THE PRACTICAL DETAILS OF PHOTOGRAPHIC PRINTING.

This Chapter is divided as follows:—

SECTION I.

Positive Printing by the direct action of Light.

This includes—the preparation of sensitive paper,—of fixing and toning Baths,—and the manipulatory details of the process.

Selection of Paper for Photographic Printing.—The ordinary varieties of paper sold in commerce are not well adapted for the production of Positive prints. Papers are manufactured purposely which are more smooth and uniform in texture. Many samples of even the finest paper are however defective, and hence each sheet should be examined separately by holding it against the light, and if spots or irregularities of texture are seen, it should be rejected. These spots usually consist of small particles of brass or iron, which, when the paper is rendered sensitive, decompose the Nitrate of Silver and leave a circular mark very noticeable after fixing.

The foreign papers, French and German, are different from the English. They are porous and sized with starch, the English being sized with gelatinous animal matter. In all cases there is a difference in smoothness between the two sides of the paper, which may be detected by holding each sheet in such a manner that the light strikes it at an angle; the wrong side is that on which dark wavy bands, of an inch to an inch and a half in breadth, are seen, caused by the strips of felt on which the paper was dried. With most qualities of paper no difficulty whatever will be experienced in detecting the broad and regular bands above referred to; but when they cannot be seen, the wrong side of the sheet may be known by wire markings crossing each other, or if the paper be wetted at the corner, one side may appear evidently smoother than the other.

PREPARATION OF SENSITIVE PAPER.

There are three principal varieties of sensitive paper in common use, viz. the Albuminized, the plain, and the Ammonio-Nitrate paper.

Formula I. Preparation of Albuminized Paper.—This includes the salting and albuminizing, and the sensitizing with Nitrate of Silver.

The Salting and Albuminizing.—Take of

Chloride of Ammonium, or Pure Chloride of Sodium	200	grains.
Water	10	fluid ounces.
Albumen	10	fluid ounces.

If distilled water cannot be procured, rain water or even common spring water[40] will answer the purpose. To obtain the Albumen, use new-laid eggs, and be careful that in opening the shell the yolk is not broken; each egg will yield about one fluid ounce of Albumen.

When the ingredients are mixed, take a bundle of quills or a fork, and beat the whole into a perfect froth. As the froth forms, it is to be skimmed off and placed in a flat dish to subside. The success of the operation depends entirely upon the manner in which this part of the process is conducted;—if the Albumen be not thoroughly beaten, flakes of animal membrane will be left in the liquid, and will cause streaks upon the paper. When the froth has partially subsided, transfer it to a tall and narrow jar, and allow to stand for several hours, that the membranous shreds may settle to the bottom. Then pour off the upper clear portion, which is fit for use. Albuminous liquids are too glutinous to run well through a paper filter, and are better cleared by subsidence.

A more simple plan than the above, and one equally efficacious, is to fill a bottle to about three parts with the salted mixture of Albumen and water, and to shake it well for ten minutes or a quarter of an hour until it loses its glutinosity and can be poured out smoothly from the neck of the bottle. It is then to be transferred to an open jar, and allowed to settle as before.

The solution prepared by the above directions will contain exactly ten grains of salt to the ounce, dissolved in an equal bulk of Albumen and water. Some operators employ the Albumen alone without an addition of water; but this commonly gives a highly varnished appearance, which is thought by most to be objectionable. Much however will depend upon the kind of paper which is employed, certain varieties taking more gloss than others; Papier Rive, for instance, often requires the Albumen to be nearly or quite undiluted.

The principal difficulty in Albuminizing paper, is to avoid the occurrence of streaky lines, which, when the paper is rendered sensitive, bronze strongly under the influence of the light. To avoid them, use the eggs quite fresh, and lower the paper on to the liquid by one steady movement; if a pause be made, a line will probably be formed. Some papers are not readily wetted by the Albumen, and when such is the case, a few drops of spirituous solution of bile, or a fragment of the prepared Ox-Gall sold by the artists' colour-men, will be found a useful adjunct. Care must be taken however not to add an excess, or the Albumen will be rendered too fluid, and will sink into the paper, leaving no gloss.

In salting and albuminizing Photographic paper by the formula above given, it is found that each quarter-sheet, measuring eleven inches by nine inches, removes one fluid drachm and a half from the bath, equivalent to about one grain and three-quarters of salt (including droppings). In salting plain paper, each quarter-sheet takes up only one drachm; so that the glutinous nature of the Albumen causes a third part more of salt to be retained by the paper.

English papers are not good for albuminizing; they do not take the Albumen properly, and curl up when laid upon the liquid: the process of toning the prints is also slow and tedious. The thin negative paper of Canson, the Papier Rive, and Papier Saxe, have succeeded with the writer better than Canson's Positive paper, which is often recommended; they have a finer texture, and give more smoothness of grain.

To apply the Albumen, pour a portion of the solution into a flat dish to the depth of half an inch. Then, having previously cut the paper to the proper size, take a sheet by the two corners, bend it into a curved form, convexity downwards, and lay it upon the Albumen, the centre part first touching the liquid, and the corners being lowered gradually. In this way all bubbles of air wall be pushed forwards and excluded. One side only of the paper is wetted: the other remains dry. Allow the sheet to rest upon the solution for one minute and a half, and then raise it off, and pin it up by two corners. If any circular spots, free from Albumen, are seen, caused by bubbles of air, replace the sheet for the same length of time as at first.

The paper must not be allowed to remain upon the salting Bath much longer than the time specified, because the solution of Albumen being alkaline (as is shown by the strong smell of Ammonia evolved on the addition of the Chloride of Ammonium) tends to remove the size from the paper and to sink in too deeply; thus losing its surface gloss.

Albuminized paper will keep a long time in a dry place. Some have recommended to press it with a heated iron, in order to coagulate the layer of Albumen upon the surface; but this precaution is unnecessary, since the coagulation is perfectly effected by the Nitrate of Silver used in the sensitizing; and it is doubtful whether a layer of dry Albumen would admit of coagulation by the simple application of a heated iron.

To render the paper sensitive.—This operation must be conducted by the light of a candle, or by yellow light. Take of

Fused Nitrate of Silver	60	grains.
Glacial Acetic Acid	1/3	minim.
Distilled Water	1	ounce.

Prepare a sufficient quantity of this solution, and lay the sheet upon it in the same manner as before. Three minutes' contact will be sufficient with the thin Negative paper, but if the Canson Positive paper be used, four or five minutes must be allowed for the decomposition. The papers are raised from the solution by a pair of bone forceps or common tweezers tipped with sealing-wax; or a pin may be used to lift up the corner, which is then taken by the finger and thumb and allowed to drain a little before again putting in the pin, otherwise a white mark will be produced upon the paper, from decomposition of the Nitrate of Silver. When the sheet is hung up, a small strip of blotting-paper suspended from the lower edge of the paper will serve to drain off the last drop of liquid.

A Bath prepared by the above formula is stronger than is really necessary. Forty grains of Nitrate to the ounce of water is abundantly sufficient if the sample be pure; but it must be borne in mind that the strength of the Bath diminishes rapidly by use, and hence, when the prints begin to be wanting in vigour, with pale shadows and perhaps a spotted appearance, an addition of Nitrate of Silver must be made. Fused Nitrate of Silver is recommended in preference to the crystallized Nitrate, on account of the latter being occasionally contaminated with an impurity alluded to at page 101. This when present will be likely to redden the pictures and to interfere with the rapidity of bronzing.

The solution of Nitrate of Silver becomes after a time discoloured by the Albumen, but may be used for sensitizing until it is nearly black. The colour can be removed by Animal Charcoal,[41] but a better plan is to use the "kaolin," or pure white china clay. This substance often contains Carbonate of Lime, and effervesces with acids: it must in such a case be purified by washing in vinegar, or the Bath will become alkaline, and dissolve off the Albumen. It has been stated that an addition of Alcohol to the Nitrate Bath prevents it discolouring with Albumen.

Sensitive albuminized paper will usually keep for several days, if protected from the light, but afterwards turns yellow from partial decomposition.

Formula II. Preparation of plain paper.—Take of

Chloride of Ammonium or Sodium	160	grains.
Purified Gelatine	20	grains.
Iceland Moss[42]	60	grains.
Water	20	ounces.

Pour boiling water upon the Moss and Gelatine and stir until the latter is dissolved, then cover the vessel and set aside until cold; add the salt, and strain.

Use Papier Saxe or Towgood's paper,[43] floated upon the salting Bath in the same manner as directed for Albumen at p. 243.

Render sensitive by floating for two or three minutes upon a solution of Nitrate of Silver, 40 grains to the ounce. Thirty grains to the ounce, or less, will be sufficient if the sample be pure; but in that case occasional additions of fresh Nitrate of Silver must be made, as the Bath loses strength.

A second Formula for plain paper.—Take of

Chloride of Ammonium	200	grains.
Citrate of Soda[44]	200	"
Gelatine	20	"
Water	20	fluid ounces.

If Towgood's or any English paper be used, the Citric Acid, Carbonate of Soda, and Gelatine may be omitted. With a foreign paper the Citrate tends to give a purple tone to the Positive, when toned by Sel d'or, but the gold toning Bath must be in active order, or the prints will be too red. The Citric Acid also should not be in excess over the alkaline Carbonate.

Render sensitive by floating for three minutes upon a Nitrate Bath of sixty grains to the ounce of water.

Formula III. Ammonio-Nitrate Paper.—This is always prepared without Albumen, which is dissolved by Ammonio-Nitrate of Silver. Take of

Chloride of Ammonium	100	grains.
Citrate of Soda	200	"
Gelatine	20	"
Water	20	fluid ounces.

Dissolve the Gelatine by the aid of heat; add the other ingredients, and filter. The solution cannot be kept longer than two or three weeks without becoming mouldy. The Saxony paper, or Towgood's English paper, may be employed; the Gelatine and Citrate being retained or omitted, according to the taste of the operator and the mode of toning which is adopted.

Render sensitive by a solution of Ammonio-Nitrate of Silver, 60 grains to the ounce of water, which is prepared as follows:—

Dissolve the Nitrate of Silver in one-half of the total quantity of water. Then take a pure solution of Ammonia and drop it in carefully, stirring meanwhile with a glass rod. A brown precipitate of Oxide of Silver first forms, but on the addition of more Ammonia it is re-dissolved.[45] When the liquid appears to be clearing up, add the Ammonia very cautiously, so as not to incur an excess. In order still further to secure the absence of free Ammonia, it is usual to direct, that when the liquid becomes perfectly clear, a drop or two of solution of Nitrate of Silver should be added until a slight turbidity is again produced. Lastly, dilute with water to the proper bulk. If the crystals of Nitrate of Silver employed contain a large excess of free Nitric Acid, no precipitate will be formed on the first addition of Ammonia. The free Nitric Acid, producing Nitrate of Ammonia with the alkali, keeps the Oxide of Silver in solution. This cause of error however is not likely to happen frequently, since the amount of Nitrate of Ammonia required to prevent all precipitation would be considerable. From the same reason, viz. the presence of Nitrate of Ammonia, it is often useless to attempt to convert an old Nitrate Bath already used for sensitizing, into Ammonio-Nitrate.

Ammonio-Nitrate of Silver should be kept in a dark place, being more prone to reduction than the Nitrate of Silver.

Sensitizing paper with Ammonio-Nitrate.—It is not usual to float the paper when, the Ammonio-Nitrate of Silver is used. If a bath of this liquid were employed, it would not only become quickly discoloured by the action of organic matter dissolved out of the papers, but would soon contain abundance of free Ammonia (see the Vocabulary, Part III., art. "Ammonio-Nitrate"); and an excess of Ammonia in the liquid produces an injurious effect by dissolving away the sensitive Chloride of Silver.

The Ammonio-Nitrate is therefore applied with a glass rod, or by brushing, and in neither case is any of the liquid which has once touched the paper allowed to return into the bottle.

Brushes are manufactured purposely for applying Silver solutions, but the hair is soon destroyed unless the brush be kept scrupulously clean. Lay the salted sheet upon blotting-paper, and wet it thoroughly by drawing the brush first lengthways and then across. Allow it to remain flat for a minute or so, in order that a sufficient quantity of the solution may be absorbed (you will see when it is evenly wet by looking along the surface), and then pin up by the corner in the usual way. If, on drying, white lines appear at the points last touched by the brush, it is probable that the Ammonio-Nitrate contains free Ammonia.

The employment of a glass rod is a very simple and economical mode of applying Silver solutions. Procure a flat piece of board somewhat smaller than the sheet to be operated on, and having turned over the edges of the paper, secure them with a pin. Next bring the board near to the corner of the table, and laying the glass rod along the edge of the paper, allow the fluid to drop into the groove so formed; then carry the rod directly across the sheet, when an even wave of fluid will be spread over the surface. A pipette made of glass tubing, when dipped into the bottle and the upper end closed with the finger, will withdraw as much of the Ammonio-Nitrate as is required; and if a scratch be made upon the tube at a point corresponding to 30 or 40 minims, it will be found sufficient for a quarter sheet of the Papier Saxe.

Ammonio-Nitrate paper, however prepared, cannot be kept many hours without becoming brown and discoloured.

Use of a solution of Oxide of Silver in Nitrate of Ammonia.—The great objection to the use of Ammonio-Nitrate of Silver is the decomposition which it sometimes experiences by keeping, metallic Silver separating and Ammonia being set free. To obviate this liberation of Ammonia, the Author employs Nitrate of Ammonia as the solvent for the Oxide of Silver. The solution is prepared as follows:—Dissolve 60 grains of Nitrate of Silver in half an ounce of water, and drop in Ammonia until the precipitated Oxide of Silver is exactly re-dissolved. Then divide this solution of Ammonio-Nitrate of Silver into two equal parts, to one of which add Nitric Acid cautiously, until a piece of immersed litmus-paper is reddened by an excess of the acid; then mix the two together, fill up to one ounce with water, and filter from the milky deposit of Chloride or Carbonate of Silver, if any be formed.

This solution of Oxide of Silver in Nitrate of Ammonia appears to possess all the advantages of the Ammonio-Nitrate without the inconvenience of liberating so much free Ammonia upon the surface of the sensitive sheets.

Hints in selecting from the above FormulÆ.—Albuminized paper is the most simple and generally useful; it is well fitted for small portraits and stereoscopic Photographs. The Ammonio-Nitrate Process requires more experience, but gives excellent results when black tones are required: it may be used for larger portraits, engravings, etc.

Plain paper rendered sensitive by floating upon a Bath of Nitrate of Silver is easier of manipulation than the Ammonio-Nitrate, and will be found to be better adapted for toning by the Sel d'or Bath (p. 267) than the Albuminized Paper.

PREPARATION OF THE FIXING AND TONING BATH.

Take of

Chloride of Gold	4	grains.
Nitrate of Silver	16	grains.
Hyposulphite of Soda[46]	4	ounces.
Water	8	fluid ounces.

Dissolve the Hyposulphite of Soda in four ounces of the water, the Chloride of Gold in three ounces, the Nitrate of Silver in the remaining ounce; then pour the diluted Chloride by degrees into the Hyposulphite, stirring with a glass rod; and afterwards the Nitrate of Silver in the same way. This order of mixing the solutions is to be strictly observed: if it were reversed, the Hyposulphite of Soda being added to the Chloride of Gold, the result would be the reduction of Metallic Gold; Hyposulphite of Gold, which is formed, being an unstable substance, and not capable of existing in contact with unaltered Chloride of Gold. If however it be dissolved by Hyposulphite of Soda immediately on its formation, it is rendered more permanent, by conversion into a double salt of Soda and Gold.

In place of Nitrate of Silver, recommended in the formula, Chloride of Silver may be used, but not Iodide of Silver, as the formation of Iodide of Sodium would be objectionable (p. 136). For the same reason it is better not to add any part of the Hyposulphite Bath used for fixing Negatives, to the Positive colouring solution.

This toning Bath is not to be employed immediately after mixing, but should be set aside until a portion of Sulphur (produced by free Hydrochloric Acid, and Tetrathionate of Soda reacting upon the Hyposulphite) has subsided. It will be very active at the expiration of a few days or a week; but upon keeping for a longer time, loses much of its efficacy by a process of spontaneous change.

The immersion of prints also lessens the quantity of Gold; and hence, when the Bath begins to work slowly, more of the Chloride must be added, the Sulphur being allowed to deposit as before. Filtration through blotting-paper will not be required.

The writer finds that after a certain time, when the Bath has been long used, and organic matters, Albumen, etc., have accumulated in it, it is better, and more economical, to throw away what remains, and to prepare a new solution. The addition of Chloride of Gold to an old Bath will not always make it work as quickly as one recently mixed.

THE MANIPULATORY DETAILS OF PHOTOGRAPHIC PRINTING.

These include—the exposure to light, or printing properly so called; the fixing and toning; and the washing, drying, and mounting of the proof.

The Exposure to Light.—For this purpose reversing frames are sold, which admit of being opened at the back, in order to examine the progress of the darkening by light, without producing any disturbance of position.

Simple squares of glass however succeed equally well, when a little experience has been acquired. They may be held together by the wooden clips sold at the American warehouses at one shilling per dozen. The lower plate should be covered with black cloth or velvet.

Supposing the frame to be employed, the shutter at the back is removed, and the Negative laid flat upon the glass, Collodion side uppermost. A sheet of sensitive paper is then placed upon the Negative, sensitive side downwards, and the whole tightly compressed by replacing and bolting down the shutter.

This operation may be conducted in the dark room; but unless the light be strong, such a precaution will not be required. The time of exposure to light varies much with the density of the Negative and the power of the actinic rays, as influenced by the season of the year and other obvious considerations. As a general rule, the best Negatives print slowly; whereas Negatives which have been under-exposed and under-developed print more quickly.

In the early spring or summer, when the light is powerful, probably about ten to fifteen minutes will be required; but from three-quarters of an hour to an hour and a half may be allowed in the winter months, even in the direct rays of the sun.

It is always easy to judge of the length of time which will be sufficient, by exposing a small slip of the sensitive paper, unshielded, to the sun's rays, and observing how long it takes to reach the coppery stage of reduction. Whatever that time may be, nearly the same will be occupied in the printing, if the Negative be a good one.

When the darkening of the paper appears to have proceeded to a considerable extent, the frame is to be taken in and the picture examined. If squares of plate glass are used to keep the Negative and sensitive paper in contact, some difficulty may be experienced at first in returning it precisely to its former position after the examination is complete, but this will easily be overcome by practice. The finger and thumb should be fixed on the lower corners or edge, and the plate raised evenly and quickly.

If the exposure to light has been sufficiently long, the print appears slightly darker than it is intended to remain. The toning Bath dissolves away the lighter shades, and reduces the intensity, for which allowance is made in the exposure to light. A little experience soon teaches what is the proper point; but much will depend upon the state of the toning Bath; and albuminized paper will require to be printed somewhat more deeply than plain paper.

If, on removal from the printing-frame, a peculiar spotted appearance is seen, produced by unequal darkening of the Chloride of Silver, either the Nitrate Bath is too weak, the sheet removed from its surface too speedily, or the paper is of inferior quality.

On the other hand, if the general aspect of the print is a rich chocolate-brown in the case of Albumen, a dark slate-blue with Ammonio-Nitrate Paper, or a reddish purple with paper prepared with Chloride and Citrate of Silver, probably the subsequent parts of the process will proceed well.

If, in the exposure to light, the shadows of the proof become very decidedly coppery before the lights are sufficiently printed, the Negative is in fault. Ammonio-Nitrate paper highly salted is particularly liable to this fault of excess of reduction, and especially so if the light be powerful; hence it is best, in the summer months, not to print by the direct rays of the Sun. This point is important also, because the excessive heat of the Sun's rays often cracks the glasses by unequal expansion, and glues the Negative firmly down to the sensitive paper. An exception however may be made in the case of Negatives of great intensity; which are printed most successfully upon, a weakly sensitized paper (p. 124) exposed to the full rays of the Sun; a feeble light not fully penetrating the dark parts.

The fixing and toning of the proof.—No injury results from postponing this part of the process for many hours, provided the print be kept in a dark place.

The mode often followed is to immerse the Positive in the Hyposulphite Bath in the state in which it comes from the printing-frame; moving it about in the liquid in order to displace air-bubbles, which, if allowed to remain, produce spots. But the Author, for reasons given in the first part of the Work (pp. 129 and 165), recommends that the print should first be washed in common water until the soluble Nitrate of Silver has been removed.[47] This is known to be the case when the liquid flows away clear; the first milkiness being caused by the soluble Carbonates and Chlorides in the water precipitating the Nitrate of Silver. Greater security is thus afforded that the print will be toned in a really permanent manner, since after removing the Nitrate of Silver from the proof, the Bath does not work quickly unless the supply of Gold be well maintained.

Immediately on coming in contact with the Hyposulphite of Soda in the fixing and toning Bath, the chocolate brown or violet tint of the Positive disappears, and leaves the image of a red tone. Albumen proofs become brick-red; Ammonio-Nitrate a sepia or brown-black. If the colour is unusually pale at this stage, probably the Silver Bath is too weak, or the quantity of Chloride of Ammonium or Sodium insufficient.

After the print has been thoroughly reddened, the toning action begins, and must be continued until the desired effect is obtained. This may happen in from ten minutes to a quarter of an hour, if the solution is in good working order and the thermometer at 60°; but much depends upon the temperature, and the activity of the Bath. English papers, and especially the same prepared with Albumen, tone more slowly than foreign papers plain salted.

The brown and purple tints are an earlier stage of coloration than the black tones, and therefore the latter require more time. It must be borne in mind however that prolonged immersion in the Bath is favourable to sulphuration and yellowness; tending also to render the image unstable and liable to fade in the half-tones. This fading may not be seen decidedly whilst the print is in the Bath, but will show itself in the after-processes of washing and drying.

The ultimate colour of the Print will vary much with the density of the Negative and the character of the subject; copies of line engravings, having but little half-tone, are easily obtained of a dark shade resembling the original impression.

Some advise that on removal from the toning Bath the Print should be soaked in new Hyposulphite for ten minutes, to complete the fixation; but this precaution is not required with a Bath of the strength given in the formula. An analysis of an old Bath which had been extensively used, indicated only ten grains of Hyposulphite of Silver to the ounce, so that it was far from saturated.

The occasional addition of fresh crystals of Hyposulphite of Soda to keep up the strength of the Bath, is useful, the exact quantity added not being material.

The washing, drying, and mounting of the Positive Proofs.—It is essential to wash out every trace of Hyposulphite of Soda from the Print if it is to be preserved from fading, and to do this properly requires considerable care.

Always wash with running water when it can be obtained, and choose a large shallow vessel exposing a considerable surface in preference to one of lesser diameter. A constant dribbling of water must be maintained for four or five hours, and the prints should not lie together too closely, or the water does not find its way between them, (see the remarks at p. 162).

When running water cannot be obtained, proceed as follows:—first wash the Prints gently, to remove the greater part of the Hyposulphite solution. Then transfer them to a large shallow pan, in which may be placed as many Prints as it will conveniently hold. Leave them in for about a quarter of an hour, with occasional movement, and then pour off the water quite dry. This point is important, viz. to drain off the last portion of liquid completely before adding fresh water. Repeat the process of changing at least five or six times, or more, according to the bulk of water, number of Prints, and degree of attention paid to them.

Lastly, proceed to remove the size from the Print by immersion in boiling water.[48] This process will give some idea of the permanency of the tints, since, if they become dull and red, and do not darken on drying, the Print is probably toned without Gold. Ammonio-Nitrate and plain paper Prints prepared on foreign papers by the modes described in this Work, may be expected to stand the test of boiling water; Albumen Prints and Positives on English paper are a little reddened, although not to an objectionable degree.

The size may also be effectually removed from the Print by the common Carbonate of Soda used in washing, although the former process is recommended as the most secure. Dissolve about a handful of the Soda in a pint of water, and when the milky deposit, if any occurs, has subsided, immerse the washed Positives for twenty minutes or half an hour. The Soda renders the paper quite porous, but produces no alteration of tint. If the process be properly performed, ink will run in attempting to write upon the back of the finished picture. After removal from the Soda Bath a second washing will be required, but the time of the first washing may be proportionally shortened. Here a difficulty will occur with many kinds of water; the Carbonate of Soda precipitating Carbonate of Lime, in the form of a white powder which obscures the picture. To obviate this, use rain water until the greater part of the alkaline salt has been removed, and do not allow a stationary layer of liquid to rest too long upon the Print. The New River water supplied to many parts of London, being comparatively soft, answers perfectly, and produces no white deposit, if the proofs are moved about occasionally.

When the Prints have been thoroughly washed, blot them off between sheets of porous paper and hang up to dry. Some press them with a hot iron, which darkens the colour slightly, but does so in an injurious manner when Hyposulphite of Soda is left in the paper.

Albumen proofs when dry are sufficiently bright without further treatment; but in the case of plain paper, salted simply, the effect is improved by laying the Print face downwards upon a square of plate-glass and rubbing the back with an agate burnisher, sold at the artists' colour-men's. This hardens the grain of the paper and brings out the details of the picture. Hot-pressing has a similar effect and is often employed.

Mount the proofs with a solution of Gelatine in hot water, freshly made; the best Scotch glue answers well. Gum water, prepared from the finest commercial gum, and free from acidity, may also be used, but it should be made very thick, that it may not sink into the paper, nor produce an unpleasant "cockling up" of the cardboard, which is caused by the damp and expanded print contracting as it dries.

Caoutchouc dissolved in mineral Naphtha to the consistence of thick glue or gold-beaters' size, is employed by many for mounting Photographic Prints; it may be obtained at the varnish shops, and is sold in tin boxes. The mode of using it is as follows:—with a broad brush made of stiff bristles, apply the cement to the back of the picture; then take a strip of glass with a straight edge, and by drawing it across the paper, scrape off as much as possible of the excess. The print will then be found to adhere very readily to the cardboard, without causing expansion or cockling; and any portion of the cement which oozes out during the pressing may, when dry, be removed with a penknife without leaving a stain.

REMARKS UPON THE WANT OF CORRESPONDENCE BETWEEN THE FORMULA OF DIFFERENT OPERATORS.

The formulÆ for Positive printing given in the works on practical Photography exhibit great variety; and it has been proposed to attempt to reduce them to more uniform proportions. This cannot however easily be done, both on account of the difference in the structure and preparation of the various Photographic papers, and also because the mode of applying the solutions is not always the same.

Take as an illustration the following process, which has long been recommended for its simplicity, and which is in every respect a good one:—Dissolve 40 grains of Chloride of Ammonium in 20 ounces of Distilled Water, and immerse about a dozen sheets of Towgood's Positive paper, removing air-bubbles with a camels'-hair brush. When the last sheet has been placed in the liquid, turn the batch over and take them out one by one, so that each sheet, remaining in the liquid at least ten minutes, may be thoroughly saturated. When dry, excite by brushing with a 40 or 60-grain solution of Ammonio-Nitrate of Silver in the usual way.

Now this formula contains less than one-fifth of the amount of salt often employed, and if a thick foreign paper sized with starch, such as Canson's Positive, were floated upon such a salting Bath, it would be difficult to obtain a good picture. By immersing however a paper sized with Gelatine like the one recommended, a much larger quantity of salt is retained upon the surface, and the film is sufficiently sensitive. There are three modes of applying solutions, viz. by brushing, floating, and immersion. The quantity of solution left on the paper varies with each, and consequently each requires a different formula. Immersion in a strong salting Bath tends to give a coarse picture wanting in definition; whereas the plan of brushing a weak salting solution, produces a paper deficient in sensitiveness, and yielding a pale red image without proper depth of shadow.

But independent of these differences, the chemical nature of the size employed also influences the toning of the Print. For instance, in the process above given, if the Positives, after having been fully toned in the Gold Bath, and washed in cold water, be treated with boiling water, the tint immediately changes to a dull red; but on blotting off between sheets of bibulous paper and pressing with a hot iron, the dark tones are restored.

This destruction of the tint by boiling water, and its restoration by dry heat, is due in great part to the animal substance employed in sizing the paper; and it will be found that prints upon a foreign paper, such as the Saxony Positive, salted with a ten-grain solution and sensitized with Ammonio-Nitrate, do not lose their tones in hot water and are not much darkened by ironing.

The peculiarity of the sizing of the English Photographic papers must therefore be borne in mind, and allowance made for the additional sensitiveness and alteration of colour which it produces. When a formula is given, the paper which is recommended for that particular formula should alone be used.

SECTION II.

Positive Printing by Development.

Negative printing processes will be found useful during the dull winter months, and at other times when the light is feeble, or when it is required to produce a large number of impressions from a Negative in a short space of time. The plan of development also enables the operator to obtain Positives of greater stability than those yielded by the direct action of light.

Three processes may be described, the first of which gives Positives of an agreeable colour, but the second, on Iodide of Silver, the greatest permanency under unfavourable conditions.

NEGATIVE PRINTING PROCESSES UPON CHLORIDE OF SILVER.

Positives may be obtained by exposing paper prepared with Chloride of Silver to the action of light until a faint image is perceptible, and subsequently developing by Gallic Acid; but in this process it is difficult to obtain sufficient contrast of light and shade; the impression, if sufficiently exposed and not too much developed, being feeble, with a want of intensity in the dark parts. By associating with the Chloride an organic salt of Silver, such as the Citrate, this difficulty may be overcome, and the shadows be brought out with great depth and distinctness.

The papers are salted with a mixed Chloride and Citrate as in the formula for the Ammonio-Nitrate Process.[49] They are then rendered sensitive upon a Bath of Nitrate of Silver containing either Citric or Acetic Acid, which are used in Negative processes to preserve the clearness of the white parts under the influence of the developer.

[49] The formula at p. 246 may be modified with advantage: use double the quantity of Gelatine, and half the amount of Citrate and Chloride.