Experiment C.—Blue Print Paper
Engineers’ drawings have for many years past been copied upon ferro-prussiate, or “blue print” paper. The original design being made in opaque ink upon tracing linen, a sheet of the sensitive paper is held against this in strong daylight until blue coloration has advanced everywhere except beneath the ink lines of the drawing. These remain yellow, or rather white, when finished, as the excess of sensitive salt is removed by washing.
Since this last operation is in itself all-sufficient to insure permanency, the simplicity is unique. In fact, the impossibilty of securing other colors than blue has been the only factor to exclude this process from far wider use.
The preparation of the sensitive surface presents no great difficulties, provided a drawing-paper of good quality be used. It should be cut into strips about 6 inches wide, which are passed one by one up and down (see [Fig. 7]) through a dish containing the following solution: 1 oz. ferri-cyanide of potash + 4 oz. water, added to 1 oz. ammonio-citrate of iron + 4 oz. water. (Note—4 oz. water = nearly 1⁄4 pint.) This must be done in very dull light—candle or paraffin oil by preference—and the wet paper pinned up to dry in a dark cupboard, hanging from the edge of a shelf or other projecting support. Then it may be cut into pieces of the required size and stored with a wrapping of tissue and brown paper in a handy box.
All manner of designs may be produced on this paper, such for example as fern leaves, lace, and embroidery. Actual sea-view photos or imitation moonlight views also look very well indeed. Another notion is to secure the copy of some picture printed in black on thin paper, which has been oiled and dried in order to render it translucent for quicker printing. The final washings should be thorough, and then the blue print will last its maker as long as the latter cares to keep it.