may be carried out by known method; either, as in the case of precipitation of the cores, by providing one precipitation component and then adding silver nitrate, or by simultaneously running in both precipitation components or by alternately adding one and the other component. Salts of polyvalent cations, e.g., bismuth salts, may advantageously be present during this precipitation. The emulsions are then processed in the usual manner by flocculation, washing, etc.

According to another embodiment of the invention, precipitation which leads to the formation of heterodisperse emulsions with irregular silver halide crystals is carried out in the presence of salts of polyvalent metal ions which are not normally used for chemical ripening. It has been found advantageous, for example, to carry out precipitation in the presence of compounds of trivalent bismuth, preferably at pH values below 5 and preferably using 0.02 to 2 millimols per mol of silver halide. Such emulsions then contain the electron traps statistically distributed within the whole volume of the grain or in a part thereof without having the usual composite grain structure.

Fogging of the silver halide grains in the photo graphic direct-positive emulsions of the invention is performed in known manner by treatment with reducing agents, preferably in the presence of water-soluble salts of metals which are more electropositive than silver.

Suitable reducing agents are e.g., tin(ll) salts such as tin(II) chloride, hydrazine or hydrazine compounds, sulfur compounds such as thiourea dioxide, phosphonium salts, e.g., tetra(hydroxymethyl) phosphonium chloride or formamidine sulflnic acid. Suitable compounds of metals which are more electropositive than silver are, for example, salts of the following noble metals: Gold such as potassium chloroaurate, gold(lll) chloride, rhodium, platinum, palladium such as ammonium hexachloropalladate and iridium such as potassium chloroiridate.

The concentrations of reducing agents and of noble metal salts used for fogging may vary within wide limits. Concentrations of about 0.0005 to about 0.06 milliequivalents of reducing agent and about 0.001 to about 0.01 millimols of noble metal salt per mol of silver halide as described in German Offenlegungsschrift No. 1.547.790, are generally sufficient. If the emulsions are too heavily fogged, they may subsequently be treated with a bleaching agent in known manner to adjust the light-sensitivity of the direct-positive emulsions to the optimum value.

Fogging may also be carried out by the known method of silver salt digestion according to Wood described in J.Phot. Science 1 (1963), page 163, at pAg values of between 2 and 5 and pH values of above 6.5.

The chemical nature of the silver salt used for the direct-positive emulsion is not in principle critical. The most suitable silver salt for any given fogged directpositive emulsion can easily be determined by a few simple tests. The shell and core of the emulsion grains may have the same or different composition as regards the silver salt. They are preferably substantially identical in composition. Fogged direct-positive silver salt emulsions preferably contain silver halides, e,g., silver bromide, which may contain a certain amount of silver iodide which is preferably up to 20 mols percent.

Direct-positive silver bromide or silver iodobromide emulsions are preferred. Silver bromide emulsions which have been converted to silver iodide on the surface have proved to be particularly suitable.

The photographic direct positive materials according to the invention have excellent sensitivity to light. The direct positive images obtained have a much flatter gradation, being improved by a factor of about 2 than that of direct-positive images obtained with known monodisperse direct-positive emulsions which have a comparable sensitivity to light and the same average grain size and silver application.

The binders used for the emulsion layer may be any of the usual hydrophilic film forming substances, e.g. proteins, particularly gelatin, alginic acid or its derivatives such as esters, amides or salts, cellulose derivatives such as carboxymethyl cellulose and cellulose sulfates, starch or its derivatives or hydrophilic synthetic binders such as polyvinyl alcohol, partly saponified polyvinyl acetate and polyvinyl pyrrolidone. The layers may also contain solutions or dispersions of other synthetic binders mixed with the hydrophilic binders, e.g. homopolymers or copolymers of acrylic or methacrylic acid or derivatives thereof such as esters, amides or nitriles, or vinyl polymers such as vinyl esters or vinyl ethers.

The fogged silver salt emulsion layers are applied to the usual supports, e.g. glass or foils of cellulose esters such as cellulose acetate or cellulose acetobutyrate or foils of polyesters, in particular of polyethylene terephthalate or polycarbonate, especially those based on bis-phenylolpropane. The support used may also be baryta-coated paper supports or paper which has been laminated with polyolefines, for example polyethylene or polypropylene.

The direct positive silver salt emulsions of the invention may be optically sensitized in the usual manner. Both desensitizers and the usual sensitizing dyes for negative emulsions may be used for these directpo sitive emulsions which contain internal ripening nuclei. According to the work carried out by SHEP- PARD, et al. (J.Phys.Chem. 50 (1964) 210), STA- NIENDA (ZeitschLphysChem. (N.F.) 32 (1962) 238) and DAHNE (Zt.wiss.Phot. (1969) 161), desensitizers are dyes whose cathodic-polarographic half wave potential measured against the normal calomel electrode, is more positive than l.0 V. Such compounds were later described in U.S. Pat. No. 3.501.306, 3.501.306 and 3.501.307. The sensitizers described in German Patent No. 1.153.246 and in U.S. Patent No. 3.314.796 are also suitable. Reference may also be made to the imidazo-quinoxaline dyes, e.g. according to Belgian Patent No. 600.253.

The emulsions may contain the usual stabilizers such as monopolar or salt-type compounds of mercury which contain aromatic or heterocyclic rings (such as mercaptotriazoles), simple mercury salts, sulfonium mercury double salts and other mercury compounds. Other suitable stabilizers are azaindenes, especially tetraor penta-azaindenes and particularly those which are substituted with hydroxyl or amino groups. Compounds of this kind have been described in the article by BlRR,Z.Wiss.Phot. 47 (1962) 2 58. Other suitable stabilizers are e.g. heterocyclic mercapto compounds such as phenyl mercaptotetrazole, quaternary benzothiazole derivatives and benzotriazole.

The emulsions may be hardened in the usual manner, for example with formaldehyde or halogenated aldehydes which contain a carboxyl group, such as mucobromic acid, diketones, methanesulfonic acid esters and dialdehydes.

Photographic materials which contain at least one of the direct-positive silver salt emulsion layers according to the invention may be used for various photographic purposes, particularly for producing continous-tone images, e.g. for direct-positive color images by the silver dye bleaching process or by a dye diffusion process, or for producing photographic color images which are produced by colorforming development by conventional methods. The materials according to the invention are also suitable for color intensifying processes or for producing vesicular images in accordance with German Offenlegungsschrigt No. 2,201,849 or U.S. Pat. application Ser. No. 322,101.

Subsequent processing of the exposed material according to the invention is performed in the usual manner. The usual black and white developers or color de velopers are suitable for development.

EXAM PLE l a. To produce a heterodisperse emulsion as staring material, a solution of 675 g of silver nitrate in 1,500 ml of water is run into a solution of 600 g of potassium bromide and 100 g of gelatin in 3,000 ml of water at 50C over a period of one minute with stirring. After digestion for 20 minutes, a further 160 g of silver nitrate in 350 ml of water are added over a period of minutes as well as a further 150 g of gelatin. The emulsion is cooled, left to solidify, washed and after being remelted it is adjusted to pAg 9 with potassium bromide solution.

b. 7 ml of a 10' molar solution of Na [Au(S O- are added to one-fifth of the emulsion described above. Ripening is carried out for 60 minutes at 50C.

c. After the addition of 1,000 ml of a 5 percent gelatin solution and ml of a 3N potassium bromide solution to the emulsion which has been treated according to b), 90 ml of a 3N silver nitrate solution are added over a period of 5 minutes at 50C with stirring. The addition of 90 ml of potassium bromide solution followed by 90 ml of silver nitrate solution is repeated 4 times. Between the individual stages of precipitation, 10 g of solid gelatin are added in each case and dissolved for about 15 minutes. The resulting emulsion is cooled, washed and after remelting is adjusted to pAg 9 with potassium bromide. The cyrstals of the resulting emulsion are irregular and have an average particle diameter of 0.5 [.L, 65 percent of the crystals being outside the range of i40 percent, based on the average diameter, that is to say outside the range of 0.3 to 0.7 a.

d. A reducing agent such as tin (ll) chloride, formamidine sulfinic acid or hydrazine and a gold compound such as AuCl are added to fog the emulsion in accordance with British Pat. No. 732,019. The fogged emulsion is cast on a cellulose acetate support. After exposure under a grey wedge, the emulsion is developed in a developer bath of the following composition:

1 g of p-methylaminophenol,

13 g of sodium sulfite,

3 g of hydroquinone,

26 g of sodium carbonate.

1 g of potassium bromide. made up to 1 litre with water.

A direct-positive density curve is obtained which has a maximum density of S 1.5 and a gradation of y 0.5 (measured in the middle straight line portion of the density curve) and a sensitivity of 2.7 relative logarithmic units.

A homodisperse direct-positive emulsion prepared by the previously customary double jet process which has a grain size of 0.5 u has a maximum density of S 1.6 and a sensitivity of 2.8 relative logarithmic units and a density curve with a gradation of y 1.8.

EXAMPLE 2 1/5 of the heterodisperse emulsion prepared according to Example 1 (a) which is used as starting material is digested for 60 minutes at 50C after the addition of 40 mg of K lrCl dissolved in 40 ml of water. The emulsion is then treated as described in Examples 1(c) and (cl). The direct-positive density curve shows a sensitivity of 2.9 relative logarithmic units and a gradation of y 0.6.

EXAMPLE 3 A further fifth of the heterodisperse emulsion prepared according to Example 1(a) is digested at 50C, first for one hour after the addition of 10 ml of a 10 molar solution of formamidine sulfinic acid and then for one hour after the addition of 10 ml of a 0.08 percent solution of gold (Ill) chloride. It is then treated as described in Example 1 (c) and (d). The direct-positive density curve shows a sensitivity of 2.7 relative logarithmic units and a gradation of y 0.5.

v EXAMPLE 4 Another fifth of the heterodisperse emulsion prepared according to Example 1(a) is treated as described in Example 1(b). It is adjusted to pH 3 with nitric acid after the addition of 1,000 ml of a 5 percent gelatin solution. 300 mg of Bi(NO.1) 3,5 H2O are then added. Precipitation is then performed as described in Example 1 (c). The subsequent procedure is the same as that described in Example 1(c) and (d).

The direct-positive density curve shows a sensitivity of 3.0 relative logarithmic units and a gradation of y 0.6.

EXAMPLE 5 A solution of 675 g of silver nitrate in 1,500 ml of water is run into a solution of 600 g of potassium bromide, 100 g of gelatin and 200 mg of Na-,,[ IrCl in 3,000 ml of water at 50C within one minute with stirring. After leaving the mixture to digest for minutes, a further 160 g of silver nitrate in 350 ml of water are added in the course of 10 minutes as well as 150 g of gelatin. The mixture is cooled, left to solidify and washed.

After remelting, precipitation is carried out as described in Example 1(c) by the repeated addition of potassium bromide solution and silver nitrate solution. The emulsion is then treated as described in Example l (c) and (d). The direct-positive density curve shows a sensitivity of 3.] relative logarithmic units and a gradation of 'y 0.4.

EXAMPLE 6 A heterodisperse emulsion is prepared in the presence of Na i lrCl as described in Example 5. Precipitation of additional silver bromide is carried out as described in Example 4, i.e. in the presence of Bi(NO .5 H O at a pH of 3.0. The emulsion is then treated as deribe xa n s. 31 .111 ..Fl CEB QIiXEQEPEIX curve shows a sensitivity of 3.7 relative logarithmic units and a gradation of y 0.6.

EXAMPLE 7 450 s it o-asfi H19 amassed t a selution o 180 g of potassium bromide and g of gelatin in 850 ml of water after adjustment to pH 3 with nitric acid. 400 ml ofa 3N silver nitrate solution are run in at 50C within one minute. The solution is then digested for 20 minutes after the addition of 45 g of gelatin. A further 100 ml of 3N silver nitrate solution are then added in the course of 10 minutes. The emulsion is left to solidify and washed in the usual manner and after remelting it is adjusted to pAg 9 with potassium bromide solution.

After fogging accomplished as described in Example 1(d), a direct-positive emulsion containing irregular silver halide crystals with an average particle size of d 0.25 t is obtained. 70 percent of the particles lie outside the range of d i 40 percent. The direct-positive density curve shows a sensitivity of 3.2 relative logarithmic units and a gradation of 'y 0.8.

EXAMPLE 8 A dilute solution containing potassium bromide and potassium iodide in a molar ratio of 4:1 was added to 1 part of the fogged direct-positive emulsion prepared according to Example 7 to adjust the pAg to 9.8. Direct-positive density curves of this emulsion which had thus been partly converted into silver iodide on the crystal surfaces indicate a sensitivity of 3.7 relative logarithmic units and a gradation of 'y 0.4.

. EXAMPLE9 Dyes I to IV represented below were added in the quantities indicated in the following table (based on 1 g of silver in the form of silver halide) to the heterodisperse fogged emulsion prepared according to Example 8. The photographic layers prepared in the conventional manner were exposed behind color separation filters and developed in the developer described in Example l. The sensitivities obtained are indicated in relative logarithmic units. Dyes I and II are sensitizers for normal negative emulsions. Dyes III and IV act as del CH:

(German Offenlegungsschrift No. 2,057,617)

Table green red Non-sensitized emulsion 1.4 1 2.2 mg of Dye I 2.7 1 1.1 mg of Dye 11 1.3 2.2 4.4 mg of Dye III 2.1 2.2 2.2 mg of Dye IV 3.0 1.2

EXAMPLE 10 450 mg of Bi(NO- .5 H O are added to an aqueous solution of g of KBr, 38 g of KJ and 30 g of gelatin in 850 ml of water (pH adjusted to 3 with nitric acid). 400 m1 of a 3N AgNO solution are added at 50C within 1' minute. After addition of 55 g of gelatin it is digested for 30 minutes. The emulsion is cooled, left to solidify, washed and after being remelted it is adjusted .to pAg 9 with aqueous potassium bromide solution.

A direct positive image with flat gradation is obtained.

What we claim is:

1. A photographic material comprising a support and a light-sensitive layer containing at least one fogged light-sensitive direct positive silver halide emulsion layer being spontaneously developable with silver halide grains containing internal electron traps wherein the improvementcomprises silver halide grains of the fogged emulsion are heterodispersed and irregular having cores which contains interval electron traps and outer shells of silver halide on the cores and having a grain size distribution wherein at least percent of the grains have a size which deviates by at least 40 percent from the average grain diameter, said outer shells of silver halide have been fogged.

2. The material of claim 1 wherein at least 20 percent of the grains have a grain size which deviates by at least 40 percent from the average grain diameter.

3. The material of claim 1 wherein the silver halide is silver bromide which may contain up to 20 mols percent of silver iodide.

4. The material of claim 1 wherein the silver halide grains are fogged with a reducing agent in the presence of a gold compound or a compound of another metal which is more electro-positive than silver.

5. The material according to claim 1 wherein the silver halide emulsion is optically sensitized.

Claims (5)

1. A PHOTOGRAPHIC MATERIAL COMPRISING A SUPPORT AND A LIGHT-SENSITIVE LAYER CONTAINING AT LEAST ONE FOGGED LIGHTSENSITIVE DIRECT POSITIVE SILVER HALIDE EMULSION LAYER BEING SPONTANEOUSLY DEVELOPABLE WITH SILVER HALIDE EMULSION LAYER BEING INTERNAL ELECTRON TRAPS WHEREIN THE IMPROVEMENT COMPRISES SILVER HALIDE GRAINS OF THE FOGGED EMULSION ARE HETERODISPERSED AND IRREGULAR HAVING CORES WHICH CONTAINS INTERVAL ELECTRON TRAPS AND OUTER SHELLS OF SILVER HALIDE ON THE CORES AND HAVING A GRAIN SIZE DISTRIBUTION WHEREIN AT LEAST 10 PERCENT OF THE GRAINS HAVE A SIZE WHICH DEVIATES BY AT LEAST 40 PERCENT FROM THE AVERAGE GRAIN DIAMETER, SAID OUTER SHELLS OF SILVER HALIDE HAVE BEEN FOGGED.

1. A photographic material comprising a support and a light-sensitive layer containing at least one fogged light-sensitive direct positive silver halide emulsion layer being spontaneously developable with silver halide grains containing internal electron traps wherein the improvement comprises silver halide grains of the fogged emulsion are heterodispersed and irregular having cores which contains interval electron traps and outer shells of silver halide on the cores and having a grain size distribution wherein at least 10 percent of the grains have a size which deviates by at least 40 percent from the average grain diameter, said outer shells of silver halide have been fogged.

2. The material of claim 1 wherein at least 20 percent of the grains have a grain size which deviates by at least 40 percent from the average grain diameter.

3. The material of claim 1 wherein the silver halide is silver bromide which may contain up to 20 mols percent of silver iodide.

4. The material of claim 1 wherein the silver halide grains are fogged with a reducing agent in the presence of a gold compound or a compound of another metal which is more electro-positive than silver.