THE FERRO-PRUSSIATE PROCESS.
The chief use of the process is for copying plans and drawings such as are used by engineers and architects, the lines coming out white on a background of blue. Very pleasing effects can, however, be got from negatives of certain subjects, notably of sea-pieces.
The paper is manufactured and sold by Marion and Co., and is ready sensitised. It is placed in a frame like albumenised paper, and the process of printing is watched in precisely the same way. Several changes of colour take place. The print becomes first yellow, then greenish, afterwards greenish-blue, deep bluish-gray, and lastly, an olive tinge with a metallic tint is acquired. The deepest shadows should reach this colour. The time taken is about four or five times as long as for albumenised paper. When the printing has gone far enough the piece of paper is placed in a dish and clean water is poured on to it. The water becomes yellowish, and the print almost instantly becomes of a bright blue colour. The washing is continued as long as the water comes off of a yellow colour, which is only a few minutes. It should not be allowed to go farther, otherwise the depth of the blue may be somewhat reduced. The print is next pressed between sheets of blotting-paper to remove surface moisture, and is hung up to dry when it is finished.
When drawings are to be copied, they are best traced first on transparent tracing-cloth or paper. It is, how ever, possible to take prints even from drawings on thick drawing-paper.
The following is the course pursued in a large drawing office attached to a mechanical engineering work : The drawings are pencilled in as usual on drawing-paper, but, instead of inking them in, a tracing on very transparent tracing-cloth is made. The thing most necessary to observe is that the ink be quite opaque. The best way to secure very white lines in the prints is to mix a little vermilion colour with the Indian ink used for the tracing.
This will not notably alter the appearance of the tracing, whilst it will stop all chemical rays. The sectioning, instead of being clone in colour, is clone in lines. The tracing is now kept in the office as a finished drawing, the pencil drawing being destroyed. When copies are wanted for the workshop or to send out they are taken on the ferro-prussiate paper. As it is difficult to turn over the very large frames which are generally used for drawings, it is best to leave a narrow margin of ferro prussiate paper beyond the tracing-cloth, so that the change of colour may be watched. Another plan is to put a very small drawing in an ordinary quarter or half plate printing frame beside the large one, and look at the progress of it from time to time.
Corrections or additions may be made on the blue paper by writing or drawing with a solution of a few grains of caustic potash to each ounce of water. The cor rection is made, and as soon afterwards as possible the part is sponged with clean water to remove superfluous potash, which would otherwise cause the line to spread.
If it be desired to have blue lines on a white ground, a double process has to be gone through. A special paper is made for taking prints with white lines on a blue ground, which, being transparent, may be printed from. In this case the exposure must be continued for several times as long as if white lines on a blue ground only are required. This print, taken on the transparent paper, is now used precisely as the tracing was, and there results a print with blue lines on a white ground.
It is almost needless to remark that the paper must be kept from any bright light except during exposure, just as sensitive albumenised paper is.