Читать книгу Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone - W. T. Wilkinson - Страница 15

For the production of suitable collotype prints, the first consideration is the negative, from which the printing surface is to be made. The requirements are, a negative in which all the half-tones, both in the lights and in the shadows, are reproduced in their proper gradations; the lights must not be too dense, or the shadows too clear; the blackest portion of the negative must be that part representing the highest light, and the only clear glass, that which represents the deepest shadow; these negatives may be either wet collodion, or on ordinary gelatine-bromide plates and reversed—i.e., the mirror must be used behind the lens. If it is desired to produce a collographic block from a negative already in existence, and which may not be retaken, then it will be necessary to make another negative. This may be done in many different ways; but whichever way is adopted, the first and most important step is the production of a transparent positive.

This transparent positive may be made on an ordinary gelatine-bromide plate, or on a gelatine-chloride plate, and then from the transparent positive a negative is made by transmitted light (in the enlarging camera), either the same size, reduced or enlarged in size; or instead of using this class of transparency, by far the finest results can be got by making the transparency in carbon tissue, which can be used for both transmitted light, for enlarging or reduction in size, and it can also be used for contact printing, when the reproduced and reversed negative is required to be the same size as the original.

On page 29 directions are given for the manipulations when gelatine dry plates are used, so here we will confine ourselves to the production of negatives from the carbon transparency. First of all, procure some carbon transparency tissue, ready sensitized, cut this into sizes at least half an inch larger all round than the negative, then cut a brown paper mask to fit all round the negative, and to protect the margin of tissue from the action of light; this acts as a safe edge to the tissue, enabling it to adhere to the glass support during the operation of development. If it is only intended to reproduce a portion of the {32} original negative, the safe edge may be made by putting the mask just round that portion of the image, but if the whole of the negative is required, put it in a printing frame with a plate glass front, fix the brown paper mask outside the negative, then place the sensitive carbon tissue the black side next the negative film, and put in the backing, and back, and fasten it up. Next expose it to the light for four or five times the time requisite to get an ordinary silver print.

The carbon tissue having been exposed to light for the proper time, it is taken into the dark-room, the edges are turned up so as to form a dish, and pinned upon a small board; it is now coated with thin plain collodion, the surplus being poured into a bottle kept for the purpose, and the board gently rocked to and fro, so as to prevent the collodion running in ridges, or streaks. Now allow the collodion to dry, then place the coated and exposed tissue in clean cold water until it is quite limp, then slide underneath it a gelatinized glass plate a little larger all round than the tissue, bring the gelatinized surface of the glass plate and the collodionized surface of exposed tissue in contact under the water, taking great care not to abrade the collodion; then lift the two out and lay the glass down upon a bench, and cover it over with a piece of thin India-rubber, or mackintosh, or oil-cloth. Hold the cover firmly on the bench with the left hand; with the right use a strong squeegee vigorously. That will expel all the water from between the two surfaces (collodion on the tissue, and gelatine on the glass), and so bring them into absolute contact; now remove the cover, lift the plate and examine the tissue through the glass, and if there are no air-bells between the two surfaces the operation has been successful; but if air-bells are present, at once plunge the plate into cold water, strip off the tissue, and try again, but be careful to be quick about it, or there is a risk of the tissue absorbing too much water, in which case great difficulty will be experienced in getting it to lie flat upon the gelatinized glass.

The gelatinized plate upon which the exposed and collodionized tissue is mounted is prepared as follows: Glass plates free from scratches and stains, are placed in a weak mixture of hydrochloric acid and water, and rubbed with a clean rag until free from grease and dirt; they are then washed under the tap, and whilst wet are covered with:

Gelatine	1	ounce.
Water	20	ounces.
Bichromate of Potash	15	grains.

Soak the gelatine in the cold water until it is soft, then melt at gentle heat, add the bichromate, and when dissolved, filter, and coat the cleaned plate two or three times, throwing the surplus away each time of coating; now stand the coated plate on a rack to dry. Any quantity of these plates may be prepared at a time, and used when required. When all the batch has been coated, the rack may be removed into a warm corner, free from dust, until the plates are dry, then expose the plates to the light for at least twenty minutes, when they are ready for use for mounting the exposed and collodionized transparency tissue upon.

After the tissue is mounted upon the glass plate, it is allowed to stand a few minutes, then it is placed in a dish containing hot water (100° F.), and as soon as the tissue compound shows signs of dissolving by exuding from under the paper, lift the paper away, then rock the dish, and, if necessary, add more hot water; then raise the plate and gently lave it with the hot water, until the whole of the soluble pigmented gelatine is washed away, leaving a transparent positive on the glass, which merely requires washing under the cold water tap, and drying, to be finished. Now examine it closely, and if it contains the whole of the details in the negative, and is free from dust spots, it may be put away to dry, but if underexposed, or overexposed, or marred by spots not in the original, then another must be done; but if care is taken to dust the surface of the tissue, and the film of the negative with a camel’s-hair brush before printing, and before collodionizing, and also to see that both the collodion and the gelatine mixture are properly filtered, and that the water in which the coated and exposed tissue is soaked is free from dirt, then there will be no difficulty in getting a perfect transparency.

The transparency being obtained, if it is desired to make a negative larger or smaller than the original, the transparency is placed in the enlarging camera with the carbon film inside, and the negative made the requisite size, either on a dry plate or on a wet collodion plate.

If a negative the same size is required, the transparency is placed face up in the carrier of the dark slide, and the surface dusted; then take a dry plate, and having also dusted its film, place it face down in contact with the transparency, then close the dark slide and remove it to the camera, which should be previously adjusted opposite a large sheet of white paper; now put in the dark slide, draw the shutter, and expose to the sheet of paper, and develop as directed on page 30.

By exposing in this manner, instead of direct to the light (day or gas), the {34} rays projected through the lens fall upon the dry plate quite parallel, so securing sharp negatives.

In making reversed negatives for collographic printing when wet collodion is used, it is not absolutely necessary to use the mirror, as they may be made upon polished glass (i.e., not albumenized) dried, then coated with gelatine, again dried, and stripped.

Some brands of dry plates can also be stripped, by first of all, after drying, coating them with India-rubber solution, then with collodion, and soaking in dilute fluoric acid; but great care and patience must be exercised, the preliminary trials being made upon negatives of no particular value.