2 BULLETIN 449, U. S. DEPARTMENT OF AGRICULTURE. 
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 polish 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 carelessly 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 electrolytic 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 principle 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 
