US1593998A - Method of depositing silver - Google Patents

Description

B. BART METHOD OF DEPOSITING SILVER July 27 1926.

Filed Nov. 10, 1925 INVENTOR Patented July 27, 1926.

UNITED STATES BLASIUS BART, OF EAST ORAN GE, NEW JERSEY.

METHOD OF DEPOSITING SILVER.

Application filed November 10, 1823. Serial No. 673,975.

.The invention relates in general to a method of depositing metals by chemically reducing the same from a solution or compound of such metals. The invention relates specifically to a method of depositing silver on different objects to be coated thereby, and in the following disclosure of the invention reference will be made specifically to an improved manner of depositing nascently formed metallic silxaer on a mold to form reflectors of the type in which the formed silver reflecting surface is backed by a strengthening layer of electrolytically deposited copper. I

In my copending application, Serial No. 387 ,974, method of forming reflecting surfaces, filed June 10, 1920,. it is therein pointed out that it was a usual practice in the art to, which thi invention relates to mix a solution of a silver salt, such as silver nitrate, with a reducing reagent, such as Rochelle salts and to'fiow the wet mixture over the object to be coated. The prior application featured an advance over this old flowing method by causing the. reacting chemicals to be discharged into air, cause them to mix and. thus react while in the.

form of a stream of separate particles and to regulate the time duration while'the reagents were in the air so as to complete the reducing reaction by theIti-me the reagents reached the object an'djiii th-isway to receive" nascently formed metallic silver, on the surface under treatment. I v v In application Serial-iNo 502,680, method of depositing silver,..filed Septen-1ber 23,- I921, the method in the; first application was modifled so'that the initial stages of the reducing reaction took place while the reagents were in liquid form and enclosed in a mixing chamber. The partially reduced substances were then sprayed onto the surface of the object under treatment and the operation was carried on in such timed duration that the metallic silver was formed and immediately received on the surface to be coated.

This nicety of time control was attained by spacing the receiving surface from the dis charging nozzle so as to insure a completion of the reducing reaction ;by the time the substances reached the surface.

Vhile the methods disclosed in these two co-pending applications are still used in certain types of work, the present disclosure features a further modification in certain details of a modified method for. depositing large numbers of articles must be coated raps idly, efliciently, and without necessity of the attention "of a skilled operator to insureuniformity of product.

The object of the present invention, like the. object of the inventions disclosed in the preceding applications is to attain a brilliant reflecting surface, of uniform high lustre simulating a hi hly polished silver surface, and to attain this result with minimum loss of reagents, rapidly, efiiciently and economically considered from both cost of reagents used and cost of skilled labor.

The present disclosure features a preliminary chemical contact of a limited amount of the silver solution and its reducing reagent continuing for a short period of time prior to its discharge in the air, as featured in application Serial No. 502,680, but instead of spraying the partially reduced substance on to the mold in this case" it is caused to flow or rather drip onto'th-e mold, initially in the form ofa continuous stream, followed as soon as themoldais'c6vered by a stream having mess rate 'o'f discharge, preferably in the forin-of'd'istinct separate drops of small indiv idual'size but following each other rapidly inorder to insure a continuity of application.

The invention thereforefeatures the application of the silver in two steps, first as a-st-ream and then as a drip-ping of a silver solution with its reducing reagent on to the surface to be coated while the reducing reaction is taking place.

The perfection in resulting products ap films of nascently precipitated metallic. silver. \Vhile it is obvious that different silver compounds may be used and the different compounds in the solutions may be of varying strength it is suggested that when silver nitrate is used, it be freshly prepared, pref the silver nitrate salts to each gallon of water. If Rochelle salts is used as the reducing-substance 4O grammes to the gallon is suggested.

At the instant the metallic silver particle erably of the proportion of-80 grammes of strikes the surface it has not lost its fluidity and as it strikes the rotating mold or other object to be covered there is apparently a spreading out of each drop over a relatively extensivesurface. It is of course obvious that the extent of surface covered will depend upon several factors, for instance. upon the mechanical speed of rotation of the mold and upon the rapidity with which the silver deposit loses its fluidity. lVhen the surface receiving the silver has inclined sides such as the parabolic molds discosed in these several applications, the flowing film of metallic silver has a tendency to flow downwardly along the revolving sides of the mold but due to the short time duration with which the liquid silver is in contact with the sides before it solidifies into the film, the lapsed time is so short that the entire surface cannot be covered with one dripping stream.

In the preceding applications it was sug gested that the nozzle apparatus be moved over the surface for the purpose of spreading the silver film. In application, Serial N 612.132, method of forming flanged reflectors filed January 11, 1923, it is suggested that the receiving surface be rotated under the discharge nozzle. This idea of rotating the receiving surface in order to spread the silver is utilized in the present disclosure for it has been found effective in spreading out and rapidly flattening the more or less rounded drops featured in this disclosure.

Accordingly the present disclosure suggests the utilization of a plurality of streams of drops directed so as to fall at different points upon the rotating mold. at variably spaced distances from the axis of rotation and so in directing the streams that they will coact to cover the entire surface under treatment with a continuous film of silver having a uniform depth.

In application, Serial No. 387.974. it was suggested that the amount of silver initially in the solution compared with the amount of reducing reagent present would be in relatively larger percentage than would be indicated by the theoretically .correct amount required by chemical ca culation-to effect the reducing reaction. This excess of silver was used so as to form the initial layer of extremely brilliant silver without perceptible-thickness. The proportion of silver to its reducing reagent was then cau ed to approach the true theoretical chemical reacting proportions so as to form the succeeding layers of less brilliancy. This same feature of reducing the proportion of silver present has been carried forward in the invention herein disclosed, except that a wider variation in proportion of silver to reagent can be practiced with the dripping method herein featured and atthe same time continue to feature the practicing of the process without material loss of reagents.

It was suggested in application, Serial No. 387,974 that the mold should be heated during the depositing, a temperature of to 90 F. being indicated, and in application Serial No. 508,680 it was suggested that the temperature condition was a vital factor in attaining the best results. In the latter application a temperature of about 60 F. was indicated. In the present disclosure the temperature conditions are still requisite to success and while the reaction may take place at practically any room temperature the best results are attained when the temperature during the entire period of the reactionis 50 to 60 F. The variations in these limits apparently depend upon unknown conditions, presumably atmospheric conditions and must be determined from time to time during the progress of the coating operations.

In practicing the operation a thin stream of the silver solution and a similar stream of the reducing reagent are caused to flow into a small mixing cup to form a shallow pool and are retained therein for a limited time, sufficient to permit some reducing action between the substance while in the form of a liquid mass. but insufficient to permit a complete reduction of all the silver present. The partially reduced substances are then caused to pass from the pool in the mixing cap through a discharge nozzle having a pin hole opening and arranged to discharge the contents of the cup, first, in an extremely thin stream and then in the form of separated drops onto the surface to be coated. However, it is within the scope of the disclosure to have the initial application in the form of large drops. The initial stream is directed onto the rotating surface, the speed of which is so regulated that the entire surface is covered usually in one or two revolutions. ered the rate of flow is reduced so that the succeeding layers are formed by dripping the silver onto the rotating mold. In practicing the process it is preferable to use the rich silver for the first two or three seconds and then to slow up the reducing activity so that succeeding layers will be formed from si'ver deposited from mixtures of solutions and reagents present in the usual reducing proportions as previously suggested.

Various other objects and advantages of the invention will be in part obvious from the suggestions herein made relative to the improved method and from an inspection of the accompanying drawings showing a means for practicing the process and in part will be morefully set forth in the following particular description of one particular method of practicing the invention. Referring to the accompanying drawing After the surface is covthere is shown largely in diagrammatic outline a suggested form of apparatus for practicing the process featured in this disclosure.

In the drawings there is shown a convex, chemically clean and preferably highly polished glass mold 10, mounted for rotary movement upon a driving shaft 11. The silver is deposited from discharge nozzles 12 and 13 through pin hole openings and these nozzles are so positioned relative'to the axis of rotation of the mold indicated by the line al-b that one of the nozzles, for instance nozzle 1.2, will cover the upper por-- tion of the mold and the other nozzle 13 will cover the lower portion of the mold. The positions of the nozzles are adjusted so that the resulting film will be continuous over the entire top of the mold.

A supply of silver 'salt' solution is contained in a reservoir tank 14 from which it is led through a main conduit 15 and hole nozzle 18 discharging into an open topmeasuring'cup 19. Two of these cups are illustrated, one for supplying the nozzle 12 and the other for supplying the nozzle 13 but it is to be understood that any number of these mixing chambers or cups may be used. Similarly the solution of reducing reagent is contained in a reservoir 20 and leads by means of a conduit] 21 and branches 22 to the mixing cups 19. Each of the branches is controlled by a valve 23 and each terminates in a discharge nozzle 24 leading into one of the cups 19. Preferably all'of the valves 17 and 23 are operatively connected so as to be opened or closed in unison and in this way the flow through the several valves is controlled by means of a single actuating handle 25. The nozzles 18 are provided with control valves 26 having two open positions in order to provide at will rates of flow through these valves. It

will be understood that the valves 17 and 26 are so designed that when both of the valves are wide open a predetermined rate of flow of the silver solution is attained to ive the predominance of silver -hereinbe ore suggested and that when the valve 17 is open and the, valve 26 shifted to its least opened position the proportion of silver discharged will be reduced to an amount corresponding to the chemical requirements of a complete reducing reaction hereinbefore suggested. If desired or necessary the nozzles 24 are provided with metering valves 27 for providing two different rates of discharge for the reducing solution and here again it is suggested that the valves 26 and 27 be controlled from a single handle 28.

- For the purpose of controlling the temperature of the substances as they are passed into the mixing chambers, the several branch conduits 16 and 22 are formed into coils 29 which extend through a water bath 30 carried in a tank 31. Under some atmospheric conditions it will be necessary to heat the water in the bath and for this purpose steam coils 36 lead into the tank at one end thereof. Under other conditions it will be necessary to cool the water in order to attain the desired temperature conditions and for this purpose the water is caused to pass from an ice tank 32 through the tank by means of a pump 33' The supply from the silver containing reservoir 14 may be cut oil from the piping In operation and assuming that the mold 10 positioned on the shaft is rotating slowlv so as not to generate excessive centrifugal forces and that the nozzle outlets 12 and 13 are disposed in proper position to distribute the' silver over the rotating mold: the handles 25 and 28 are manipulated so as to widely open all of thevalves and thus 7 cause thin streams of the silver solution and the reducing reagent to collect in the bottom of each of the cups 19 in the form of a shallow pool 37. The outlets 12 and 13 from the cups are so proportioned that the reagents are discharged from the cups each in a fine stream which may be continuous but preferably is regarded as a stream of drops and these drops fall onto the revolving mold. Even though the upper nozzle 12 discharges on the mold below the apex the film spreads over the entire top of the mold prism. The initial operation is continued for a brief period of time, usually two or three seconds, and the valves are then shifted by means of the handle 28 so as to reduce the proportion of silver to reagent present and the dripping operation is continued with the proportions so changed and until the desired thickness of coating is attained.

Following the usual practice the glass mold with its coating of silver is positioned in an electrolytic tank and a backing coating of copper or other material is positioned thereon, after which the reflector or other article so formed is removed from the mold and otherwise treated by processes forming no part of the present disclosure. I

Having thus described my invention, I claim:

1. In the art of coating articles with silver, the method which consists in directing a thin stream of a silver solution and a thin stream of a reducing reagent therefor into a mixing chamber open to the air, permitting the solution and its reagent to remain as a small body of liquid in the bottom of said chamber for a period of time insufiicient to permit a complete reduction of all of the silver present, permitting the contents of the chamber to pass therefrom in a thin continuous stream of small separate drops and to cause the same to drop through space on to the articlebeing coated, the sequence of steps being timed so that the reducing reaction is completed approximately at the instant the drop strikes the object whereby the articles are coated with nascently formed metallic silver substantially free of any unreduced silver solution.

2. In the art of coating articles with silver, the method which consists in dire-ting a thin stream of a silver solution and a thin stream of a reducing reagent therefor into a mixing chamber open to the air, permitting the solution and its reagent to remain as a small body of liquid in the bottom of said chamber for a period of time insuflicient to permit a complete reduction of all of the silver present, permitting the contents of the chamber to pass therefrom in a' thin continuous stream of small separate drops and to cause the same to drop through space on to the article being coated, the sequence of steps being timed so that the reducing rcaction is completed approximately at the in- I stant the drop strikes the object whereby the article is coated with nascently formed metallic silver substantially free of any unreduced silver solution, and simultaneously rotating the article to insure a rapid spreading of'each drop as it strikes over a material area of surface being coated.

3. In the art of coating articles with silver, the method which consists in continuously mixing a limited amount of a'silver solution and a reducing reagent therefore, and dripping the mixture drop by drop over the surface of the article being coated, said solution and reagent being in chemically reacting contact for a period just sufiicient to insure a complete reduction of the silver present and the resulting metallic silverbeing immediately deposited on the article substantially as soon as formed.

4. In\the art of coating articles. with silver, the method which consists in mixinga thin stream of a silver solution with a thin stream of a reducing reagent therefor, said solution and reagent being mixed in the proportion determined by chemical calculations as necessary to effect a complete reduction of all of the silver present, and causing the mixture to drip, drop by drop, through air on to the article being coated.

5. In the art of'coating with silver an object having an inclined side, the method which consists in directing a plurality of" streams of a mixture of silver solution and a reducing reagent therefore on to said inclmed side at verticallyv spaced points,'r'egulating the-streams so that a continuous film will be formed on the side and each stream will flow down the side and cover its apportioned area by the time the mixture has completed its reducing reaction and lost its fluidity.

6. In the art of coating with silver an object having an inclined side, the method which consists in directing aplurality of streams of a mixture of silver solution and a reducing reagenttherefore on to said in clined side at relatively spaced points, regulating the time duration during which the solution and reagent of each stream are in chemically reacting contact so that the re duction will be complete by the time each stream flows down the side and covers its apportioned area, said streams coating to spread a uniform and continuous film over the entire surface being coated.

7. In the art of coating an article with silver, the'method which consists in rotating the article, directing a plurality of thin streams, each comprising a mixture of a silver solution and a reducing reagent therefor, onto different portions of the rotating article to insure a quick spreading of the resulting metallic silver over the article being coated before the deposited silver has an opportunity to thicken at any place, the reducing activity of the mixture and the speed of rotation being regulated to insure the formation of an extremely thin but continuous film of metallic silver. 1

8. In" the art of coating with silver a dome-shaped article, the method of direct ing on different points of the side of the article a plurality of streams of a mixture of silver solution and a reducing reagent therefore and permitting the mixture to flow down the side. of the article while rotating the article past said points, the mixture of each stream being regulated in the time duration in which the solution and reagent thereof are in chemically reacting contact to insure a fluid "state while the stream is coveringits apportioned area on the side and a precipitation of substantially all-of the silver present when said area is covered and said streams coacting collectively to completely cover the side with a film of metallic silver.

9. In the art of coating a dome shaped article with silventhe method which consists in dripping a mixture of a silver solution and a reducing reagent therefor on the side of said article while the article is moving past the point of application of the drip whereby each drop of the dripping mixture is caused to's'pread over the side while-flowlng downthe side, said mixture being sup plied in such small amounts that a thin film is formed over the side prior'to theter-mination of the reduc ng reaction;-

10. In the art of coating articlswith'silver, the method whifth consists in causing small drops onto the article being coated while rotating the article to cause the drops to spread out over the surface of the article during the final period of the reducing reaction.

12. In the art of reducing silver, the process which consists in causing'a stream of silver solution to react chemically with a stream of a reducing reagent therefor in liquid form during the initial stages of the reducing reaction and'causing the mixture to assume the form of separate drops falling through space during the final stages of the reducing reaction.

13. In the art of coating objects with silver, the process which includes the step of causing drops of a mixture of a silver solution and a reducing reagent therefor to fall through air onto the object being coated while completing the reducing reaction.

14. In the art of coating objects with silver, the method which consists'in dripping on the article a mixture of a solution of silver and a reducing reagent therefor, directing the drops of the mixture during the reducing reaction to spread rapidly over the surface, initially with a film having a relatively high metallic silver content'and following the deposition of the initial film with superposed films in which the metallic silver content is less than that comprising the initial film.

15. In the art of coating articles with silver, the method which consists in dripping on to the surface to be coated a mixture of a silver solution and a. reducing reagent therefor for about two seconds and.

until an initial thin metallic silver layer is formed, changing the proportion of silver present. in the mixture and continuing said dripping with said changed mixture to form superimposed layers of metallic silver on the initial layer. 7

16. In the art of coating articles with silver, the method which consists in initially directing onto the surface being coated, a mixture of a. sllver solutlon and a reducing reagent therefor in which the silver content is relatively high to insure a high brilliant metallic silver deposit limiting the ini-, tial depositing to a few seconds. reducing the amount of silver present in the. mixture and depositing the resulting metallic silver on the initial layer to form until the desired thickness of silver is attained.

17. In the art of coating articles with silver, the method which consists in mixing a thin stream of a silver solution with a thin stream of a reducing reagent therefor, said solution and reagent being mixed in the proportions determined by chemical calculations as necessary to effect a complete reduction of all of the silver present and causing the mixture to drip. drop by drop, through air on to the article being coated while maintaining the reaction at a temperature of 50 to 65 F.

18. In the art of coating an article with silver, the method which consists in rotating the article, directing a plurality of thin streams, each comprising a mixture of a. silver solution and a reducing reagent therefor. on to different portions of the rotating article to insure a quick spreading of the resulting metallic silver over the article being coated before the deposited silver has an opportunity to thicken at any place the reducing activity of the mixture and the speed of rotation being regulated to insure the formation of an extremely thin but continuous film of metallic silver while main- 'taining the mixture at a temperature of 50 to F. during the entire period of the reducing reaction.

19. In the artof coating articles with silver, the method which consists in directing on to the article a continuous stream of a mixture of a silver solution and a reducing reagent therefore, continuing the application by means of said stream until the article is completely covered and then directing on to the resulting silver covering a stream of separate drops of a mixture of a silver solution and a reducing reagent therefore.

20. In the art of coating articles with silver, the method which includes the steps of first directing a continuous stream of a silver mixture on to the article and then directing a seriesof drops of said mixture on to the coating formed by the depositing from the stream.

Signed at New York city, in the county of New York and State of New York, this 31st day of October, A. D. 1923.

BLASIUS BART.

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