Читать книгу The Art and Practice of Silver Printing - H. P. Robinson - Страница 6

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In printing on albumenized paper we must divide the operations, and give a detailed account of each. In case the reader may desire to prepare his own paper, we give the following formula and directions.

To prepare the albumen, procure a sufficient number of eggs, remembering that the white of a large egg will be about a fluid ounce; have a cup to collect the yolks, and a four-ounce measure at hand. Give the centre of the egg a smart blow against the top of the cup. The shell can now be readily pulled in two, the yolk remaining unbroken with part of the albumen in one half, and the rest of the albumen in the other half of the shell. Take the halves, one in each hand, and pour the albumen from one to the other, holding them over the small measure. As the operation continues, the yolk will gradually separate, the white falling into the vessel below. If conducted with care, the whole of the latter will be collected without breaking the yolk. If the yolk break, some will be sure to find its way into the measure along with the white, and this, together with the white speck known as the tread, must be rigorously taken out by means of a spoon. The uncontaminated white is then poured into a large jar. If the operator carefully collects the white of each egg into the four-ounce ounce measure first, he will find his labour much diminished, as it is awkward to get out the small pieces of yolk from a large quantity of albumen. The eggs are thus broken, and the white collected till there is a sufficient quantity for the purpose in hand. Suppose we are going to make up 20 ounces of solution, then about 18 ounces of white of egg must be found in the jar. One point to settle is the amount of salt to be used to each ounce of albumen. It must be recollected that a medium quantity is the best for medium negatives; anything between 20 and 40 grains per ounce may be used. We prefer ourselves about 25. Supposing this quantity to be used, we proceed to dissolve 500 grains of chloride of ammonium in 2 ounces of water, and add it to the albumen. It has been proved that as regards colour of the picture, it does not matter what chloride is used. To prevent crystallization, it is better to use ammonium, which contains a greater amount of chlorine than do sodium or potassium chlorides. It must now be beaten up till it is in a froth. This breaks up the fibrous matter, and on subsidence the liquid will be found to be limpid. The most convenient implement with which to beat up the albumen is the American egg-beater. Three or four minutes' work is quite sufficient to make the whole into a froth. An ordinary culinary whisk, such as is used in the kitchen, may also be put into requisition, or, in default of that, a bundle of quill pens. A lesson in producing a froth can be learnt from the cook of the establishment. When the salted albumen has settled it must be filtered, which, perhaps, is best effected through a sponge, though glass-wool is a capital substitute. In either case a small, loosely-fitting plug is placed in the neck of an ordinary funnel, and, after rinsing with cold water, the albumen is poured in, and allowed to filter through slowly. It is advisable to avoid bubbles as far as possible, and the accompanying arrangement will be found to avoid their formation. The funnel is placed in the position shown (fig. 1); the capillary attraction between it and the glass will cause the drops to trickle down the side, and collect, without bubbles, at the bottom. This little contrivance will be found of use in other operations besides that of silver printing, and should be made a note of. The albumen may also be filtered through one, two, or three thicknesses of muslin, according to its fineness, tied over the mouth of a bottle or beaker of which the bottom has been removed. The albumen is placed in a vessel slightly larger than the filter, which is allowed to sink gradually. When full it is withdrawn, and the fluid poured into the dish. By this plan upward filtration is established. The fluid may be poured into the filter itself, and used in the ordinary manner.[6]

Fig. 1.

Fig. 2.

On a larger scale, white of eggs in a fresh condition can be obtained from egg merchants who utilize the yolks by selling them to the grocers and confectioners. Albumen can be obtained by the gallon in this condition, according to the price of eggs. It will be evident that there is considerable economy in taking the whites wholesale. As a rule, about three gallons of albumen will coat two reams of albumenized paper. Mr. England (to whom we are indebted for so many of our remarks on albumenizing paper) procures about the latter quantity at a time, and beats it up mechanically in a large vat holding some fifty gallons, in order to allow space for the froth. He allows the albumen to rest four days before employing it, and filters it through three thicknesses of flannel.

The quality of paper to be used varies considerably with the custom of the printer. Thus, in some countries, we find a much thinner paper used than in England. The great desideratum is that it should be perfectly opaque to transmitted light. A good test of this is to make a couple of black ink marks on a piece of white paper, and then press down firmly the paper it is proposed to employ over this. If the black ink marks are indistinguishable, the paper will do as regards this quality, as the light reflected from the surface which gives the impression of whiteness to the eye is much stronger than the light which penetrates through it, and is absorbed by the black lines. As to quality, it is best to trust to the manufacturer, those known as Saxe and Rives papers answering better than any other that we know of. The Rives is, when moist, a paper which is more easily torn than the Saxe, and, consequently, we recommend that the former be employed for small work, such as portraits, and the latter for large landscape prints.

In regard to the sizes to be albumenized, it must be left to the operator to say what will be the most useful to him. It is rarely advisable to albumenize less than a half sheet of paper, the whole size of which is about 22 by 18 inches; 11 by 18 is not an inconvenient size to manipulate. At any rate, a dish larger each way by a couple of inches than the paper must be procured, and put on a level table. The temperature of the room should be at least 90°, in fact, the hotter it is the more glossy will be the resulting paper. The solution, free from bubbles, is poured in, and should be of a depth of at least ¹/₂ an inch. Suppose the smaller size to be coated, before commencing, the paper is taken by the two opposite corners, the hands brought together, and the convex side brought on to the surface of the fluid; the hands are then separated, and the paper will gradually float on the surface. One corner should be gradually raised to see that all air-bubbles are absent. If there be any, they should be broken with the point of a glass rod, and the paper again lowered. Bubbles can usually be seen through the paper, and, instead of raising it, a few gentle taps with the finger over the spot will generally move the bubble to the edge of the paper. In practice, some have found it well to moisten the surface of the paper with a damp sponge, and when quite surface dry to albumenize it. This should, however, be unnecessary. The sheet should remain on the albumen a little over a minute, when it could be gently raised by one corner and allowed to drain over a basin; it is then caught by a couple of American clips and hung up to dry.[7]

Fig. 3.

Supposing a whole sheet is to be coated, it will be found more convenient to take the sheet by the corners of one end, one in each hand, and to lower the surface near the end of the dish, and gradually draw the paper over the side of the dish till the whole surface is flat. Bubbles can be got rid of as shown above.

Two large dishes are usually employed, and by the time the second sheet is floated in the second dish, the first sheet of paper is ready for removal from the first dish. The sheets, when slowly removed from the bath, are allowed to drain a few seconds, and then thrown over wooden rods of some two inches in diameter, which are removed to a rack, and placed near a trough to collect the drainings.[8] When drained sufficiently the rods are removed to other racks, and the paper allowed to dry spontaneously.

It is the practice of some albumenized paper manufacturers to hang the sheets over a line, uncoated side next to the line; but this is a mistake, as it will nearly always be found, on sensitising the paper and exposing it, that a mark is left across the paper corresponding to the part where the string touched the back of the paper.

In practice we have found that each sheet of paper takes up about ¹/₃ oz. of solution, and, of course, its equivalent quantity of salt. The principal difficulty in albumenizing paper is the occurrence of lines on the paper in the direction in which it was placed on the surface of the albumen. Any arrest of motion in floating the paper will cause them, but more usually it is due to imperfect beating up of the solution. Some papers are not readily coated with albumen, in which case the remedy given above may prove effectual; or a little solution of oxgall may be equally well applied. A want of gloss in the dried albumen may be due to too long a floating on the fluid, or to floating and drying the paper in too low a temperature. The explanation of the first cause is that albumen, when fresh, has an alkaline reaction, due to the presence of a small quantity of soda, which may be said to be its base, and any alkali will dissolve the gelatinous sizing of a paper. When the sizing is dissolved, instead of remaining on the surface, the albumen sinks into the paper, and thereby the gloss is lost.

When albumen is stale it no longer possesses this alkaline reaction, but has an acid reaction quite visible on the application of blue litmus paper to it; the blue colour disappears and is replaced by a red tint. When in the alkaline state, the paper is much more difficult to coat, but an acid condition means the production of inferior tones.

Rolling the Paper.—The paper, when dried, is often rolled with a heavy pressure to improve the gloss; a copper-plate press is found to answer admirably, placing the albumenized side next the bed. This rolling should not be necessary if attention be paid to the temperature of the preparation room. The higher the temperature the finer will be the gloss, as we have already said.