US2684328A - Treatment of silver to inhibit tarnishing - Google Patents

Description

Patented July 20, 1954 TREATMENT OF SILVER T'O INHIBIT TARNISHING Carroll F. Matthews, Constantia, and John C. Sawyer, Oneida, N. Y., assignors to Oneida Ltd., Oneida, N. Y., a corporation of New York No Drawing. Application October 27, 1949, Serial No. 123,982

12 Claims. 1

This invention relates to the inhibiting of tarnish on articles having a silver surface, and more particularly to the inhibiting of tarnish on such articles by means of certain fluoborate solutions.

in the past considerable effort has been made to prevent or to retard the development of silver tarnish and various means have been attempted to obtain this desired result, such as by coating the silver surfaces with various materials. No tarnish prevention coating has yet been develwhich is fully satisfactory. Coating materials, such as lacquers, have the disadvantage of being relatively difficult to apply, as well as expensive, while other methods are not sufficiently permanent or have other disadvantages.

It an object of this invention to inhibit the formation of tarnish in a relatively simple, yet effective manner.

It is another object of this invention to inhibit the formation of silver tarnish for long periods of time at relatively small cost.

These and other objects will become apparent the following disclosure.

riefly, these objects may be attained by contacting the silver article to be treated in a solution of a fiuoborate of a metal more electropositive than silver. An example of such a metal is tin.

in the treatment of silver articles, silver plated articles and articles whose surface is composed of an alloy which is predominately silver, the article is contacted briefly with an aqueous metallic :fiuoborate solution. Concentrations as low as 1% show marked effect while increasing concentrations up to about 9 per cent give increasing protection from tarnish. Concentrations in excess of 9 per cent, while satisfactory, appear to be little if any more elfective than those of about 9 per cent.

The time of immersion can be as short as one second, although a longer time is preferred and imparts better protection. Various experiments have shown that very satisfactory results are obtained by a 10 second dip in a 9 per cent solution, although the time of immersion appears to of little consequence so long as the surface chemically wet.

Instead of dipping the article having a silver surface in the fiuoborate solution, the fiuoborate may be used as an electrolyte and the article to be treated as a cathode. In such a case a relatively small current, such as about 2 amperes per square foot, applied for a brief period is sufiicient to cause the article to be very satisfactorily resistant to tarnish. The current and time should be adjusted so that insufficient material is plated out on the cathode to be visible.

Extensive work has shown that stannous flue-- borate is outstanding in its ability to retard tarnish and, in fact, considerably superior to other fiuoborates, although the following fluoborates have also been found to protect silver surfaces from tarnish. They are:

We do not know Why a brief exposure of the silver surface to an aqueous solution of a fluoborate of a metal more electropositive than silver decreases the activity of the silver surface. It is possible that a molecular layer of a silver compound is formed which ties up any free valences present in surface silver atoms. Any film that forms, however, is invisible and does not affect the lustre of the silver, nor are the films detectible by ordinary chemical means.

A further unexplained fact is that stannous fluoborate is nearly doubly as effective as any of the other fiuoborates as measured by an arbitrary scale.

The field of commercial silverware production is largely divided between so-called sterling silver and plated silverware. Sterling silver is an alloy consisting of 92.5 parts of silver and 7.5 parts of alloying metal. Other silver alloys containing varying but small amounts of copper, cadmium and other metals are manufactured on a lesser scale. We have found that we can protect silver alloy articles, of which a predominant part is silver, against tarnish by our process. ihese alloys include particularly silver alloy articles containing 90 per cent or more of silver such as sterling silver. Silver plated articles i. e. articles whose surface is entirely silver, are also substantially protected by our process.

The following is a series of tests given as examples to show the effect of stannous fluoborate on electroplated and on sterling silver articles. In each case the samples were first cleaned to re move grease, buffing compounds and other surface contamination in order to permit the fiucborate solution to contact the surface.

Example I A silver plated spoon was immersed for 5 secends in a 23% solution of stannous fluoborate, removed, rinsed in clean Water and dried. To test the resistance of the treated spoon to tarnish it was placed in a desiccator of 9 liters capacity together with ml. of saturated aqueous hydrogen sulfide solution slightly acidified with hydrochloric acid for two hours; at the end of that time no tarnish was apparent. A similar but untreated sample as a control was exposed to hydrogen sulfide at the same time and under the same conditions and became covered with a purple tarnish tinged with blue.

Exam le II Experiment I was repeated using sterling silver test pieces. The same procedure, solutions, etc. were followed except that the tarnishing test time was twelve minutes. Results:

Control: Golden and purple tarnish, Treated sample: Very slight tarnish on handle embossing only.

Example III Various concentrations of stannous fluoborate were prepared as indicated below and were used to show the effect of concentration on silver plated and sterling silver spoons. The general procedure described for Example I was followed in each case, using a time of dip of 5 seconds and a tarnishing exposure time of 44 minutes.

The results of this test are recorded below:

Control plated spoon-edeep blue and purple tarnish. Control sterling spoondeep blue tarnish. Treated plated spoon:

1% SnCBFQ light brown tarnish. 4.6% Sn(BF4)2no tarnish. 9.2% Sn(BF4)2no tarnish. 23.0% Sn(BF4)z.n0 tarnish. Treated sterling spoon:

1% Sn(BF4)2--blue grey tarnish. 4.6% SnCBFQ 2.-light blue tarnish. 9.2% Sn(BF4)2light blue tarnish. 23.0% Sn(BF4)z-light grey tarnish.

A comparison of the control spoons with the treated spoons shows that a startling improvement is realized by the fiuoborate dip even when the concentration is as low as l In addition to the forced 0r accelerated tests described above, a time test was also made.

Finishedsterling and plated teaspoons were employed for the test, and prepared in accordance with Example III, i. e. each of four sterling silver spoons and each of four silver plated spoons were dipped in 1%, 4.6%, 9.2% and 23.0% stannous iluoborate solutions respectively and one each of sterling and plated spoons were selected as controls. The eight treated and two untreated spoons were suspended by threads for two and one-half months in normal atmosphere. At the end of that time, the untreated spoons were badly tarnished having a deep brown tarnish.

The treated plated spoons all showed some brown tarnish, but were distinctly less tarnished than the untreated spoons. The treated sterling spoons required close examination to detect any tarnish or change in color.

This time test clearly shows that the fluoborate dip is of real value as a tarnish inhibiting agent.

Example IV Both electroplated and sterling silver spoons were immersed in a 4.6% stannous fiuoborate solution for five seconds. During that time the spoons wer made the cathode of an electrolytic cell and subjected to, an applied current density of 1.8 amperes per square foot. After treatment the spoons were rinsed, dried, and immersed in a saturated and acidified hydrogen sulfide solution for 10 minutes. Results:

The electroplated silver spoon used as a control was covered with a deep blue tarnish while the treated sample, while generally free from tarnish, had some slight spots of brown tarnish.

The sterling control was covered with a dark blue gray tarnish as contrasted with the treated sample which had a light gray tarnish.

While gradations are difiicult to describe in words, the electrolytically treated samples appeared to be better than corresponding dipped samples.

The treatment of articles having silver surfaces with fiuoborates as described above finds an important use in the prevention of tarnish on articles which are infrequently used as in jewellers displays; here such articles may be shown to their best advantage for long periods of time. It may also be applied as a treatment to finished silver articles prior to packaging to prevent tarnish during storage, and as a treatment to prevent tarnish to silver articles between manufacturing operations, such as occurs many times during the interval between the finishing operations and final inspection and between final inspection and packaging. It may also be used as a household treatment to the good silverware that is used only occasionally, before putting it away after using so that cleaning or polishing will not be required before subsequent use.

We claim:

1. A process for inhibiting the formation of silver tarnish which comprises contacting a silver tarnish-susceptible article with an aqueous solution consisting essentially or fiuoborate of a metal more electropositive than silver for a time sufiicient to render said article ta'nish resistant and insufficient to deposit a v detectable layer of said metal on said article, said article having a surface consisting at least in part of a metal of the group consisting of silver and alloys of silver wherein silver is the predominant metal, said solution having a concentration of at least one per cent of said fiuoborate.

2. A process for inhibiting the formation of silver tarnish which comprises dipping a silver tarnish-susceptible article into an aqueous solution consisting essentially of a iiuoborate or a metal more electropositive than silver for a time sufiicient to render said article tarnish resistant and insufficient to deposit a visibly detectable layer of said metal on said article, said article having a surface consisting at least in part of a metal of the group consisting of silver and alloys of silver wherein silver is the predominant metal, said solution having a concentration of at least one per cent of said iiuoborate.

3. A process for inhibiting the formation of silver tarnish which comprises, contacting a sterling silver article with an aqueou solution consisting essentially of a fluoborate or" a metal more electropositive than silver for a time sufiicient to render said articl tarnish resistant and insuilicient to deposit a visibly detectable layer of said metal on said article, said solution having a concentration of at least one per cent of said fluoborate.

4. A process for inhibitin the formation of silver tarnish which comprises contacting a sterling silver article with an aqueous solution consisting essentially of stannous flucborate for a time sufficient to render said article tarnish resistant and insufiicient to deposit a visibly detectable layer of said metal on said article, said solution having a concentration of at least one per cent of said fluoborate.

5. The process in accordance with claim 4 wherein the solution has a concentration of at least nine per cent.

6. A process for inhibiting the formation of silver tarnish which comprises contacting an article having a, silver surface with an aqueous solution consisting essentially of a fluoborate of a metal more electropositive than silver for a time sufficient to render said article tarnish resistant and insufficient to deposit a. visibly detectable layer of said metal on said article, said solution having a concentration of at least one per cent of said fluoborate.

7. A process for inhibiting the formation of silver tarnish which comprises contacting an article having a silver surface with an aqueous solution consisting essentially of stannous fluoborate for a time suificient to render said article tarnish resistant and insufficient to deposit a visibly detectable layer of said metal on said article, said solution having a concentration of at least one per cent.

8. The process in accordance with claim 7 wherein the solution has a concentration of at least 9 per cent.

9. A process for inhibiting tarnish which comprises the steps of preparin an electrolytic cell, the electrolyte for said cell being an aqueous solution consisting essentially of a fluoborate, said solution having a concentration of at least 1% of a fluoborate of a metal more electropositive than silver, and having as a cathode an article having a surface consisting at least in part of a metal of the group consisting of silver and alloys of silver wherein silver is the predominant metal, causing a current to flow in said cell but for an insuiicient time to form a visible deposit.

10. The process in accordance with claim 9 wherein the fluoborate of a metal more electropositive than silver is stannous fluoborate.

11. The process in accordance with claim 10 wherein the cathode is a sterling silver article.

12. The process in accordance with claim 10 wherein the cathode is a silver plated article.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,827,2'7 Mason Oct. 13,1931 2,117,657 Ganlofi et al. May 17, 1938 OTHER REFERENCES Modern Electroplating, special volume, published in 1942 by The Electrochemical Society; pages 311, 312 and 316.

Claims (1)

1. A PROCESS FOR INHIBITING THE FORMATION OF SILVER TARNISH WHICH COMPRISES CONTACTING A SILVER TARNISH-SUSCEPTIBLE ARTICLE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A FLUOBORATE OF A METAL MORE ELECTROPOSITIVE THAN SILVER FOR A TIME SUFFICIENT TO RENDER SAID ARTICLE TARNISH RESISTANT AND INSUFFICIENT TO DEPOSIT A VISIBLY DETECTABLE LAYER OF SAID METAL ON SAID ARTICLE, SAID ARTICLE HAVING A SURFACE CONSISTING AT LEAST IN PART OF A METAL OF THE GROUP CONSISTING OF SILVER AND ALLOYS OF SILVER WHEREIN SILVER IS THE PREDOMINANT METAL, SAID SOLUTION HAVING A CONCENTRATION OF AT LEAST ONE PER CENT OF SAID FLUOBORATE.