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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1272, January 19, 1883
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The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band
Band 27.1883
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Mungo Ponton, noticed that a sheet of writing paper soaked in a solution of this salt turned brown when exposed to light, and that a paper so prepared placed under a copperplate engraving, and exposed to the sun’s rays for some time, yields a negative copy. This copy can be freed from the soluble chromate by washing in water, and thus fixed—-i.e., protected from further alteration. Then E. Becquered discovered that the turning dark, under the action of light, of the paper soaked in the bichromate was connected with the presence of size in the paper. This, then, is the principle on which the process is based, that organic sub stances such as albumen or gelatine, mixed with bichromate of potassium or ammonia, become insoluble in warm water after exposure to light. Carbon prints, as most of you know, are printed direct from the negative in frames the same as silver prints. The sensitive paper used is called carbon tissue. Why it is so called I do not know ; perhaps Mr. Swan gave it this name because of its gela tinous composition. In Germany it is called Kohle-Papier (carbon paper). It is prepared by coating apiece of paper with a mixture of pigmented gelatine and bichromate of potassium. To make it in large quantities, a machine is required, but as some of you might wish to make it yourselves, I will describe a method by which small quantities may be prepared by anyone. To begin with, a solution composed of the following should be pre pared :— Nelson’s No. 2 flake gelatine 4 ounces Sugar ... J ounce Glycerine 1 to 2 ounces Water ... 1 pint In damp weather 1 ounce of glycerine will be quite sufficient, but in dry weather nearly 2 ounces will be required to keep the tissue soft, as it is apt to get very dry, and crack. This must be allowed to stand till the gelatine has absorbed the water and become quite soft. The vessel containing these ingredients must then be placed in water about 120° Fahr, till the gelatine is dissolved. Great care must be taken that the temperature is not allowed to rise above 120“ at the very most, as the setting quality of the gelatine becomes very much deteriorated, as dry-plate makers know to their cost. When the gelatine is quite dissolved, the solution must be well switched up with a fork or a small egg-beater. It is then allowed to stand till all the air-bubbles come to the surface, when they are removed with small strips of paper. The colouring matter must next be prepared. There are two rules which must always be borne in mind in choosing the colouring matter. The first is, that the colour must be a pigment, and not a dye—that is, a colour which is capable of being ground to an impalpable powder, but which will not dye or stain either the gelatine or the paper. And the second is, that the colour must be permanent. Every one knows how some colours fade away when exposed to strong lights. Lamp-black or Indian ink, which is pure carbon, the most permanent thing in nature at ordinary temperatures, usually forms the basis of the colour ; hence the name carbon process. When lamp-black or Indian ink is used alone, the finished print has a dirty greenish appearance, owing to the action of the chromic acid. To counteract this, other colours must be used. I give a few formulae for producing different colours which may be useful to you. For the ordinary photographic tone use Indian ink 4 grains Carmine lake ... 3 Indian red 5 ” or this may be used- Bone black grains Carmine lake ... 3 „ Burnt umber 2 „ Indigo 1 grain Dark brown tones are obtained by using a mixture of— Vandyke brown 2 grains Venetian red 3 „ Indigo ... ... ••• ••• ••• 1 grain Carmine lake 1 » Bone black 15 grains A very pleasant red brown'is obtained by a mixture of— China ink ...' 3 grains Carmine lake 4 ,, Vandyke brown 4 „ Bed chalk tissue is prepared by mixing—• Indian ink 4 grains Brunt sienna 3 „ Venetian red 2 „ , Tissue for making transparencies, either for the lantern or for enlarging from, can be prepared by using a warm-toned sample of Indian ink, either alone, or mixed with a little Indian red or carmine lake. You will have seen by this time that any colour may be pro duced that is desired, and variations in the colour shades in the formul I have quoted may be obtained at will by giving pro minence to the colour required in the mixture containing it. Very beautiful effects can be obtained by using two colours, one on the top of the other. This is done by coating the tissue (say) with a very dark, almost black, colour ; and after this is dry, or nearly so, giving it a second coat, very thin, of warm brown colour. The warm colour being on the surface gives very delicate tone in the half-tones and high lights, and the thicker black coating gives beautiful deep black shadows. To return to our stock gelatine, which we have almost forgotten. About 60 grains of dry colour will be required for the quantity which we have prepared. When the colours to be used have been decided upon, they must be powdered as fine as possible in a mortar, and then mixed with enough water to form a thick paste, and a very little of the stock gelatine. This paste is then thoroughly ground on a smooth paint slab with a muller, in the same way that a painter grinds his colours. The colour is then added in small proportions at a time to the warm gelatine solu tion, which must be kept thoroughly stirred while the pigment is added, in order that the latter may be evenly distributed through every part of the gelatine. The bichromate of potas sium may be added now, but it is much better to prepare the tissue insensitive, and sensitize it as it is wanted, as it will only keep good about a week, or at most a fortnight; J oz. of very finely-powdered bichromate would be required to render the gelatine sensitive. It must be added like the colour, with con tinued stirring. The mixture is filtered or strained through fine muslin, and is then ready for coating. There are two methods by which the amateur can do this : it can be floated on the mixture—or rather, drawn over its surface—or it can be placed on a glass plate which has been covered with the gelatine. The first is preferable for the manufacture of tissue in large quantities, and the second is best when only small quantities are required, as it needs no special utensils, and only so much of the gelatine mixture is used as is necessary for coating the tissue to be made. I will describe the latter first. The gela tine mixture must be put on warm ; the bottle containing it is therefore placed in warm water, or the gelatine is kept fluid in some other way. A sheet of plate glass is carefully cleaned, allowed to stand in rain water, and then rubbed with ox-gall. It is next placed in a perfectly horizontal position. The gelatine is poured on to the middle of it, and by a gentle lifting and lowering action it is made to flow over the surface as quickly and evenly as possible. Air-bubbles are got rid of with a fine brush or a bit of paper. The gelatine soon sets. Before it dries, a piece of paper is so far damped that, without being exactly wet, it is still thoroughly saturated with moisture. The moistened paper is now carefully and gradually laid down on the glass plate, commencing with one corner, so as to avoid air-bubbles. After a short time a knife is passed along the edges of the plate, and with the required care, the paper is then lifted off evenly and not too slowly. The gelatine adheres to damp paper much better than to dry ; the damped paper is also smoother. To prepare a whole sheet of paper 17 by 23 inches in the manner just described, requires from seven to nine ounces of the gelatine mixture. The paper, by the way, must be rather porous and under-sized. To prepare the paper on a flat dish is not so easily done as with albumenized paper, by merely floating it; it requires to be drawn over the warm mix ture. The dish which holds the gelatine is therefore narrow, and only corresponds in width with the paper to be drawn over it. The gelatine is kept in solution by placing the vessel containing it in a second vessel which is filled with warm water. The quicker the paper is drawn over the gelatine, the thicker the coating is; and the slower, the thinner it is. If the tissue is then hung up at night in a room where a fire has been burning all day, it ought to be dry next morning. When dry, the tissue is sensi tized by soaking in a solution of bichromate of potassium. As the paper is but little sensitive in the wet state, it can be prepared in subdued daylight; but before it begins to dry, the room must be quite darkened, as the dry carbon tissue is much more sensitive to white light than the sensitized paper for silver prints. Another reason for drying the tissue in abso lute darkness is, that no change manifests itself to the eye when
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