2 BULLETIK^ 449, U. S. DEPAETMEIs'T OF AGEICULTURE. 



of these salts. Iron rust, then, consists principally of ferric oxid 

 in admixture with varying amounts of basic ferric oxid. In the 

 case of metals like copper and zinc, and alloys like brass and bronze, 

 basic salts are also formed. For example, copper reacts with water 

 vapor and carbon dioxid in the air to form a basic carbonate, while 

 in the presence of weak organic acids it forms salts like basic copper 

 acetate (verdigris). 



Unlike most other metals, silver and gold are not tarnished by the 

 oxygen, water vapor, or carbon dioxid present in the air, or by the 

 action of weak organic acids. Silver, however, readily forms black 

 silver sulphid on coming in contact with sulphur compounds, small 

 quantities of which are found in the air as the result of burning coal 

 and illuminating gas, while larger amounts occur in vulcanized 

 rubber, wool, and foods like eggs. The problem, of cleaning silver 

 involves the removal of the tarnish of silver sulphid by some method 

 which will also restore the j^olish to the surface of the metal. 



The two general methods for cleaning silver are polishing with a 

 finely divided abrasive material to cut away the tarnish mechanically 

 and the use of suitable chemical compounds to dissolve the coating of 

 silver sulphid. The first method is the more common one, and com- 

 mercial silver polishes usually contain a suitable abrasive, such as 

 tripoli, rouge, double-floated silica, kieselguhr, whiting, or pumice. 

 which are prepared in the form of a powder, a cake, or a suspension 

 in some liquid. As silver is a comparatively soft metal, and since 

 the process of cleaning depends essentially on the cutting away of the 

 tarnish by the sharp particles of the polishing powder, care must be 

 taken to choose an abrasive so finely powdered that it will not scratch 

 the silver. Solvent polishes are often used by jewelers and in hotels 

 and restaurants where large quantities of silver must be cleaned. 

 As a rule these consist principally of potassium cyanid and some- 

 times contain ammonia, both of which dissolve the sulphid coating 

 and give the silver a beautiful satin finish. As cyanids are extremely 

 poisonous and very dangerous when carelessh^ handled, they should 

 not be commonly used for cleaning purposes. 



A few years ago the so-called electrolytic method for cleaning silver 

 was introduced to the public and several forms of cleaners, based 

 on the electrohi:ic principle, are now to be found on the market. 

 In this method the silver is cleaned by bringing it into actual con- 

 tact with aluminum in a solution of an electrolyte. As this form 

 of cleaning is becoming quite extensively used and questions are 

 frequently asked about its efficiency and its effect on the silver, in- 

 formation is desirable regarding the prineiple and details of the 

 process. The main object of this investigation, therefore, was to de- 

 termine the value of the method and the factors necessary for its 



