THE FERRO-PRUSSIATE, OR BLUE PROCESS.
The preparation of the paper by this process is very simple, and requires very few manipulations, while the results, when prints are made, are pleasing and lasting. Prints in blue are so easily made as to be p208 extremely convenient for making proofs from negatives, and they are well adapted for mottoes, plans, drawings, manuscript, circulars, and for representations of scenery, boats, machinery, etc., etc., and for engravers' use.
Ferro-prussiate paper can be procured from E. & H. T. Anthony & Co., or can be easily made by the following formula:
Float the paper until it lies quite flat upon a solution prepared as follows:
| 1. Water | 2 ounces fluid. |
| Red prussiate of potash | 120 grains. |
| 2. Water. | 2 ounces. |
| Ammonia citrate of iron | 140 grains. |
When these two are dissolved, mix them together and filter into a clean bottle.
The solution should not be exposed to a strong light, and the paper must be floated on it in a very subdued light, and in the same manner as paper is floated on a silver solution. When it no longer curls, but lies flat on the solution, take it by the corners and raise it slowly from contact, and hang it up to dry in a dark place. When dry, it can be used at once, or may be kept for future use by rolling it, prepared surface in, and placing it in a tin box or other receptacle, free from light and dampness.
To make a print on this paper, place the prepared surface in contact with the negative in a printing frame and expose to sunlight.
The time of exposure will vary according to the density of the negative and the intensity of the light. The rule is to allow the light to act long enough for the portions which first turn blue to become gray, with p209 a slight metallic luster. At this point remove the paper from the frame and place it in a dish of clean water.
It now gradually becomes a rich blue throughout, except the parts which should remain white. Change the water from time to time, until there remains no discoloration in the whites; dry, and the picture requires no further treatment.