Georg Fritz

The processes of photo-lithography previously described are based upon the printing on to bichromated gelatine or albumen paper development with a greasy ink and subsequent transference of the greasy print to plate or stone. A process may also be used in which the stone or plate is printed on direct. There are in this various methods, of which the principal are printing on to light-sensitive asphalt, or an organic substance, gelatine or albumen, in combination with a bichromate salt. The reason is the high sensitiveness, which is possible, of the substances used.

Only that part of the asphalt is sensitive or useable for photo-lithographic work which does not dissolve in ether. The insensitive part must, therefore, be separated out, which is done by finely powdering the asphalt, sifting it through a fine sieve, and then treating it with ether. All that is soluble in ether goes into solution; the marc which is not soluble in ether is then dissolved in benzole and used as wanted.

After standing for about twenty-four hours the ether contains all soluble insensitive constituents of the asphalt; the pitch-like residue in the flask, after pouring off the solution, is again treated with ether, so that all turpentine is extracted and any stray insensitive particles are dissolved. The pitchy residue is now removed from the flask and allowed to stand in a porcelain dish in a warm place, with frequent stirring, till it is quite freed from ether, and has formed a hard, brittle, black shining substance, which can be easily broken up into powder with the hand. This product is now the light-sensitive asphalt, which is dissolved for use in anhydrous benzole. As benzole is only to be obtained anhydrous with difficulty, some chloroform is added to the solution in order to prevent the running together in drying, whence the film would become unequal and patchy.

The coating of the stone or plate with the asphalt film is very simple; it should be placed horizontal, and then whirled rapidly on a whirler.

Printing under a clear negative takes from a quarter to half an hour in direct sunlight.

The picture is developed with turpentine.

The process is as follows:—100 g. of raw Syrian asphalt are boiled in a retort with an equal quantity of raw pseudo-cumene, which has the formula C₆H₃(CH₃)₃, and a boiling-point of about 170° C., with 12 g. of flowers of sulphur, which should have been previously dissolved in the pseudo-cumene. When after about three or four hours’ boiling the evolution of sulphuretted hydrogen has ceased, the pseudo-cumene is distilled off and the black pitchy residue dissolved in benzole in the proportion of 4:100, and used for the preparation of the plate or stone.

The sulphurized asphalt prepared in this way is almost insoluble in ether, but dissolves fairly readily in benzole, toluene, xylene, cumene, and turpentine, and is very sensitive to light.

With this asphalt good prints are obtained even in bad, cloudy weather. For use four parts of the sulphurized asphalt are dissolved in 100 parts of benzole, the solution filtered and diluted till a thin film on a zinc plate shows a golden yellow colour. Exposure of the asphalt solution in an open flask for from half to one hour in sunlight is advisable.

For developing the asphalt image rectified oil of turpentine free {62} from acid is used; the best is French or Austrian. The development may be accelerated by adding Hungarian or Russian turpentine, which, however, attack the asphalt image; it can be restrained by the addition of ligroin, benzine, or wood oil to the turpentine.

Zinc plates are best developed in a dish, stones by making a wax margin round the drawing. The oil of turpentine is poured on and rocked to and fro. Rubbing with a pad of cotton wool, etc., should be avoided. After development the plate should be well washed under a stream of water, and before gumming, the plates should be exposed for some time to light, as this makes the ground more resistant to the etching.

The solution must be thin, so that it flows over the stone well and does not form unequally-covered patches.

The subsequent manipulations must be conducted in the dark.

The stone which is to be used must be well ground, and polished with dry pumice stone, well dusted, and be absolutely flat. It ought not to have any depressions, as the negative would not lie in contact there. The places would not print sharp, and therefore a bad result would be obtained.

The necessary quantity of the solution is now poured into the middle of a well-polished stone, and distributed as evenly as possible by rocking backwards and forwards. When thoroughly distributed the stone should be placed on the whirler and kept for some time in motion. The asphalt solution must be as thin as possible, so that it appears yellowish-brown and the colour of the stone appears through it. If the stone is only covered all over the film can be very thin; on the other hand, if it is too thick it will not print through, and will develop badly, as too much of the asphalt will dissolve. If zinc plates are used for printing they should be prepared as previously described on p. 6. Zinc plates for deep etching must be well ground and well polished, and ought not to repel water. The zinc plates are coated the same as the stone.

If it is printed too long the stone will not develop at all, or only in parts; if it is printed too little the drawing wholly or partially dissolves. In both cases an unsatisfactory result will be obtained. Negatives very clear in the lines of the drawing, and well covered in the ground, are here more essential for success than with the chrome gelatine process.

The very sensitive kinds of asphalt of Husnik and Valenta are much more readily soluble than the ordinary Syrian asphalt. In developing one must go carefully to work, and not allow the turpentine to stay on the stone a long time, but to pour it over the stone, which should be inclined so that it can run off. This can be repeated if required. It can be well and safely developed if half and half of turpentine and linseed oil is used, which, however, has the disadvantage that the stone becomes greasy, but this can be taken off by the expert printer.

After developing the stone should be well washed under a strong stream of water, then allowed to dry in the light, as by this the asphalt film becomes more capable of standing the etching, and then be coated with neutral or very slightly acidulated gum. After gumming, the stone is again dried and allowed to stand for some hours to rest, after which it can be inked up and etched, and finally sharp etched.

The zinc plate is, after development, also well washed, then allowed to dry, treated with thin gum solution, then with an {64} etching solution of tincture of galls or gallic acid and phosphoric acid, and inked up.

Prints on zinc plates for deep etching are gummed after developing, and if the asphalt film is perfect without rubbing up with a pad, which thickens the drawing, etched for some minutes in very dilute nitric acid.

Many lithographers are accustomed, and indeed it is necessary with some transfers, to rub the prints up with greasy ink. The process is as follows:—Greasy transfer ink is diluted with some turpentine, and after the stone has been gummed and allowed to dry, then washed, the drawing is wiped over with a soft pad with this dilute ink, when the ink adheres to the drawing, strengthens it, and if great care is not taken, thickens it also. With asphalt prints this is absolutely useless, and for other direct prints, as well also as for good chromated gelatine prints, in most cases unnecessary. The exposed asphalt combines so intimately with the stone that strengthening with a greasy ink is quite unnecessary, is indeed absolutely purposeless, as the ink can neither penetrate the hard asphalt film nor combine with it. The film of asphalt alone is so resistant to every etching solution that it is for this reason absolutely useless to strengthen it.

The solutions which have been specially used in our establishment consist of—

Solution A is strongly sensitive to light, and is used for coating the stone or zinc plate; with solution B the stone is coated before developing. The details of the process are as follows:—

If a zinc plate be used it must be well prepared and be rendered free from grease. To prove this the plate is flowed over with distilled water, and when this spreads equally all over and is not repelled from any part, the solution can be poured on. If, however, the plate does not willingly take water everywhere, it must be again cleaned or polished. The other operations are the same as for the stone.

The picture is faintly visible. If the negative was too thin, or if it be over-printed, the image develops badly or not at all. With under-printing the film washes right away.

Immediately after printing the stone or plate is coated with solution B, which is distributed as evenly as possible by rocking backwards and forwards. It is not necessary, however, to obtain even distribution with a whirler. The whole of the coated surface becomes deep violet.

R. J. Sachers, of Toronto, Canada, published a sensitive solution which has the following composition:—

This solution is brown, not very transparent, with an olive green tinge, and must stand and settle at least twelve hours before it can be decanted, filtered, and used. The time of exposure is about—

The image appears a bright brown on the golden yellow ground of the plate. The development is effected in an aqueous solution of methyl violet, when the image assumes a beautiful blue violet colour. The developed image is hardened with alcohol. The results by this method are very good, and although this is recommended for half-tone etching, it is also suitable for photo-lithography.

Excellent results also attend the sensitive “Marine glue solution,” prepared by A. Hebensperger, of Munich, to which a second solution called “Chlorogutt-losung” is added. The sensitiveness to light is very high, and the treatment about the same as with the first described substance. The “marine glue solution” consists of albumen, bichromate of soda and ammonia. The dye solution of Victoria blue dissolved in benzine.

The process published by Waterhouse consists of coating a prepared zinc plate with a thin chromated solution, and then drying in a few minutes with a gentle heat—

For use mix—

This is printed under a reversed negative for about six minutes in the sun. The plate is then laid for about half an hour in cold water in order to remove the chromium salts, and then cleansed from all soluble parts with a pad, and after well washing set up to dry. It is then inked up with transfer ink, and after about fifteen minutes washed with turpentine. This process, as, indeed, all direct printing processes, offers many advantages for preparing colour plates from a coloured object with one negative. The register of the separate plates is very exact, which is not always the case with a transfer process. In reproducing line drawings the unnecessary parts on the stone or plate can be covered.