Page:The museum, (Jackson, Marget Talbot, 1917).djvu/172

146 M. A. Richter. The other forms are the so-called "cracking" patina, the warty patina, the scaly patina and the pale blue patina. The most complete and exhaustive study of this subject will be found in the little book called "The Preservation of Antiquities," by Dr. Friedrich Rathgen, mentioned above.

.—The tin disease is another very serious malady which is so far little understood. It comes about through exposing objects containing tin to too great cold and consists in the reduction of the metal tin into the salt tin. The salt cannot, however, be changed back into a metal without bringing the substance to the melting point. It is obvious, therefore, that this would be an impossible method to pursue with museum pieces which are preserved for their shape or modelling. Should the little white particles which denote the presence of the disease appear on any museum piece, care must be taken that the temperature of the room in which the piece is kept should never go below 60° Fahrenheit.

.—Museum silver can best be cleaned by the use of ammonia, a soft brush, very hot water and a soft chamois skin. A brilliant polish is not desirable. The usual cleaning compounds all scratch the surface more or less.

.—Certain general principles