PART III.PHOTO-ENGRAVING ON COPPER.CHAPTER I.SUBJECTS IN LINE.
In printing from copper plates, the ink, instead of being spread on the surface of the plate by means of a flexible roller as in ordinary type printing, is smeared over the previously warmed plate and thereby forced into the deep parts of the plate. Then, after the surface is polished, the paper picks out the ink from these cavities and so forms the picture. Therefore, instead of using a negative for producing the resist, a transparency is required; and for the production of subjects in line, the transparency must fulfil all the conditions laid down as required in a negative for the production of prints for zinc etching and for photo-litho. transfers, and must be from a subject in line or in stipple.
The negative is made the exact size required, by the wet collodion process (the lens being used without the reversing mirror). This negative is intensified by immersion in the bichloride of mercury solution, and after well washing is blackened by the application of ammonia, as directed in Part I. of this book, under the head of photographic manipulation.
The negative must be varnished, and the margin outside the actual subject required to be shown in the copper plate, must be masked by means of strips of tin-foil cut perfectly straight and laid in position squarely and symmetrically. The object is to make this margin appear in the transparency as perfectly clear glass.
The masked negative is next placed in the carrier of the dark slide, film up; then a couple of thicknesses of blotting paper about the eighth of an inch square are laid on the extreme corners of the negative. Upon these pieces of blotting paper a sensitive wet collodion plate is placed face down. The little squares of blotting paper serve to prevent the sensitive plate from touching the negative.{120}
The camera, with the lens out of focus, should be pointed to a large sheet of white paper; insert the dark slide, draw the shutter, uncap the lens, and the light reflected from the white sheet through the lens, through the negative upon the sensitive plate, will result, after development, in a transparent positive of the negative.
The time of exposure will, of course, vary and can only be found by experiment. The operations of development, etc., are carried on as for a negative.
This method of making transparencies in the camera yields the best possible results, as the rays of light are projected by the lens perfectly parallel, and the resulting picture is far crisper than it is possible to obtain by exposing in the ordinary manner.
If the negative to be reproduced is already made, and is larger than required, mask the margin by means of the tin-foil, place it in the holder of an enlarging camera, film towards the lens if the negative was made without a mirror; but if the mirror was used and the negative is a reversed one, the film must be placed away from the lens.
The camera is now adjusted so as to get the image the size required, then a sensitive wet collodion plate is used to receive that image and produce the transparency.
The transparency for printing upon a copper plate must, when laid upon a piece of white paper, allow every detail, however minute, to be perfectly distinct by reflected light. If this is not so it will be impossible to get a good engraving on the copper plate.
The transparency being secured, a plate of highly polished copper is cleaned with a little fine whiting and alcohol, then flooded with water. Take care that all grease has been removed (which will be detected by the water running in streaks); next wipe the plate carefully with a clean, soft cloth, and place it in the jaws of the whirler mentioned in the chapter on the apparatus for zinc printing. In the dark-room coat the copper plate with prepared bitumen (see chapter on engraving on zinc in half-tone), and set the whirler in motion. When the film of bitumen is dry, place it in contact with the transparency in a printing frame, and expose it to the light (sunlight, if possible).
The time of exposure will depend very greatly upon the time of year and day, but, in direct sunlight, ten to fifteen minutes will be about the time.
After exposure, the copper plate is removed from the frame, and placed in a dish containing turpentine. Keep it rocking until the image is developed by the unaltered bitumen being washed away, then remove the plate from the dish,{121}and wash it well under the tap. When quite dry, coat the back and edges of the copper with ordinary black varnish laid on with a camel’s-hair brush, so as to prevent the mordant to be used from attacking the back.
Alterations and additions may now be made by means of transfer ink thinned with turpentine, applied by a sable brush, or by means of an etching needle or scraper, remembering that the etching needle or scraper will make a black line and the transfer ink will cause a white mark, or exactly the reverse of a type block. The plate is now ready for etching in a
Saturated solution of Perchloride of Iron2ounces.Water10ounces.
Saturated solution of Perchloride of Iron2ounces.Water10ounces.
Use a porcelain dish for the solution. Immerse the plate, and keep it rocking for about ten or fifteen minutes, then remove it, and wash it under the tap, rubbing it gently with cotton-wool. After drying it, warm it slightly, then roll up with a glazed leather roller, or a smooth India-rubber roller charged with the hard ink used during the clearing-up of the zinc block, viz.:
Beeswax1⁄2ounce.Resin1ounce.Shoemaker’s Wax1ounce.Litho-printing Ink, black2ounces.
Beeswax1⁄2ounce.Resin1ounce.Shoemaker’s Wax1ounce.Litho-printing Ink, black2ounces.
Melt together. When cool, mix with turpentine to the consistency of soft soap.
The roller is charged with this ink, and the plate warmed and rolled up, so as to close up the high lights and half tones, leaving only the deepest portions bare. Immerse in the etching fluid again for about three minutes, rocking during the immersion, and finally wash the plate.
Next remove the resist of bitumen, by means of turpentine, and take proofs in the press. If deeper etching is required, proceed as before.
The following is equally satisfactory, and a little quicker:
Fuming Muriatic Acid (sp. gr. 1.190)10parts.Water70parts.To this is added a boiling solution ofChlorate of Potash2parts.Water20parts.
To this is added a boiling solution of
This is diluted for use by adding 1 part to from 100 to 400 parts of water.
The time of immersion in either of these mordants is ruled by the temperature of the room and of the liquids used. One or two careful trials will soon give the desired information.