Читать книгу Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone - W. T. Wilkinson - Страница 12

The glass used as a support for the collodion film should be thin flatted crown of the best quality, for the negatives, as a rule, are not required again after once the zinc is etched, so that the film may be washed off, and the glass used again, which cannot be done so often if the glass be of bad quality; flatness is a sine qua non, as it has to be subjected to heavy pressure between a zinc plate and the front glass of a printing frame.

As a good and suitable negative is the foundation of success in the subsequent processes, so is a properly cleaned glass plate the foundation of a successful negative; unless the glass is chemically clean, the collodion film will either be stained or marked, or it will split off in drying, therefore care and attention are requisite; even in preparing a piece of glass for the reception of the film of collodion care is especially requisite, as the want of it cannot be seen until after all the operations, therefore the labor and time are not thrown away.

There are two methods of cleaning the glass plate: one quite mechanical; the other is a mixture of the mechanical and the chemical. Both are good. The second is perhaps the best, as it is simple and more certain.

In the first method the glass, when new, merely requires polishing with clean wash leather kept for this purpose.

The best way of doing this is on a flat board 24 by 18 inches, or thereabouts, {22} covered with good American oil-cloth, glazed side out. A plate laid upon this will not slip about when being polished.

Both sides and edges of the plate must be well cleaned, as one of the most prolific causes of derangement of the silver bath is from organic matter introduced by imperfectly cleaned glass plates.

One side of the plate, if gently rubbed with the thumb nail, will be found smoother than the other, and is the suitable side for the support of the collodion film; this side is polished with the leather until, upon breathing gently upon it, the film of moisture is quite free from marks or streaks, not only in the centre but at the sides and corners.

It is absolutely necessary that the plate be chemically clean all over, else the film of collodion, upon drying, will split, and the negative be spoilt.

When the plate is clean, an edging of India-rubber dissolved in benzole, about an eighth of an inch wide, is run all round the edge, by means of a small camel’s-hair brush tied to a stick, the end of the stick being level with the point of the brush. The stick acts as a guard against the brush going too far from the edge.

In the second method, the new glass plate is soaked an hour or two in a mixture of 5 ounces of common spirits of salt (hydrochloric acid) in 80 ounces of water. After both sides of the plate are rubbed with a rag, and well washed under the tap, it is put in a dish of clean water. When all the plates have been so treated, they are taken out singly, again well rubbed on both sides with another rag, and well washed; then the smoothest side having been selected, it is twice flooded with a mixture of albumen and water, draining the surplus into the sink each time of flooding. The plate is then placed upon a rack to drain and dry; the drying should be effected in a current of hot air freed from all dust.

The albumen mixture is composed of the white of one egg well beaten, then diluted with 40 ounces of water and ten drops of liquor ammonia added, and again beaten. It must then be very carefully filtered through cotton-wool, and used as above.

In placing the albumenized plates upon the drying rack, care must be taken to face the coated sides all one way, as when dry there is nothing to indicate to the eye which is the back and which the front.

The albumenized plates, when dry, should have the backs cleaned with a cloth or a leather before using, so as to clear away any albumen which may have adhered during coating. {23}

Albumenized plates stored in a dry place will keep good for months, and do not require any edging with India-rubber, as the most horny collodion film will not split off an albumenized plate.

Glass plates that have been used, and not varnished, are freed from the films by being immersed in a mixture of hydrochloric acid and water for a few hours, then washed under the tap, well rubbed with a rag, and put into clean water, from which they are taken singly and coated with albumen; or they may be allowed to dry, and are then polished by means of the wash leather.

Negatives that have been varnished are cleaned by first immersing in a hot solution of common washing soda until the film comes off, then wash them under the tap, rubbing well with a rag, then immerse them in the acid solution so as to neutralize the alkali. Finally they may be treated as above.

The films that are removed from the glass plates must not be thrown away, but should be carefully collected, dried, and sent to a refiner with the other silver waste; a good plan for saving these films, and at the same time keeping the acid and soda solutions clear of them, is to filter each solution occasionally through an old felt hat suspended conveniently over the tanks, using one for each of the tanks.

The polishing of the glass plate must not be done in the dark-room, or the crop of comets, spots, etc., upon the negatives will be exasperating in the extreme; let this operation be conducted in a clean, quiet corner of the studio, where no other operation will disturb, and where the fluff from dusters and leathers will not interfere with anything else.