Modern Mechanical Engineering Vol-I

THE PHOTOGRAPHIC ROOM

43

the ideal system is to have a continually running supply, so that the bath is
kept constantly fresh. The photograph is immersed, and a small hose
made to play on it to drive off every particle of surface chemical. Separate
baths should be kept for black-and-white prints and the ordinary blue prints,
if satisfactory results are to be obtained. When thoroughly washed, the
print should be drawn through a squeegee, permanently attached to the
side of the bath, to take off as much of the surplus water as is possible. If
the groundwork of black-and-white prints comes up slightly muddied, it
may be chemically treated to bring it up white, but great care must be exer-
cised in this treatment lest the black lines of the drawing should get obliterated.
It may be of some interest to the operator, or draughtsmen with some
knowledge of chemistry or photography, to briefly indicate the chemical
reactions with either the ferro-prussiate blue paper with white lines or the
ferro-gallic white paper with black lines.
In the former the paper is coated with potassium ferricyanide and
ferric compound of iron. When exposed to the influence of actinic light,
either from the sun or the electric arc, part of the iron in the sensitive
compounds is changed from the ferric to the ferrous condition, which with
potassium ferricyanide gives an insoluble blue compound which is pre-
cipitated on the paper. Side by side with this reaction a portion of the
potassium ferricyanide is reduced to potassium ferrocyanide, which, with
the unchanged ferric iron, also deposits a blue compound on the paper.
The net result is that a complex mixture of blue compounds is laid down
on that portion of the paper, i.e. the background, which has been sub-
mitted to the ultra-violet rays. The portions unaffected because of the
protection afforded by the ink on the tracing are washed away in the bath,
leaving the white lines on a blue ground.
Similarly with ferro-gallic photo paper, i.e. paper which gives black
lines on a white ground. In this case the paper is coated originally with
a solution of iron salts, the ferric compound being reduced by the action
of light to the ferrous state. The paper is now treated with a solution of
gallic acid, which changes the ferric iron on the parts shielded from light
into a bluish black compound. The exposed portion, where the trans-
formation from the ferric to the ferrous state has taken place, is unaffected.
In water bath, or one bath ferro-gallic paper, the coated material
carries its own developer in the form of a powder on the surface. With
this paper, on immersion in water, after printing, the ferrous salt, with
the gallic acid in contact with it, is washed away, leaving fixed on the
paper the black compound of ferric iron and gallic acid.
Drying of Prints.—The prints are caught by spring clips at the edges,
and hung up to dry. It is better to let the prints dry naturally, as artificial
drying is apt to distort them badly, especially where they dry last.
Register of Prints.—A photo register is kept by the photographer,
showing when each print was sent out, and what was its destination.