It is absolutely necessary to store all papers for platinum printing in an air-tight tin containing chloride of calcium, which must be dried by heating from time to time.  For the cold bath, however, it is important to have moisture present during printing, or it may be after printing and before development.  If the paper is left in a dampish room for fifteen minutes, it should be sufficient.  Prints made by exposing damp paper, or damping dry paper just before development, must be developed within one hour if the maximum of vigor is desired; by delaying the development some hours, the prints in the meantime being stored in a drawer so that they may retain their moisture, an increase of half tone and warmth of color will be obtained.  If it should be necessary to delay development for a day or two, the prints must be dried before a fire soon after being removed from the frames, and then stored in a calcium tube until wanted for development.

While printing, the lemon color of the paper receives a grayish colored image, which, although faint, can, with practice, be judged as easily as silver printing.

The developer consists of oxalate of potash and potassic chloro-platinite—­about thirty grains of the platinum salt to half an ounce of oxalate forming about six ounces of solution; a great many variations, however, may be made in the proportions of platinum salt and oxalate, and different effects secured.  Development is effected by sliding the print face downward on to the developer, which must be rocked after the development of each print to avoid scum marks.  To clear the prints they are washed in three or four baths of a weak solution of hydrochloric acid after leaving the developer, to remove all traces of the iron salts, and finally washed for a quarter of an hour in three changes of water; they are then finished, and may be dried between clean blotting paper.

Pizzighelli’s process differs from the above in being one that prints fully out in the frame without development; the paper contains the platinum and iron salts as well as the developer, and so prints and develops at the same time.  Although excellent prints can be produced with it, for general work the results of the paper, as at present made, will not compare with the hot and cold bath processes.  It is, however, excellent for printing from very dense negatives, and occasional negatives that seem extremely suitable for it.  The paper should be breathed on before printing, as if it is quite dry the printing will be very slow and irregular.  The best conditions for the preparation of the paper have scarcely been decided upon yet, and it is not quite fair to judge the process.  The prints are cleared in the acid baths and washed for about a quarter of an hour.

The sepia and black hot bath processes are much alike in the general treatment.  There are, however, some special precautions to be observed with the sepia paper, the chief being to protect it from any but the faintest rays of light; the prints, unlike the black ones, may be affected by light when in the acid bath.  A special solution must be added to the developer to keep the lights pure.  Over-exposure cannot be corrected by using a cooler bath, as is the case with the black prints, and the paper does not remain good so long.