This section is from the American Hand Book of the Daguerreotype, by Samuel D. Humphrey. Published S. D. Humphrey, 37 Lispenard Street 1858.
There is probably no one cause of complaint so general as "what makes those black specks?" There are several causes which produce them, and probably the most general are dust, rouge, or a spray of moisture on the plate. It this be the case, there is no solution which can remove them, as they have prevented a chemical action with the silver, and their removal would only expose the surface of the plate which in itself would afford a contrast with the impression. Another and less dangerous source of these specks is organic matter contained in the solution employed in dissolving the chemicals, or the water in washing. much of the hyposulphite of soda in market contains a sulphuret, which, coming in contact with the silver surface, immediately causes oxidation. Such spots, as well also as most all others found on the plate after it has been exposed in the camera, can be removed by the following, solution: To one ounce of water add a piece of cyanide of potassium the size of a pea; filter the solution and apply by pouring it on the surface of the plate. In all cases the plate should first be wet with water. Apply a gentle heat, and soon the spots disappear, leaving the impression clear and free from all organic matter.
In the absence of cyanide of potassium, a solution of pure hyposulphite of soda will answer as a fair substitute.