van Monckhoven, Désiré van. A Popular Treatise on Photography. Translated By W.H. Thornthwaite. London, 1863.
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THEORETICALLY, the printing process is one of the greatest simplicity:-One side of a sheet of paper is first imbued with a solution of common salt, and dried; it is then treated with a solution of nitrate of silver. A white substance, called chloride of silver, is formed in the texture of the paper, which has the singular property of becoming rapidly darkened on exposure to the sun; it is on this account that when it is prepared the operation should be performed in the absence of daylight, and the sensitive paper preserved in a dark box.
It is evident that if paper so prepared be placed behind a negative, and exposed to daylight, a positive image will be produced on the paper. And, in order to its preservation, it will be necessary to remove the whole of the chloride of silver unacted on by light, without affecting those portions which constitute the image; this is readily accomplished by soaking the proof for a quarter of an hour in a solution of hyposulphite of soda, and then washing most copiously in water to remove every trace of this fixing agent from the texture of the paper.
The proof thus completed is the final result of all the operations which have been previously described.
The foregoing description gives the general principles of the printing process; a successful result, however, can only be attained by the exercise of the greatest care and attention to many important details, which will be treated of under the following heads:
Suitable paper for positive proofs is now manufactured expressly for the purpose, and can readily be obtained both in England and oil the continent. It is not., however, every sheet in the ream as obtained from the manufacturers that ought to be employed. Those that are uneven in texture, and blemished with black and other spots, must be rejected.
While selecting paper it is important to touch only the edges of the sheets, because, however dry the fingers may be, they always leave a slight imprint which, sooner or later, will produce a stain.
It is also necessary to ascertain the right side of the paper to which the chemicals should be applied; this may be clone by examining the paper by reflected light, and is that side most uniform in its character and freest from lines. Each sheet, as selected, should have the wrong side distinguished by a pencil mark.
Positive paper should be moderately thick. A ream should weigh about 22 lbs. the size being 22 inches by 17 inches; but, generally speaking, for albumenised paper, the ream should not weigh less than 24 lbs., especially if it be required highly glazed; while for ordinary salted paper a weight of 18 lbs. to the ream will be found sufficient. The paper may be salted with various chlorides, such as the chlorides of barium, strontium, potassium, ammonium, &c.; and although it was once thought that each salt imparted a characteristic tint to the proof, a contrary opinion is now entertained; however, chloride of sodium, or common salt, which is always to he obtained in a fair state of parity, is now generally used.
The salt is dissolved in distilled, or rain-water, in the proportion of 1.2 grains to the ounce, and the solution filtered into a dish large enough to hold a whole sheet of paper, and measuring, therefore, about 24 inches by 19 inches. The preparation of whole sheets of paper at one time will generally be found most advantageous, and they are afterwards easily cut to any smaller sizes that may be required.
The paper is spread out or floated on this solution in the following manner:--The opposite ends of the sheet are held, and the paper bent as represented in Fig. 82, and the middle of the paper being brought into contact with the liquid, the two ends are regularly lowered until the whole of one side just floats on the surface. The side of the paper previously marled with a, pencil should be uppermost.
Fig. 82. Salting the Paper.
The paper ought to remain on this bath five minutes in winter and three minutes in summer, at the end of which time it is withdrawn by means of a pair of horn or boxwood forceps and held for a little while over the dish, to drain; it is then attached to two clips, or other means of suspension, and hung on a line to dry (Fig. 84).
If the operator, or rather amateur, should not be often performing this operation it may be found more convenient to use smaller sheets, and to suspend them in the manner indicated in Fig. 83; whichever method of suspension, however, is adopted, a small piece of white bibulous paper should be attached to the lower corner or corners, in order to aid the draining of the salt solution.
Paper thus salted is preserved in a portfolio, and, as it does not undergo any alteration by keeping, it may be prepared in large quantities at a time.
Albumenised paper yields positive proofs of great delicacy and fine colour. It, s, however, a little more difficult to prepare.
Fig. 83. Hanging the Paper to Dry.
White of egg is beaten into a snow-white froth, adding, for each 10 eggs, 150 grains of salt, reduced to fine powder; then, when the albumen has accumulated by being allowed to remain at rest (see Fig. 74, and pages 75, 76), it is poured into a large flat porcelain tray, and used in the same manner as the ordinary salting-bath, except that the viscous nature of the albumen requires greater care in floating.
It is very difficult to albumenise large sheets of paper. The first operation should be the removal of bubbles by means of a card, and this must be done very effectually, because wherever there is a bubble the liquid does not touch the paper, and the consequence is a white spot in the print.
It is advisable to keep the paper which it is intended to albumenise in a damp place for a few days before using, otherwise it gets into folds on the liquid, and bubbles are sure to form. When, however, the paper is damp it spreads with the greatest facility on the albumen bath, and no fear need be entertained as to the formation of the fatal bubbles. These precautions are specially applicable to large sheets, the small ones being comparatively easy of manipulation.
Fig. 85. Drying of Large sheets.
The large sheets are very difficult to suspend. An excellent plan is to attach the sheets, on being withdrawn from the bath, to frames of cardboard by the aid of pins as shown in Fig. 85. In this way the tearing of the sheet, and its frequent falling to the ground, is avoided.
There is not much purely albumenised paper used that which has been prepared with a mixture of albumen and water being preferred, on account of the more moderate brilliancy given to the sheets. From 10 to 40 per cent. of water may be added to the pure albumen; but the more water, the less brilliant the paper.
For portraiture a paper prepared with pure albumen should not be used, on account of its excessive brilliancy; but rather plain salted paper, or paper prepared by floating on a mixture of equal volumes of albumen and water containing 4 per cent. of salt. Albumenised paper should be preserved in tin or zinc boxes, closed from contact with a damp atmosphere, for if it absorbs moisture it becomes rapidly changed, especially in summer time. Whether the amateur prepares his own paper, or whether be buys it, there are always some bad sheets. Sometimes there are great lines arising from the draining of the albumen towards one point, sometimes the gloss is greater at one place than another, without the reason being very obvious.
This operation should be performed in the dark, or at all events in a moderate light. Daylight does not act by any means so energetically on positive paper as on collodion; nevertheless, it is advisable to have a room specially arranged for the purpose.
To this end the windows should be furnished with yellow blinds or frames, which will admit a large body of light, having no action on the prepared paper.
Paper is sensitised by floating it for three or four minutes on a silver-bath of 20 per cent.
|Recrystallised Nitrate of Silver||4 ounces.|
This solution is poured into a perfectly clean dish, and the paper floated thereon, in the manner shown at Fig. 82.
If the proportion of nitrate of silver be much reduced the proofs will be wanting in vigour; from which circumstance, as often as 100 sheets of paper, 9 inches by 7, have been sensitised, half an ounce of nitrate of silver should be added to the bath.
In order to avoid soiling the fingers, and also the stains which would be produced by the contact of the clips, it is best to turn up a corner of the sheet (Fig. 86), which prevents its absorbing any of the nitrate of silver, and may, therefore, be taken hold of for hanging up the sheet. Instead of hanging the sheets of paper by hooks (Fig. 87), they can be more conveniently suspended by some little articles, supplied with India-rubber springs, known as American Clips (Fig. 88).
It is only necessary to press on the ends of these clips in order to open them, and to withdraw the pressure after inserting the paper, which is then held with the greatest tenacity. Figs. 83 and 84 show how these improved clips are used.
Small pieces of white blotting-paper should be attached to the corners of the sheet from which the solution is draining, and the drops of nitrate of silver may be collected in test-glasses (Figs. 83 and 84), although the drops so collected serve only for transformation into chloride of silver.
The dry sensitised sheets should be used within a day or two after preparation, unless preserved in a box with chloride of calcium, to be presently described.
The silver-bath may, of course, be used until it is exhausted, remembering always to add the half ounce of nitrate of silver for every 100 sheets 9 inches by 7, as before mentioned. It must, however, be filtered every tune it is desired to sensitise a fresh batch of paper.
A special bath should be used for albumenised paper, to which should be added one-fortieth of its weight of kaolin, which should always be kept at the bottom of the bottle, shaking it up from time to time, and regularly allowing it to settle. This kaolin maintains the bath in a colourless condition, for without it the solution would acquire a deep yellow colour which would be communicated to the proofs.
Fig. 89, Apparatus for preserving Sensitised Papers.
When a number of sheets have been sensitised, they are placed in portfolios, one on the other, handling them only by the edges, in order to avoid contact of the fingers, which produces stains. They should be used within three days.
If it be desired to preserve sensitised paper for some months, an apparatus must be employed consisting of a box with an air-tight cover, and containing some dried chloride of calcium. Figs. 89 and 90 represent two forms of the apparatus. Fig. 89 consists of a square zinc or tin box, about six inches deep, furnished with a false bottom, on the top of which is placed the sensitised paper, and underneath a metal or porcelain basin filled with pieces of dried chloride of calcium; the whole is closed by a well-fitting lid, which can be rendered air-tight by pasting a strip of paper around the joining.
The other form, Fig. 90, is a very convenient one for travelling. The outer cylinder B and cover A are made of zinc or tin plate; the inner core E D is formed of canvas or metal gauze, and is filled with pieces of dried chloride of calcium. A sheet of blotting-paper is first rolled round this core, and afterwards the sheets of sensitised paper; it is then put into the cylinder, the lid adjusted, and a band of vulcanised India-rubber placed over the joining, as shown at C, which renders the whole air-tight.
Whenever any of the sheets of paper are required to be taken from either form of apparatus, it should be closed again as quickly as possible, because the chloride of calcium very rapidly attracts moisture from the air; from which circumstance its peculiar action of keeping perfectly dry the air in the interior of the boxes depends.
When the chloride of calcium has been some time in use, it becomes covered with moisture, and eventually converted into a liquid, and its action destroyed. So soon as any moisture is perceived, it ought to be placed in a moderately hot oven, in a porcelain or iron vessel, until perfectly dry, when its properties will be entirely restored. Thus the same quantity of chloride of calcium can be used over and over again for any length of time required.
Fig. 91. Printing Frame.
A printing frame is procured, furnished with screws for large plates, and with springs for small negatives. The instrument (Fig. 91) consists of a simple wooden frame, at the bottom of which is a strong piece of plate glass, which should be always well cleaned on both sides before using. The negative is placed on this glass with the plain side downwards, while the sensitised side of the paper is brought into contact with the collodion film, which is, of course, uppermost.
In order to maintain perfect contact between the sensitised paper and the negative, the printing-frame is furnished with a hinged board lined with cloth or felt, and kept in its place by transverse bars with springs or screws. If it be desired to examine the progress of the printing, one of the transverse bars is removed, the hinged board is opened, and one half of the paper bent back, as shown in Fig. 92. This operation should, of course, not be performed in the direct sun-light, but in the shade.
Fig. 92. How to examine the Action of Light on the Paper.
A consideration of the following list of successive changes, through which the paper passes, will guide the operator in judging when a print has been sufficiently exposed.
The paper turns--
It is not sufficient that the picture when examined, as shown in Fig. 92, should seem to be printed deep enough, because the fixing agent removes a great deal of the depth of the colour; on this account, and in order that the finished print should possess the proper depth of tone, it is necessary that the tint imparted by exposure in the printing frame should be much deeper than is required for the finished picture.
The time, then, for withdrawing the paper from the action of the sun or light, depends solely on the method of fixing, and is best learned by experience.
Pressure-frames should be made with the greatest care, for if, for example, the back for pressing the paper against the glass should not be quite flat, the paper will in some places be away from the negative, while in other parts the contact will be complete, and the resulting positive image will bear evidence of unequal sharpness, thus marring the beauty of the picture.
Professional photographers, who generally have a great many negatives to print from, arrange the pressure-frames on a framework, so constructed as to admit of the angle of inclination being altered in such a way as to receive the solar rays perpendicularly.
Fig. 93. Vignette Glasses for Graduated Backgrounds.
The more vigorous and fully developed the negative, the more brilliant and satisfactory is the resulting positive proof. But if the negative be a weak one, it is possible, notwithstanding, to produce a passable print from it by prolonging the action of the solar rays. To this end the frame is exposed not to the direct rays of the sun, but to diffused daylight. Or better still, the printing frame may he covered, as shown in Fig. 94 by a light frame, upon which is stretched a piece of thick white paper. When it is desired to produce portraits with a white background, glasses with yellow borders;and white centres are employed, called vignette glasses, as shown in Fig. 93.
It will be understood that the yellow portion of these glasses prevents the passage of the chemical rays, and as the action of light on the chloride of silver takes place only through the colourless portion of the glass, that is to say, in the centre, it follows that from an ordinary negative a picture may be obtained with a white background.
Fig. 94. Moveable Framework for Printing Positives.
When the background of a negative is not sufficiently developed, or is defective through any other cause, it becomes very difficult to remedy. Some persons paint in the background, others cut out the figure from a positive print and paste the matrix, or background portion, on the negative; but in either case the outline of the picture is always hard and disagreeable. A better plan is to cover the pressure frame with the vignette glass, Fig. 93, taking care, of coarse, to place the oval in such a position that only the bust shall be printed, while the defective background is as much as possible mashed.
The following is the method adopted when it is desired to produce the effect of clouds in a proof; firstly, a negative is taken with the ordinary exposure, in which the sky will be perfectly black; secondly, another negative is taken with such a short exposure that nothing appears when the developing solution is poured on but the sky. With this short exposure the effect of clouds is very successfully obtained.
A positive being printed from the first negative will have, of course, a white sky; this being placed under the second, or cloud negative, the foreground of which is covered with cotton wool in such a way that light can pass only through that. portion of the glass bearing the image of the clouds, The outline of the objects in the landscape is covered. with cotton wool, the fibres of which have been well pulled out, in order that the hardness which would be otherwise produced may be moderated and toned down. An exposure of a few minutes will be found sufficient to produce the desired effect.
To produce a similar effect artificially is even still more easy. In taking the negative it is arranged in the exposure that the sky shall not be too deep, and the desired cloud effects are painted, not on the film, but on the other side of the glass. It should be remembered, however, that the effect produced in the positive proof will be the reverse of what is painted on the glass.
A recently varnished negative should not be immediately printed from under the direct rays of the sun, for however good the varnish employed may be, it invariably sticks a little when freshly applied. Instead of exposing to direct sunlight, use diffused daylight, or better still, cover the pressure-frame with a case or covering of white paper.
Paper should be made perfectly dry after sensitising, and before being placed in contact with the negative, otherwise the moisture in the fibres of the paper will be volatilised by the sun's heat, and condense on the negative, which will soon become blackened by the nitrate of silver of the paper. When this is discovered in time, the spots so produced may be removed by cyanide of potassium; the operation, however, requires great dexterity and judgment.
Negatives should never be allowed to remain all night in contact with paper at all damp, as the depression of temperature induces a condensation of moisture on the surface of the glass, and brings about the same untoward result as occurs under the circumstances detailed in the preceding paragraph.
Unusual precautions are necessary in the use of albumenised paper, which easily sticks to the negative, and shows stains wherever it is touched. It should be handled only by the extreme edges.
It is very important that the woodwork of pressure-frames should be well put together, so that no warping shall occur through changes of temperature or differences in the hygrometric condition of the atmosphere; otherwise the prints will not be equally sharp over the entire surface, owing to the unequal contact between the paper and glass induced by the warping. Moreover, the negatives are very likely to become cracked from the same cause.
A great many sheets of positive paper may be prepared at a time, on the sole condition of keeping them in a box with chloride of calcium, as described at page 105. This box, however, ought not to be opened oftener than is absolutely necessary, but enough paper should be withdrawn at one time to serve for two days' consumption.
Before placing the negative in the pressure-frame, the plate-glass should be carefully wiped, in order to avoid the accumulation of dust on the proof.
When the paper is placed in the frame the screws must be tightly adjusted, so as to secure it against the slightest movement when the hinged back is opened to examine the progress of the printing.
The frame itself should be a little larger than the hinged back, in order that the latter may open without difficulty. If it be so tight as to require on effort to open it, there will almost invariably occur a displacement of the negative, and a consequent doubling of the outline in the proof.
The process of toning is employed to impart a pleasing tint to the proof, and the object of fixing is to remove the unaltered chloride of silver by immersion in a solution of hyposulphite of soda.
In order to avoid constantly touching the proof during these operations, horn or boxwood forceps are employed, or glass rods bent as shown in Figs. 95 and 96.
Many formulae have been given for toning and fixing the prints, in some of which the two operations are con ducted simultaneously; in others the fixing is done be fore the toning, and in a third the toning is effected before the fixing. The latter process, however, is the one by which the best results and the greatest permanency are obtained. It consists of four operations: 1. Washing the print, to remove the free nitrate of silver adhering to the surface, and which has remained unchanged by light. 2. Toning in a weak solution of chloride of gold, to which a small quantity of carbonate of soda has been added. 3. Washing again to get rid of the chloride of gold. 4. Fixing in a solution of hyposulphite of soda.
The chloride of calcium box (Fig. 89) serves as well for the preservation of proofs which have been exposed to light as for sensitised paper. As it is desirable to fix a large number of proofs at one tune, it is recommended that the operator should at the outset provide himself with three, at least, of the chloride of calcium boxes; one for preserving the white chloride, another for the paper ready for immediate use, and the third for paper which has been exposed under a negative.
The room set apart for the production of positive proofs should be on the ground-floor, and the panes of;lass darkened with yellow paper; it should be furnished with several large wooden troughs for washing, and an assortment of porcelain or brown stoneware trays for hyposulphite solutions, &,c. There should also be an abundant supply of water.
The proofs being taken either from the pressure-frame or the chloride of calcium box, are plunged one after the other into a porcelain pan containing rain-water, which is changed for every hundred prints. This bath should be at least one foot deep.
By far the greater quantity of the nitrate of silver withdrawn from the sensitising bath still remains on the surface of the paper after exposure to light. The bath of rain-water dissolves the nitrate of silver, and when the water is thoroughly charged therewith, it may be treated as described at page 75, by throwing in an excess of common salt, which produces a precipitate of chloride of silver.
There should not be more than ten prints washed at a time in this bath, and special care should be taken to prevent any hyposulphite of soda from falling therein, on which account the bath may be advantageously covered with a plate of glass, and the prints touched only with boxwood forceps.
When the prints have been for about ten minutes in the water, they are taken out one at a time and immersed in the toning-bath. To prepare the toning-bath, male the two following standard solutions:--
|No. 1||Bicarbonate Soda||2 drachms.|
|No. 2||Chloride Gold||30 grains.|
|Distilled mater||32 ounces.|
Take one quart of water and dissolve in it. two drachma of common salt; then add one ounce of solution No. 1 and one ounce of No. 2. This toning-bath should be kept about ten minutes before being used.
The first prints which are immersed in the bath after the addition of the chloride of gold tone rapidly; but in proportion as the gold is taken up, the toning proceeds more slowly, when more of solution No. 2 will have to be added. To accelerate the toning in winter the solution may be slightly warmed by setting the dish on a sheet of plate iron heated with a spirit lamp.
The chloride of sodium or common salt is added to the bath to transform into chloride the nitrate of silver which has not been washed out, and which would otherwise decompose the chloride of gold.
The prints being put into the toning-bath will first become red, and will then pass successively through all the intermediate tints between red and black. The toning can be stopped at any stage by immersing the print in a tray of clean water, which should be kept at hand for the purpose.
The real colour is only seen after immersion in the hyposulphite fixing solution, and it is at this stage only that the operator can judge to what tint he should tone his pictures, in order that they should finally possess the desired colour. As a general rule the bluish black changes to a pure black in the hyposulphite; less toning than is required to give the bluish black produces prints more or less brown; if the toning is pushed further than the bluish black, the prints will be ashy coloured and flat when dry.
Prints which have to be toned black must be printed deeper than those which have to be toned purple or brown, for the toning-bath has also a bleaching action.
Prints on albumenised paper require for toning a larger quantity of gold than prints on ordinary salted paper.
The toning-bath may be used over and over again, by adding from time to time more gold and bicarbonate of soda; but, as hyposulphite of soda is liable to be accidentally introduced into it, it is advisable, where a large number of prints are produced, to make a new one every day.
In order to judge of the colour of the proof it should be withdrawn from the gold-bath by means of a boxwood forceps, as on account of the yellow colour of the bath, it is difficult to form an accurate estimate while the print remains therein. The same remark may be applied to the colour of the panes of glass in the printing room. It has been said that they should be covered with yellow paper, but this applies, of course, only to those panes which illuminate that part of the room in which the prints are either withdrawn from the pressure-frame or placed therein, or where the first washing with plain water takes place. All the other operations may be safely conducted by daylight, although it will be found quite as well to preserve the whole of the workshop from the direct action of the solar rays by means of blinds.
The prints, as soon as they are taken out of the toning-bath, are immersed in water; they are then washed several times and put into the fixing-bath, which is composed as follows
|Hyposulphite of Soda||8 ounces.|
The fixing will generally require from ten to fifteen minutes; but it is easy to understand that the longer the bath has been in use, the more will its capacity for dissolving chloride of silver become exhausted, and the longer will the print require immersion, so that, one pound of hyposulphite of soda will not fix more than one hundred prints, 8½ in. X 6½ in.; but as this salt is very cheap, to use it liberally will make but a fractional increase in the cost of the prints.
The hyposulphite bath should, then, be often changed, and when out of use it may be kept for the purpose of extracting the silver.
The proof which comes from the last bath is now rinsed in a wooden trough filled with water, to remove the excess of adherent hyposulphite; then allowed to soak for two hours in a second trough, and for three hours in a third. There should be an extra trough or dish for every twenty-five prints of 8½ in. X 6½ in. When the prints are allowed to remain longer in the water than is specified above, they lose their beautiful colour and become yellow.
It is sometimes desired to finish the washing of a print somewhat hurriedly, and yet to ensure the complete removal of every trace of hyposulphite of soda; when this is the case, the proof is placed on a glass plate of suitable size. and moved about under a stream of water from a tap falling from a height of about six feet. The motion imparted to the glass should be so regulated as to allow the water to fall on every part of the proof in succession, without tearing it or causing its separation from the glass. Five minutes of this energetic washing removes the hyposulphite more effectually than five hours soaking in the troughs of water, although it is obvious that the consumption of water is much greater. A badly fixed or imperfectly washed print undergoes spontaneous alteration after a few weeks.
The following may be received as practical rules to avoid the fading of prints.
As silver forms, with hyposulphite of soda, a double salt, slightly soluble in a feeble excess of hyposulphite, of soda, but on the contrary very soluble in a concentrated solution,-and as this double hyposulphite of silver and soda decomposes with great facility,-it is very important that concentrated solutions should be used, in order that the double salt shall be dissolved as soon as formed.
A bath such as that of which the formula has been given, must not be used too long, and for the same reason too many proofs must not be immersed therein at one time, especially if there be not sufficient space between each proof to allow the bath to exert its full solvent action.
All the formulae which prescribe the addition of an alkali or an acid to the bath should be unhesitatingly rejected. The chloride of gold of commerce is sometimes very acid, in which case it may be replaced, if desired, by a white salt known by the name of Sel d'Or de Fordos et Gelis; this must be very much diluted before mixing with the hyposulphite of soda.
If the proof, through insufficient washing, contains hyposulphite of soda, it will have, when dry, a sweet taste, and will fade in a very short time.
Finally, all fermented or fermentable substances for mounting photographs, should be rejected. Nothing but recently prepared starch-paste should be used, and the proof should be allowed to dry very rapidly when mounted, in order that no acid principles may be developed by fermentation.
In rainy climates in winter-in England, for example -it is sometimes necessary to expose the paper for a whole day to the light before a positive can be obtained. Albumenised paper being much more rapid than simple Salted paper, will therefore be preferred, although even this will sometimes fail to produce a positive in a reasonable time in which case recourse may be had to the following expedient.
Plain paper being cut to the proper dimensions, and the reverse side marked with a pencil, is allowed to float for about one minute on a bath composed of 300 grains of iodide of potassium dissolved in 35 ounces of water. This bath serves until it is exhausted, and the paper when dry and placed in a portfolio will keep a long time. The paper so prepared is rendered sensitive to light by floating (in the dark, of course) on a bath of nitrate of silver for one minute. The bath is composed of
|Fused Nitrate Silver||30 grains.|
|Glacial Acetic Acid||300 grains.|
The paper is withdrawn from the bath, well drained, and rapidly dried between folds of blotting-paper, in order to remove the excess of solution, and prevent its touching the back of the paper. This operation is repeated with a second and even a third sheet of blotting-paper. It is necessary that the sensitised sheet should be damp, but not to such a degree as to stain the negative, upon which it should be pressed very lightly.
The exposure to diffused daylight varies between five and fifteen seconds; a long exposure yields a flat picture, deficient in brilliancy-in a short one the sky only appears.
The paper is carefully removed from the negative, and placed on a sheet of glass with the sensitised side uppermost. A solution is then made of 15 grains of gallic acid in 35 ounces of warm water, to which is added 3½ drachms of glacial acetic acid. This is filtered and spread very quickly by means of a brush or glass rod over the sensitised side of the sheet. The image appears very rapidly, and as soon as the desired effect is arrived at the paper is plunged into water to arrest the action of the developer.
After washing, it is toned with chloride of gold, as described at page 113, and it is then washed for two hours in a large trough of water.
Whatever process may have been adopted in the production of the positive print, it has finally to be pasted on Bristol board, and rolled.
Nothing is more simple than the mounting of a proof It is cut into any suitable or desired form, and the back covered with starch-paste. It is then spread on the Bristol board, a piece of blotting-paper laid over it, and the whole rubbed with the hand in every direction, in order to ensure complete contact between the print and the board. This being done, it is allowed to dry in the air, and subsequently rolled in a lithographic or other press, by which it acquires a finishing gloss, which is entirely absent in unrolled prints.
Poirier's press, represented in Fig. 97, is very applicable for this purpose. It is composed of a steel roller, E, the axle blocks of which are moveable in a vertical direction; double levers are employed to make the necessary adjustment, and the power is applied at b b. It is sufficient to move one only of these heads in order to move the other, there being an arrangement of cog-wheels for this purpose. However, for delicate adjustments, the power is best applied to each screw-head separately, which, in turning, depresses two screws attached to the axle-blocks, these latter sliding between two edges of cast-iron.
The other part of the apparatus consists of a plate of planed polished steel, a a, or of a very fiat lithographic stone, which receives its backward and forward motion from a toothed wheel, moved by a still smaller one, which in its turn is moved by a long lever or handle.
Upon this steel plate two very smooth and specially prepared cards are placed, between which the proof to be rolled is laid. The screws being adjusted, and the motion imparted, the steel plate moves on, turning the cylinder at the same time. An enormous pressure is thus brought to bear uniformly over the whole surface of the card, and after two or three movements to and fro, the screws b b are slightly tightened, and the rolling repeated.
The proof thus becomes pressed into the cardboard, and acquires a splendid gloss. To mount. stereoscopic views and cartes de visite, they should be cut by means of a steel or glass mould and a good knife.
It is a good plan to paste a great many proofs on one card, in order to effect the glazing by one operation.
Fig. 97. Rolling Press.
A beautiful gloss can be imparted to stereoscopic and carte de visite proofs by varnishing them. This varnish is made as follows:--One ounce of white lac is dissolved in ten ounces of warm alcohol, and after allowing the bottle to stand for several weeks, the clear portion is decanted for use.
This varnish is then applied in a similar manner to that adopted by French polishers in polishing cabinet work; it has been introduced into commerce under the name of Crystal Enamel.
The picture having been mounted, is sized with a warm solution made by dissolving 10 grains of Swinborne's gelatine in 1 ounce of water, and either hot-pressed or burnished with an agate burnisher. It is then coated with the crystal enamel, applied in the following manner:--A tampion of cotton-wool saturated with the liquid is wrapped in a piece of clean white calico rag, the outer surface of which is touched with a drop or two of linseed oil: this is gently and evenly applied with a circular motion over the whole surface to be enamelled, until the picture becomes brilliant. It is, lastly, finished off by applying, in the same manner, alcohol and linseed oil.