Apart from the mechanical operation of tracing, there are several methods by which facsimile copies of drawings can be produced with a very slight expenditure of labour and at small cost. These will now be described.

(1) Cyano-type, or ferro-prussiate paper. This is prepared by covering one side of the sheet with a mixture of red prussiate of potash (potassium ferrocyanide) and iron peroxide; under the influence of light, i. e. under the white portions of the drawing to be copied, the ferric compound is reduced to the state of a ferrous salt, which gives with the red prussiate of potash an intense blue coloration, analogous to Prussian blue. This coloration is not produced in the portions of the sensitive paper protected from the light by the black lines of the drawing to be copied, and on washing the print the design appears in white lines on a blue ground. The formula for preparing the sensitive paper is as follows: - Dissolve 10 dr. red prussiate of potash (ferrocyanide) in 4 oz. water; dissolve separately 15 dr. ammonio-citrate of iron in 4 oz. water; filter the 2 solutions through ordinary filtering-paper, and mix. Filter again into a large flat dish, and float each sheet of paper to be sensitised for 2 minutes on the surface of the liquid, without allowing any of this to run over the back of the paper. Hang up the sheets in a dark place to dry, and keep from light and dampness until used. They will retain sensitiveness for a long time. The paper being ready, the copy is easily made.

Procure either a heavy sheet of plate glass, or a photographer's printing frame, and lay the drawing to be copied with the face against the glass; on the back of the drawing, lay the prepared side of the sensitive paper, place upon it a piece of thick felt, and replace the cover of the printing frame, or in some other way press the felt and papers firmly against the glass. Expose, glass side up, to sunshine or diffused daylight, for a time, varying, with the intensity of the light and the thickness of the paper bearing the original drawing, from minutes to hours. It is better to give too much than too little exposure, as the colour of a dark impression can be reduced by long washing, while a feeble print is irremediably spoiled. By leaving a bit of the sensitive paper projecting from under the glass, the progress of the coloration can be observed. When the exposure has continued long enough, the frame is opened and the sensitive sheet is withdrawn and thrown into a pan of water, to be replaced immediately by another, if several copies are desired, so that the exposure of the second may be in progress while the first is being washed and fixed.

The water dissolves out the excess of the reagents used in the preparation of the paper, and after several washings with fresh water the print loses its sensitiveness and becomes permanent. It is advantageous, after several washings with water, to pass over the wet surface a weak solution of chlorine or of hydrochloric acid, 3 or 4 parts acid to 100 of water, which gives brilliancy and solidity to the blue tint, and prevents it from being washed out by long soaking. This should be followed by 2 or 3 rinsings with fresh water, and the print may then be hung up to dry, or placed between sheets of blotting-paper. This mode of reproduction, whose simplicity has led to its adoption in many offices, has the inconvenience of giving a copy in white lines on blue ground, which fatigues the eye in some cases, while the application of other colours is impracticable. By repeating and reversing the process, copying the white line print first obtained on another sensitive sheet, a positive picture, representing the black lines of the original by blue lines on white ground, can be obtained; or the same result may be reached by a different mode of treating the sensitive paper.

This latter may also be made by brushing it over with a solution of ferric oxalate (10 gr. to the oz.); the ferric oxalate is prepared by saturating a hot aqueous solution of oxalic acid with ferric oxide. A better sensitising solution may be made by mixing 437 gr. ammonium oxalate, 386 gr. oxalic acid, and 6 oz. water, heating to boiling-point, and stirring in as much hydrated iron peroxide as it will dissolve.

(2) Several varieties of paper called "cyanoferric," or "gommoferric," are sold, which have the property of giving a positive image. The mode of preparation is nearly the same for all: 3 solutions, 1 of 60 oz. gum arabic in 300 of water; 1 of 40 oz. amrno-niacal citrate of iron in 80 of water; .1 of 25 oz. iron perchloride in 50 of water, are allowed to settle until clear, then decanted, mixed, and poured into a shallow dish, the sheets being floated on the surface as before, and hung up to dry. The solution soon becomes turbid, and must be used immediately; but the paper once dry is not subject to change, unless exposed to light or moisture. The reactions involved in the printing process are more complex than in the first process, but present no particular difficulty. Under the influence of light and of the organic acid (citric), the iron perchloride is reduced to protochloride, and, on being subjected to the action of potassium ferrocyanide, the portions not reduced by the action of the light, that is, the lines corresponding to the black lines of the original drawing, alone exhibit the blue coloration. The gum plays also an important part in the process by becoming less soluble in the parts exposed to light, so as to repel in those portions the ferrocyanide solution.

The mode of printing is exactly the same as before, but the paper is more sensitive, and the exposure varies from a few seconds in sunshine to 15 or 20 minutes in the shade. The exact period must be tested by exposing at the same time a slip of the sensitive paper under a piece of paper similar to that on which the original drawing is executed, and ruled with fine lines, so that bits can be torn off at intervals, and tested in the developing bath of potassium ferrocyanide. If the exposure is incomplete, the paper will become blue all over in the ferrocyanide bath; if it has been too prolonged, no blue whatever will make its appearance, but the paper will remain white; if it is just long enough, the lines alone will be developed in blue on a white ground. During the tests of the trial bits, the printing frame should be covered with an opaque screen to prevent the exposure from proceeding further. After the exact point is reached, the print is removed from the frame and floated for a few moments on a bath of saturated solution of potassium ferrocyanide, about 1 oz. of the solid crystals to 4 of water. On raising it, the design will be seen in dark-blue lines on white ground.

It is necessary to prevent the liquid from flowing over the back of the paper, which it would cover with a blue stain, and to prevent this the edges of the print are turned up all round. On lifting a corner, the progress of the development may be watched. As soon as the lines are sufficiently dark, or blue specks begin to show themselves in the white parts, the process must be immediately arrested by placing the sheet on a bath of pure water. If, as often happens, a blue tint then begins to spread all over the paper, it may be immersed in a mixture of 3 parts sulphuric or 8 of hydrochloric acid, to 100 of water. After leaving it in this acidulated liquid for 10 or 15 minutes, the design will seem to clear, and the sheet may then be rinsed in a large basin of water, or under a faucet furnished with a sprinkling nozzle, and a soft brush used to clear away any remaining clouds of blue; and finally, the paper hung up to dry. The ferrocyanide bath is not subject to change, and may be used to the last. If it begins to crystallise by evaporation, a few drops of water may be added. The specks of blue which are formed in this bath, if not removed by the subsequent washings, may be taken out at any time by touching them with a weak solution of soda or potash carbonate.

The prints may be coloured in the usual way.