 |
|
Apothecaries' weight is used throughout this article. In case of liquids the abbreviation oz. signifies fluidounce.
1. By nitre process: Oil of vitriol (s. g. 1.70) 6 oz.; dried nitrate of potash, 3 1/2 oz.; water, 1 oz. Mix the acid and the water in a porcelain vessel, and add the pulverized nitre, gradually stirring with a glass rod until the lumps disappear and the mixture becomes transparent. Place a thermometer in the mixture, and when it indicates between 145° and 150° Fahr., the cotton should be immersed. Take 60 grs. of clean cotton, and separate it into 10 or 12 bolls, and immerse the bolls separately, and leaving the whole in the mixture for 10 minutes. Should the temperature fall to 140°, float the cup on boiling water, and maintain it between 140° and 150°. At the expiration of 10 minutes lift the cotton with glass rods, and squeeze out the acid quickly and dash the mass into a large vessel of clean, cold water, separating the mass so as to wash it thoroughly and quickly. Complete the washing by immersion for several hours in running water, then spread it out to dry spontaneously.
2. By mixed acids: Oil of vitriol (s.g. 1.845), 18 oz.; nitric acid (s. g. 1.457), 6 oz.; water, 5 oz. Mix the nitric acid and water in a porcelain vessel, then add the oil of vitriol and mix thoroughly and allow the mixture to cool to 150° Fahr., when immerse the cotton. Take 300 grs. of clean cotton, well loosened, and immerse piecemeal, so as to saturate thoroughly with the acids. Allow the whole to remain 7 minutes, after which time lift it out with the rods and wash it precisely as directed in last process.
Pure alcohol (s. g. 0.805), 10 oz.; pure ether (s. g. 0.725), 20 oz.; prepared cotton, 300 grs. Pour the alcohol into a 40 oz. glass bottle, add the cotton, and shake until the cotton is thoroughly wetted; then add the ether, shake well and set away in a cool, dark place for several weeks to settle. Mark "Plain Collodion." In very warm weather increase the proportion of alcohol by addition a day before use or at time of iodizing.
Iodide of ammonium, 90 grs.; iodide of cadmium, 30 grs.; bromide of ammonium, 40 grs.; alcohol (8.10), 10 oz.
Or, iodide of magnesium, 200 grs.; bromide of cadmium, 50 grs.; alcohol (810), 10 oz.
Pulverize the salts, and add gradually to the alcohol, commencing with the bromide; shake until completely dissolved, and set away in a dark place. Mark "Iodizing Solution."
Plain collodion, 3 oz.; iodizing solution, 1 oz. Mix and set away in a dark, cool place 12 hours before using.
Any of the ordinary iodides or bromides can be substituted in the above formula.
For positives: Pure re-crystallized nitrate of silver (437 gr.), 1 oz.; pure nitric acid, 3 minims; alcohol, 2 drs; distilled water, 10 oz.; iodide of potassium, 1 gr. Dissolve the nitrate of silver in 3 oz. of the distilled water, add the iodide of potassium, shake and allow to settle; test for acidity with blue litmus paper, and, if present, neutralize carefully with a solution of carbonate of soda. When neutral, add the remaining 7 oz. of distilled water, filter and add the alcohol and nitric acid, and the bath is ready for use.
For negatives: Pure recrystallized nitrate of silver, 1 oz.; glacial acetic acid, 5 minims; alcohol, 3 drs.; iodide of potassium, 1 gr.; distilled water, 10 oz. Dissolve the silver in 8 oz., and treat precisely as for bath for negatives, observing that it is to be acidified with glacial acetic acid in place of nitric acid.
These baths should be kept in a dark place, and always show an acid test. When out of order, boil for a few minutes, add one-tenth volume of distilled water and restore to the original strength by adding strong solution of crystallized nitrate of silver in distilled water, and acidifying with the proper acids.
For positives: 1. Pure sulphate of iron, 150 grs.; glacial acetic acid, 6 fl. drs.; water, 10 oz.; nitric acid, 2 minims. Dissolve the sulphate of iron in the water, and add the acetic and nitric acid and cork tightly.
2. Sulphate of iron, 480 grs.; nitrate of baryta, 320 grs.; alcohol, 1 oz.; nitric acid, 30 minims; water, 10 oz. Powder the nitrate of baryta and dissolve in the water warmed; when dissolved, add the powdered sulphate of iron stirring for a few minutes, filter, and when the liquid becomes cold, add the nitric acid and alcohol separately. Bottle and cork tightly
For negatives: 1. Pyrogallic acid, 10 grs.; glacial acetic acid, 3 drs.; distilled water, 10 oz. Dissolve the pyrogallic acid in the water, add the glacial acetic acid, cork tightly.
2. Sulphate of iron and ammonium, 2 oz., or sulphate of iron, 1 1/2 oz.; glacial acetic acid, 1/2 oz., or acetic acid No. 8, 1 1/2 oz.; alcohol, 1/2 oz.; distilled water, 10 oz. Add to the distilled water in the order indicated, the iron-salt to be first dissolved. In warm weather this developer requires dilution, and must be washed from the plate the instant the details appear.
For positives: Cyanide of potassium, 120 grs.; nitrate of silver, 6 grs.; water, 10 oz. When this solution requires more than a minute or two to clear the picture, add a small amount of cyanide of potassium.
[This solution is highly poisonous, and should not be allowed to touch unsound skin, nor should the fumes be breathed.]
For negatives: The above solution of cyanide of potassium answers very well, or hyposulphite of soda, 10 oz.; water, 10 oz.; ether, 30 minims.
Should be employed after the picture has been developed, fixed and carefully washed:
Nitrate of silver, 200 grs.; distilled water, 10 ounces.
Or, Bi-chloride of mercury, 30 grs.; distilled water, 40 oz.
Before washing the glasses, each square should be roughened on the edges and at the corners by means of a file, whetstone, or a sheet of emery paper; or more simply by drawing the edges of two plates across each other.
A cream of Tripoli powder and spirits of wine with a little ammonia added, is commonly employed. A tuft of cotton is to be dipped in this mixture and the glasses well rubbed with it for a few minutes.
After wiping the glass carefully, complete the process by polishing with an old silk handkerchief, avoiding contact with the skin of the hand. Leather may be used instead of silk for the final polishing; they must be first beaten, then washed in pure water, dried in the sun and well pulled out until they are soft and yielding. Before deciding that the glass is clean, hold it in an angular position and breathe upon it.
Other modes of cleaning glasses have been recommended, of which, perhaps, the residues of collodion are the most simple. Add a fluidrachm of water to the ounce of collodion until the pyroxyline begins to form a white deposit not redissolved on agitation: this will prevent the collodion from evaporating too quickly. Then pour a little upon the glass and rub with a tuft of wool or piece of cambric. Clean off with a second piece, and finish with cloth and leathers as before, no water will be required.
New glass plates are frequently dotted on the surface with little gritty particles, which consist of carbonate of lime. They are not removed by potash or any alkali, but dissolve readily in a diluted acid, such as oil of vitriol, with about four parts of water added, or dilute nitric acid.
Water, 1 pt.; sulphuric acid, 1/2 oz.; bichromate potash, 1/2 oz. The glass plates, varnished or otherwise, are left, say 10 or 12 hours or as much longer as desired, in this solution, and then rinsed in clean water and wiped or rubbed dry with soft white paper. It removes nitrate of silver stains from the fingers.
When the collodion is properly cleared from sediment, take a glass plate, previously cleaned, and wipe it gently with a broad camel's-hair brush in order to remove any particles of dust which may have subsequently collected. If it be a plate of moderate size it may be held by the corners in a horizontal position, between the forefinger and thumb of the left hand. The collodion is to be poured on steadily until a circular pool is formed extending nearly to the edges of the glass,
By a slight inclination of the plate the fluid is made to flow towards the corner marked 1 in the above diagram, until it nearly touches the thumb by which the glass is held; from corner 1 it passes to corner 2 held by the forefinger, from 2 to 3 and lastly, the excess poured back into the bottle from the corner marked No. 4. It is next to be held over the bottle for a moment until it nearly ceases to drip and then by raising the thumb a little the direction of the plate is changed so as to give a rocking movement, which makes the diagonal lines coalesce and produces a smooth surface.
The operation of coating a plate with collodion must not be done hurriedly, and nothing is required to ensure success but steadiness of hand and a sufficiency of the fluid poured in the first instance upon the plate.
With regard to the time which ought to elapse between coating and dipping, observe the following: After exposing a layer of collidion to the air for a short time, the greater part of the ether evaporates and leaves the pyroxyline in a state in which it is neither wet nor dry; but receives the impression of the finger without adhering to it. This is termed setting, and when it takes place the time has come for submitting the plate to the action of the bath.
When the plate is ready, rest it upon the glass dipper, collodion side uppermost, and lower it into the solution by a slow and steady movement and let it remain until the oily appearance on the surface disappears.
After the plate has been taken out of the bath it should be exposed and developed with all convenient despatch, otherwise the film will become partially dry, the developing solutions will not flow easily, and the negative will be weak and metallic.
The pyrogallic acid solution having been previously measured out (about 3 drs. for a stereoscopic plate or a plate 5 X 4, 1 oz. for a 9 X 7, and 12 drs. for a plate l0 X 8), hold the glass in the band in the same manner as when coating it with collodion, and flow the liquid on evenly.
This reducing agent developes the picture so rapidly, when the collodion contains only iodide, that its employment requires care. The solution should be thrown on to the plate rather quickly and with a sweep. In the course of a few seconds the image appears in all its parts, and the liquid on the film shows signs of commencing turbidity. When this happens, the developer must be poured off immediately, and the plate washed with water, otherwise the shadows will be misty from adherent particles of silver. Plates developed with sulphate of iron may be further blackened by washing the film and pouring over it a 20 gr. solution of nitrate of silver, followed by a second application of sulphate of iron. A better process, however, is to wash away all traces of the iron salt by a stream of water applied for a few seconds, and then to mix the ordinary solution of pyrogallic acid with nitrate of silver.
Wash the film gently with water, and pour the solution of hyposulphite on and off until the whole of the iodide has been cleared away. The solution of hyposulphite becomes nearly black after a time, but this is of no importance, some operators considering that even a preliminary washing to remove the pyrogallic acid solution is unnecessary. A thorough washing after the hyposulphite will be essential. After the negative has been washed flow it over with a solution of gum Arabic, stand it on clean blotting-paper to dry; otherwise dust will ascend the film by capillary action, and give an appearance as if impurities had drained down from above. It is important that the plate should be dried by artificial heat before varnishing, and the negative will also look more neat if a damp cloth be first run along the edge with the finger and thumb, so as to scrape away the collodion, and leave a clear margin of 1/8 to 1/4 of an inch all round. After doing so remove with the nail or a camel's-hair brush any loose or detached particles of collodion, which otherwise might be washed on to the image by the varnish, and produce a spot.
Take a photograph of a printed sheet with the full aperture of the portrait lens, the central letters being carefully focussed as before. Then examine at what part of the plate the greatest amount of distinctness of outline is to be found. It will sometimes, happen that whereas the exact centre was focussed visually, the letters on a spot midway between the centre and edge are the sharpest in the photograph. In that case the chemical focus is longer than the other, and by a distance equivalent to, but in the opposite direction of, the space through which the lens has to be moved, in order to define those particulars sharply to the eye.
In developing a glass positive, the solution of sulphate of iron should be flowed evenly over the film, and in some quantity, so as to wash off a portion of nitrate of silver into the sink.
Forty grs. of pyrogallic acid being dissolved in 1 oz. of glacial acetic acid, and 20 minims added to 1 oz. of water when required for use. The strong solution becomes black from decomposition in the course of a few weeks, but when diluted it has only a faint yellow tint, and is tolerably effective in bringing out the image. In place of acetic acid strong alcohol may be used as a solvent, 1/2 a dr. of pyrogallic acid being dissolved in 1 oz. of spirit, and 20 minims of the resulting liquid added to each oz. of acidified water. No attempt must be made, however, to combine acetic acid and spirit in one solution, since abundance of acetic ether would be generated by so doing. These plans of preparing a concentrated developer are useful for a few weeks' keeping, but are not recommended for an unlimited time
To photograph a full-sized steel engraving on a plate not larger than 7X6 or 5X4, is a very simple operation, and no special directions will be needed. Remove the engraving from its frame (the glass would cause irregular reflection), and suspend it vertically and in a reversed condition, in a good diffused light, placing a black cloth behind it, if any bright reflecting surface be presented to the lens. Point at it a camera mounted with a portrait-lens, and if the image upon the ground glass appears misty towards the edge, make a diaphragm, and place it in front of the anterior glass. Instead of a portrait-lens an ordinary view-lens may be used, and as the field to be covered is small, a comparatively large diaphragm will suffice.
Be careful not to over-expose the plate, develop with pyrogallic acid, and fix with hyposulphite.
The additional deposit obtained, as above described, will often be found sufficient, and when such is the case the method is to be preferred, because the half-tones of a photograph are easily obliterated by too much intensifying. In the case of large copies of maps taken with orthoscopic lenses of long focus, the iodine method proves insufficient, and the fine lines of the drawing become partly obliterated during the prolonged development. In such a case it is advised to develop the plate only partially in the first instance, and to fix it with cyanide of potassium; then to intensify twice with pyrogallic acid, citric acid, and nitrate of silver, and lastly to treat the plate with the two following liquids: No 1. Iodine, 6 grs.; iodide of potassium, 12 grs,; water, 6 oz. No. 2. Sulphide of potassium, 1 dr.; water, 6 oz.
Apply No. 1, either in the yellow room, or in the daylight, until the whole of the image is converted into iodide of silver, and the deposit appears yellow throughout. Then pour water on the image from a jug, and apply No. 2, which must be allowed to remain until the yellow color changes to a deep reddish-brown. Lastly, dry the plate, and varnish in the usual way.
Photographs for the refracting stereoscope are taken with small lenses of about 4 1/2 inches focus. For portraits a camera may advantageously be fitted with two double-combination lenses of 1 3/4 inches diameter, exactly equal in focal length and in rapidity of action. The caps are removed simultaneously, and the pictures impressed at the same instant. The centres of the lenses may be separated by 3 inches, when the camera is placed at about 6 or 8 feet from the sitter. Pictures taken with binocular camera of this kind require to be mounted in a reversed position to that which they occupy on the glass, or a pseudoscopic effect will be produced. The negatives may be cut in half, the right half being printed on the left side, or the finished prints may be removed before mounting.
Sensitive films prepared by Russell's Tannin Process. Place the negative and the prepared plate in contact, and squeeze them together in an ordinary pressure-frame; not too strongly, however, or it will probably be found, after throwing off a few impressions, that the negative has been scratched. Lay a strip of black velvet behind the sensitive film to absorb stray light.
Take of hyposulphite of soda, 1 oz.; water, 6 oz. Dissolve without filtering, and preserve the solution in a stock-bottle ready for use.
Formula 1. - Take of chloride of ammonium, 200 grs.; water 5 oz.; albumen, 15 oz. Chloride of barium is sometimes used in salting paper, instead of chloride of ammonium, but is contraindicated when the alkaline gold-toning process is adopted, since the carbonate of soda would throw down carbonate of baryta in the paper.
When pure albumen is used without water, from 5 to 8 grs. of salt to each ounce will be sufficient. The less the quantity of salt the warmer the color, but it must not be so far reduced as to injure the contrast and depth of shadow in the print.
If distilled water cannot be procured, rain-water, or even common spring-water, will often answer the purpose. For the albumen use eggs nearly fresh, and be careful that in opening the shell the yolk be not broken. Each egg will yield about one fluidounce 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 very much upon this part of the process, for, if the albumen be not thoroughly beaten, cakes of animal membrane will be left in the liquid, and will cause streaks upon the paper. When the froth has partially subsided transfer the liquid part 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 will be fit for use. Albuminous liquids are too glutinous to run well through a paper filter, and are better cleared by subsidence. Lower the paper on the liquid by one steady movement, since, if a pause be made, a line will 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 oxgall sold by the artiste' colormen, 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.
This operation must be conducted by the light of a candle or by yellow light.
Take of nitrate of silver, 60 grs.; distilled water, 1 oz. Prepare a sufficient quantity of this solution, and pour it out into a porcelain dish. After it has been a short time in use, the albumen, dissolved out of the papers, will cause a greasy scum to form upon the liquid, which, if allowed to remain, produces marble stains upon the sensitive paper; it must therefore be removed by folding a strip of blottingpaper the exact breadth of the dish, and drawing it lightly along the surface; lay the sheet upon the solution in the same manner as above described for the albumen. Three minutes' contact will be sufficient with thin paper, but if a thick paper be used 4 or 5 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 sealingwax; or a pin may be used to lift up the corner, which is to be held by the finger and thumb, and allowed to drain a little before again putting in the pin, otherwise a 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.
Take of chloride of ammonium, 200 grs.; citrate of soda, 200 grs; gelatine, 20 grs.; water, 20 oz. To prepare the citrate of soda dissolve 112 grs. of citric acid in 20 oz. of water, and add 133 grs. of the dried bicarbonate or sesquicarbonate of soda used for effervescing draughts. Supposing the citric acid to be adulterated with tartaric acid, the above quantity of carbonate of soda would be too great, and free alkaline carbonate would then remain in the liquid after the neutralization was complete. The size of the paper would be liable to suffer in such a case, and the print would not be clean and bright. Excess of citric acid, on the other hand, gives very clean pictures, but they are too pale and red, without depth of shadow. Amateurs, desirous of simplifying the formula, may substitute an equal weight of "Rochelle salt" for the citrate of soda. This substance is a tartrate of potash and soda, and is sold by druggists in large crystals. Both tartrates and citrates are used for the purpose of giving a red and warm tone to the prints.
Render sensitive by floating for 2 or 3 minutes upon the solution of nitrate of silver employed for the albuminized paper.
This is always prepared without albumen, which is dissolved by ammonio-nitrate of silver,
Take of chloride of ammonium, 40 grs.; gelatine, 20 grs.; water, 20 oz. Dissolve by the aid of heat, and filter when cold.
Take 10 or 12 sheets of thin Saxe paper, and, having marked the right side, immerse them bodily in the liquid, 1 by 1, with care to remove air-bubbles, then turn the batch over, and remove them singly, beginning with the sheet first immersed.
Each paper will thus be a similar length of time in the salting liquid.
This salting solution is very weak, but it must be borne in mind that the papers being immersed will take up a large quantity, and also that the ammonio-nitrate process requires less salt, inasmuch as the silver solution is to be laid on with a brush.
An ammonio-nitrate paper, yielding a very rich color, is made by salting Towgood's paper, or Papier Saxe, with a mixed chloride and citrate, in quantity exactly one-half of that advised in the last page, and afterwards sensitizing it with an 80 gr. solution of ammonio-nitrate.
Render sensitive by a solution of ammonio-nitrate of silver, 60 grs. to the oz. of water, 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 redissolved. 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. From 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. Or, dissolve 60 grs. of nitrate of silver in 1/2 oz. of water, and drop in ammonia until the precipitated oxide of silver is exactly redissolved; then divide this solution of ammonio-nitrate of silver into 2 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; them mix the two together, fill up to 1 oz. of water, and filter from the milky deposit of chloride or carbonate of silver, if any be formed.
Ammonio-nitrate of silver should be kept in a dark place, being more prone to reduction than the nitrate of silver.
It is not usual to float the paper when the ammonionitrate of silver is used. Brushes are manufactured expressly for applying silver solutions, but the hair is soon destroyed unless the brush is kept scrupulously clean. Lay the salted sheet upon blotting-paper, and wet it thoroughly by drawing the brush first lengthwise 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 paper was too highly salted, or that the ammonio-nitrate contained free ammonia.
Ammonio-nitrate paper is more prone to spontaneous decomposition than either albuminized or plain paper; hence it cannot be kept many hours after sensitizing without turning yellow.
No. 1. Solution of chloride of gold, 1 dr.; sesquicarbonate of soda, 10 grs.; distilled water, 6 oz. No. 2. Solution of chloride of gold, 1 dr.; ordinary phosphate of soda, 20 grs.; distilled water, 2 oz..
Examine the chloride of gold, and if its solution immediately colors litmus-paper of a bright red, add to each grain a fragment of carbonate of soda about the size of a pin's head.
The solution of chloride of gold contains a grain to each fluidrachm of water, and will keep for an unlimited time without appreciable change, previously to the addition of the carbonate of soda. The toning baths, however, must not be kept ready mixed, since they gradually become colorless and eventually lose their toning properties in great measure. A useful simplification, substituting measure for weight, consists in having always on hand an aqueous solution of carbonate of soda containing 20 grs. to the oz.; or for the second formula, a solution of phosphate of soda containing 40 grs. to the oz. Half an oz. of the alkaline liquid would then in each case require a fluidrachm of the solution of chloride of gold and a subsequent dilution with water to the full amount given in the formula.
Sensitive papers ought not to be exposed in the frame until they are quite dry. The shutter at the back of the frame 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, next comes a layer of thick felt; and the whole is then tightly compressed by replacing and bolting down the shutter. The amount of pressure required is not very considerable, but if the springs of the frame become too weak after a time, a few pieces of mill-board may be placed beneath them.
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 weather.
If the exposure to light has been correct 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 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.
If, in the exposure to ordinary diffused daylight, the shadows of the proof became very decidedly coppery before the lights are sufficiently printed, the negative is in fault. Ammonio-nitrate paper highly salted is particularly liable to this excess of reduction, and especially so if the light is powerful.
The print should be first washed in common water until the soluble nitrate of silver is removed. 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. Ten minutes in water running slowly from a tap will be sufficient to cleanse a print from nitrate of silver; or three or four changes in a dish, pouring off quite dry between each change. It is an advantage to finish off with a solution of salt (2 grs. to the oz.) Pour the toning bath out into a flat dish, and put the prints into it 2 or 3 at a time, waving the dish meanwhile backwards and forwards to secure a constant movement. Continue to keep the prints moving, and watch the changes in color.
If the prints are removed as soon as the blue color of the gold is seen, they will usually change in the fixing bath to a warm shade of brown, but when left for 2 or 3 minutes longer in the toning bath, the darker tint becomes permanent.
One oz. of hyposulphite of soda dissolved in 3 oz. of water would fix two batches of stereoscopic prints, 20 in each batch. Allow the prints to remain in the fixing bath for 20 minutes, with occasional movement after which they may be transferred to a dish of clean water.
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 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 4 or 5 hours, and the prints should not lie together too closely, or the water will not find its way between them.
When the prints have been thoroughly washed, blot them off between sheets of porous paper and hang them up to dry.
Mount the proofs with a solution of gelatine in hot water, freshly made; gum-water, prepared from the finest commercial gum, and free from acidity, may also be used, but it should be made very thick, so as not to sink into the paper, or produce "cockling up" of the cardboard on drying.
Negative printing processes will be found useful during the dull winter months, and at other times when the light is feeble, or where it is required to produce a large number of impressions from a negative in a short space of time. The proofs, however, as thus obtained, are not equal to direct sun-prints in beauty and gradation of tone.
Take of iodide of potassium, 120 grs.; bromide of ammonium, 30 grs.; water, 20 oz.
Float the paper on the iodizing bath until it ceases to curl up, and lies flat upon the liquid: then pin up to dry in the usual way.
Render sensitive upon a bath of aceto-nitrate of silver, containing 30 grs. of nitrate of silver with 30 minims of glacial acetic acid to each oz. of water.
Place the dried sheets in contact with the negative in a pressure frame, and expose to a feeble light. About 30 seconds will be an average time upon a dull winter's day, when it would be impossible to paint in the ordinary way. Develop by immersion in a saturated solution of gallic acid. After the picture is fully brought out, wash in cold and subsequently in warm water, to remove the gallic acid, which, if allowed to remain, would discolor the hyposulphite bath. Fix the print in a solution of hyposulphite of soda, one part to two of water, continuing the action until the yellow color of the iodide disappears. Wash thoroughly in plenty of water.
To salt the papers, prepare the following solution: Chloride of ammonium, 100 grs.; citric acid, 56 grs.; sesquicarbonate of soda, 66 grs; gelatine, 40 grs.; distilled water, 20 oz. Float the sheets for 1 minute; render sensitive upon a bath of aceto-nitrate of silver. Take of nitrate of silver 30 grs.; glacial acetic acid, 30 minims; water, 1 oz. Float the papers upon this bath for 3 minutes, and suspend them to dry. The exposure to light is conducted in the ordinary printing frame.
The developing solution is prepared as follows: Gallic acid, 2 grs.; water, 1 oz. Filter through blotting-paper, to remove floating particles, which would produce spots in this process; pour the solution of gallic acid into a glat dish, and immerse the prints, 2 or 3 at a time, moving them about, and using a glass rod to remove air-bubbles; wash the prints for several minutes in 1 or 2 changes of water, in order to extract the gallic acid; tone in bath of chloride of gold and phosphate of soda; fix in hyposulphite, and wash.
In order to clean the plates, make a stock solution of 2 oz. of bichromate of potash, 2 oz. of sulphuric acid, and 1 qt. of water (a green quart wine bottle may be used). Pour the solution into a shallow tray; soak in it the plates to be cleansed, and ofterwards wash the plates in water and dry them; then pour on each plate a small quantity of old collodion; rub it all over the plate with a tuft of cotton, and then clean it off with a dry linen cloth. It will perfect the cleaning if the plate be dipped in pure water and again dried with a clean linen cloth. The bichromate of potash solution may be filtered back into the bottle for future use.
1. Cover the plate with a bromised collodion. Take care, in all cases, gently to oscillate on running off the excess of collodion, or white marks, visible in a subsequent stage of the process, will be formed.
2. When the film is somewhat fixed, put the plate, by the aid of a dipper, in a well-bath of a 50 gr. nitrate of silver solution, and let it remain in the solution 10 minutes.
3. Put the plate in a well-bath of distilled water until what are called the "greasy" marks disappear.
4. Put the plate in a well-bath of a 10 gr. solution of bromide of ammonium, or cadmium, or magnesium, or of potassium. It may be best to adopt that bromide which enters into the preperation of the collodion. Let the plate remain in this solution about 1 minute. Occasionally filter this bath solution, in order to remove the excess of bromide of silver which is deposited in the bath. Keep a stock bottle of this bromide solution to supply the waste drippings.
5. Put the plate for a short time in a well-bath of distilled water.
6. Wipe the back of the plate, and let it drain for a short time, and then place it in a well-bath of 10 or 15 grs. of tannin to the oz. of distilled water; let the plate remain in this solution 4 or 5 minutes, and then, on removing it from this bath, let it rest on one end, to dry. When dry, it is ready for the camera, and it will keep a long time. It should be rapidly and evenly dried, and a stone bottle of hot water may be so used as to hasten the drying. Tannin which dissolves most easily in water, is to be preferred. Add to the tannin bath 3 or 4 drops of creosote, and it will keep clear. A little experience will show how long a time the plate must be exposed to light in the camera. It will be best, in the beginning, not to make a very short exposure; harsh pictures are the result of long exposures, and soft pictures of short exposures to the light. These plates will bear a considerable amount of exposure in the camera without being fogged - even for 10 minutes.
1. The film sometimes appears to be in a rotten state, and to crack up when water is poured on it. This will not happen if a small quantity of alcohol be added to the water solution first poured over it.
2. Take a solution of 2 grs. to the oz. of water of carbonate of ammonia; add to it some alcohol, and flush the plate with it; pour this off into a glass, and add to it from 1/2 gr. to 1 gr. of pyro-gallic acid, in solution, and pour it back again on the plate. This picture will instantly appear, and the details may be allowed to come out well. The image will not be dark.
3. Pour off the alkaline pyro-solution, and gently wash the plate; add a few drops of citric acid (1 or two grs. of citric acid to the oz. of distilled water); pour it over the plate, and run off; or pour an acid pyro-solution over the plate instead of acid solution alone. Again gently pour distilled water on the plate, and run it off.
4. It will be found to be convenient to dissolve 1 dr. of pyrogallic acid in 1 oz. of alcohol, and to add to this 1 oz. of distilled water. Use 1 dr. graduate, and 16 minims of this solution will give 1 gr. of pyrogallic acid; also have a 2 oz. bottle of distilled water and dissolve in it 2 drs. of citric acid; 8 minims of this will be 1 gr. of citric acid. Have also an oz. bottle of a 10 gr. solution of nitrate of silver: and lastly have at band a pint bottle of distilled water, 1 or 2 glass oz. graduates, and 3 or 4 developingglasses, which hold about a fluidounce.
5. The development of a plate is completed by solutions of 1/2 gr. to 1 gr. of pyrogallic acid, and 1 gr. or more of citric acid to the oz. of water, and the addition of 1 drop to 3, 4, or more of the 10 gr. nitrate of silver solution. If the picture is strongly out when the ammonia is washed off, let it be dosed with an excess of acid silver; if feeble, let an excess of pyrogallic acid be used. If there be a sign of fogging when pushing the development with an excess of nitrate of silver, immediately pour off the developer, gently wash the plate and flush it with the 8 gr. bromide solution. The image, as it at first shows its strength or weakness, very soon teaches the manner in which the plate is to be dealt with.
Lastly. Gently wash the plate, fix the image with a strong solution of hyposulphite of soda, and, when the picture is "cleared," wash it with distilled or common water, and dry it.
The three steps which have made the tannin process of Major Russell simple, clean, and certain, are: First, the use of the bromide bath; second, the use of the tannin bath; and third, the use of the acid solution, either alone or mixed with pyrogallic acid, after the alkaline solution of carbonate of ammonia has been washed off, not forgetting the mixture of some alcohol with the first application of water to the film on the development.
The directions for coating and dipping in the bath are the same as for "Russell's Process." After removing the stereoscopic plate from the nitrate bath, stand it vertically on blotting paper and allow it to drain as long as it would be required for a wet plate, wiping the back meanwhile in the usual manner.
Then take it in the left hand, either by one corner or with the pneumatic holder, and having arranged the yellow light so that it falls nicely upon the surface of the film; hold it quite level and pour on 1/2 oz. of water, waving it backwards and forwards. It should not be poured on entirely at one spot, or too much of the nitrate of silver would be displaced, producing a circular mark of imperfect development. As, however, a vacant space of a quarter of an inch in breadth is usually allowed at each end of a stereoscopic plate; this will be found convenient for pouring on the water which must be allowed to run into each corner until greasiness has disappeared. It will probably touch the fingers by which the plate is held, and when they are not scrupulously clean, stains will, in consequence, result. After pouring away the water (which ought then to measure at least 3 drs. if the operation was well performed), drain the plate for an instant, and it will be ready for the albumen.
Take the white of a fresh egg and add to it 1 oz. of distilled water with 3 drops of strong ammonia. Shake for 10 minutes in a bottle, and pour out the liquid on a filter previously wetted.
Apply the filtered albumen twice or thrice to the film in the same manner as collodion. Then pour water over the plate for a minute, in order to remove the excess of albumen, and afterwards rear the glass on end to drain upon blotting paper. When the plates are perfectly dry they are ready for exposure.
This process is not quick as regards development. When pyrogallic acid is used, each drachm of the liquid is previously mixed with about 5 minims of a solution of nitrate of silver, containing 20 grains to the oz. of water. The film is wetted and the mixed developer poured on and off from a glass measure.
Fix with hyposulphite of soda.
This part of the process must be conducted with care.
Employ 2 nitrate baths in this process, one for the collodion and the other for the second film, viz. the albumen. The albumen nitrate bath must contain free acetic acid, and therefore if one solution be employed for both films it should be a bath of aceto-nitrate.
Sensitize the collodion in the ordinary way, holding it rather longer than usual before dipping.
Having arranged 2 dishes of common water, side by side, lay the plate face uppermost in the first dish, and wave the water backwards and forwards for about 30 seconds. Then put the plate into the second dish and leave it whilst another glass is being coated and immersed in the sensitizing bath. Now drain plate No. 1 closely on blotting-paper, and it will be ready for the iodized albumen. Plate No. 2 remains in the sensitizing bath until the first glass has been coated with albumen and placed to dry.
Take of albumen, 3 oz.; distilled water, 1 oz.; strong ammonia, 10 minims; iodide of potassium, 10 grs.; bromide of ammonium, 10 grs.
First mix the ammonia and the water, then add the other ingredients and shake together in a bottle. Iodide of ammonium may be advantageously used.
For a stereoscopic size, measure out 1 dr. of the albumen and pour it on and off twice to displace the surface water of the washed collodion film. Then apply a second quantity.
Stand the albuminized plates vertically on blotting paper to drain and dry.
Take an ordinary collodion negative bath and add to each fluidounce 30 minims of glacial arctic acid, keep it in glass or gutta percha and continue to use it until it has become blackened by the action of the albumen. The film of albumen must be rendered quite dry by holding it to the fire before it is dipped in the aceto-nitrate bath. Leave them in the bath any time between 30 seconds and 2 minutes, and then remove the wash with water. Use two dishes for washing and allow 20 or 30 seconds in each dish; then rear up again to dry, and the plates will be ready for exposure. Artificial heat may be used with advantage.
The most successful operators in Taupenot's process give a very long exposure.
Prepare a saturated solution of gallic acid in distilled water, adding 4 grs. to each oz. Filter this developer through paper.
The solution of nitrate of silver for use with the gallic acid may be made of the strength of 20 grs. to the ounce.
Add 15 minims to 1 oz. of gallic acid solution. Previous to the application of the developer the surface of the film must be moistened with water.
From half an hour to an hour must be allowed for the full development with gallic acid. Fix with a saturated solution of hyposulphite.
The plates for these pictures are sold as porcelain, but are really an opaque white glass. The plate is coated with collodion, rendered sensitive in the usual way, and the image received from a negative adjusted as follows:
The negative is placed at one end of a box, the other end of which joins the front end of a camera having a quarter tube. The object of this box is to cut off extraneous light. The negative is placed toward the sun, or a piece of white paper placed in the sun. The image formed by the negative is focussed on the ground glass of the camera, in the usual manner. The box in front should admit of being varied in length, in order to alter the size of the picture, by changing the distance of the negative from the lens.
The white sensitive plate is then exposed and developed with the ordinary iron developer. After washing off with the developer, fix with a weak solution of cyanide of potassium.
The picture requires to be darkened with a weak solution of bichloride of mercury chloride of gold, or chloride of platinum. As soon as the picture appears of the proper shade arrest the action of the strenghthening agent, by washing the plate under a copious stream of water.
Dissolve the ordinary commercial nitrate of uranium in distilled water; add ammonia till no more precipitate is formed. This precipitate is uranate of ammonia, insoluble in water. Wash in several waters, and then redissolve in nitric acid. The solution, crystallized and dried over a water-bath, constitutes the "uranammonium nitrium" of the specification, and is probably a double salt of uranium and ammonia. If it be very acid, dissolve in distilled water and again crystallize.
To a saturated solution of the above salt in 6 oz. of distilled water add 220 grs. of nitrate of silver, previously dissolved in 1 oz. of distilled water and intimately mix. This mixture, when crystallized over a water-bath, is the salt used to sensitize the collodion. It may be kept in a bottle, but not necessarily in a dark place, because it is only sensitive to light when brought into contact with organic substances.
Dissolve 3 oz. of this salt in 10 oz. of alcohol, to which 5 drs. of distilled water and a few drops of nitric acid have been added. This is the sensitizing liquor, and does not require to be kept in the dark.
The collodion may be the ordinary plain preparation, containing about 5 1/2 grs. pyroxyline (not powdery) to the ounce of ether and alcohol, mixed in the proportion of 6 to 2; but, in order to give it an organic reaction with the sensitizing salt, it is necessary to add to each pint of the collodion about 10 drops of so-called "harz oel," which is thus prepared: Take equal parts of castor oil and Canada balsam; dissolve the latter in sufficient ether to enable it, when mixed with the castor oil, to pass through a filter. After filtration, evaporate the mixture over a water-bath till it is of the ordinary consistence of a fatty oil. The collodion is now ready for being sensitized.
To every 3 oz. of the collodion thus prepared add from 1 to 1 1/2 oz. of the sensitizing solution with a few drops of nitric acid, and intimately mix. The collodion is now sensitive to light, and must be kept in the dark.
This is an important preliminary to prevent the image from sinking into the body of the paper. It is effected either by a tolerably strong solution of starch, Iceland or Irish moss, beaten up with about one-eighth its bulk of albumen and a few grains of acetate of lead. The paper may be floated on this in the usual way, and when dry is ready for the reception of the sensitive collodion.
The paper is pinned down by 3 of its corners to a flat board, a little larger than the sheet to be collodionized. The collodion is poured on in the usual way, and the excess run off at the unpinned corner into the stock-bottle. This may be done with great deliberation, and more may be poured on to any part, where there is a deficiency, without much chance of its running in ridges, as would inevitably be the case if a glass plate were so treated. Hang up to dry in a dark room, and it is fit for the printing-frame.
When the picture is printed to the proper strength, that is to say, to the depth which it is wished finally to retain - for uranic developed pictures lose very slightly in the toning and fixing process - place it in a bath containing distilled water 40 oz., acetic acid 1 oz., and hydrochloric acid 1 oz., for the space of 10 minutes. The object of this bath is to remove the uranic salts insoluble in water. The prints are afterwards washed in several waters before being placed in the toning solution.
The ordinary alkaline gold-bath answers very satisfactorily, and tones uranic prints much more rapidly than similar prints on chlorized paper.
If the picture be toned in hyposulphite of soda and gold no further fixing will be necessary; but if in the usual alkaline gold-bath, they will have to pass through a hyposulphite or sulphocyanide solution, to remove the silver salts insoluble in water. When thoroughly washed the process is complete.
Take of bromide of cadmium, 6 grs.; bromide of ammonium, 2 grs.; soluble cotton, 6 grs.; ether and alcohol, each, 1/2 oz. Mix, and after standing a week, filter. Then take 12 grs. crust. nitrate of silver, in fine powder; add to it a drop or two of water, so as to produce a kind of pulp. Then in a chemically dark room, mix the collodion and silver, stirring as the mixture is poured into the bottle intended for its reception. Then shake up well, and allow it to stand.
Take perfectly clean plate-glass, free from scratches; tip the edges for 1/8 of an inch with a solution of 1 gr. of India-rubber in 1 oz. of benzine. Then coat with the collodiobromide of silver, allow it to set the usual time, and place it in a dish of water until the greasy appearance has vanished. Warm water is preferable when it can be procured. When the water flows freely over the film, take the plate out of the dish, wipe the back, and drain for a moment upon a piece of blotting-paper. Then place in the dark slide for exposure in the camera. Expose a little longer than wet collodion, with the nitrate-bath. Wet the film with a little water, and pour over it protosulphate of iron, 25 grs.; glacial acetic acid, 25 minims; water, 1 oz. To 3 drs. of which, 2 drops of a 20 gr. solution of nitrate of silver. Fix with cyanide of potassium 20 grs. to the oz. Intensification may be accomplished by any of the means adapted to wet plates.
Coat the plates as directed in the last paragraph and place them in a tank of water. Take the plates out of the tank in rotation, and place them in water as hot as the hand can hear, for about 30 seconds, and then into a bath of tannin solution of 15 grs. to the ounce of water well filtered. The following tannin solution is preferable: Tannin, 10 grs.; gallic acid, 5 grs.; water, 1 oz.; grape sugar, 5 grs.; alcohol, 10 minims. Dissolve the tannin in a portion of the water and filter; dissolve the gallic acid in another portion by the aid of heat, and filter; mix the two, add the grape sugar, and when dissolved, filter; then add the alcohol. If the plate be allowed to remain in the above solution three minutes, and is properly exposed, very little intensification will be necessary. Dry the plates evenly and quickly, and expose about half the usual time.
Prepare the following solutions:
1. Alcohol and water, each, 1/2 oz.
2. Carbonate of ammonia, 40 grs. to water, 20 oz.
3. Pyrogallic acid, 96 grs. to alcohol, 1 oz.
4. Bromide of potassium, 10 grs. to water, 1 oz.
5. Nitrate of silver, 30 grs.; citric acid, 15 grs.; distilled water, 1 oz.
Pour over the dry plate once or twice, enough of No. 1 to cover it, and return to the bottle for use in the next plate. Then place the plate in a dish of water until the greasy appearance has vanished. Then pour evenly, enough of No. 2 with a few drops of No. 3 and two drops of No. 4 added, and wave to and fro with a rocking motion.
The image should very soon appear, and may be developed until the shadows become slightly tinged. Then wash the surface and back of the plate freely with water, and rinse with a little very dilute acetic acid; say 2 drops of glacial acid to the ounce. Wash again, and if any intensification is required, it may be accomplished by adding to 2 drs. of water 3 drops of No. 3 and 3 of No. 5 solution. When dense enough, wash and fix with cyanide, 20 grs. to the ounce of water.
1. Universal clouding from over exposure or diffused light in preparation or development of the plate, or alkalinity of the bath, or too much nitric acid in bath, or organic matter in the bath, or the use of colorless collodion; also vapors of ammonia or sulphuretted hydrogen. Such negatives may sometimes be recovered by the application of a weak solution of iodine, followed by hyposulphite of soda.
2. Spots upon the plate from excess of bromide of potassium in the collodion, impure nitrate of silver in the bath, super-saturation of the bath with iodide of silver, dust upon the glass or coating, the concentration of nitrate of silver by drying before exposure.
3. Curtain-like marks upon the edge, from the plate being too dry before dipping, not long enough in the bath to remove the greasy appearance.
4. Wavy lines, from the use of a glutinous, thick collodion from want of rocking when pouring off the collodion (common with cadmium sensitigus).
5. Rottenness of film, from bad cotton or dipping too soon after proving before properly set.
6. Oily lines, from the removal from the bath too soon.
7. Curved lines, from the developer not covering the whole plate immediately.
8. Silver stains, from reversing the plate between the bath and slide.
9. Yellow patches, imperfect removal of the iodide of silver in the fixing bath.
10. Scum upon the surface upon removal from the bath, over iodized collodion.
11. Image black and white without half tones, from under-exposure in the camera.
12. Collodion curls from the glass upon drying from dirty glass, insufficient alcohol in the collodion, want of roughness of the edges of the glass.
13. Blueness of film, want of iodizer in collodion.
14. Crystals on film when dried, hyposulphite not washed entirely out.
15. Developer flows greasily, from want of alcohol in developer.
16. Circular transparent spots of large size, from pouring on the developer at one place.
1. Marbling and spots, from weak nitrate-bath.
2. Marbling after toning, from the prints overlying each other.
3. Spots by transmitted light, from imperfect removal of the silver salt.
4. Cold and faded appearance, from weakness of bath or excess of chloride in paper.
5. Yellowness, from acidity of bath.
6. Bronzing of deep shadows, negative is too transparent.
Throw down the silver as a chloride by muriatic acid; settle, pour off the clear part, and wash the precipitate; place the chloride in a dish, together with some bars of zinc, and pour over it sulphuric acid largely diluted with water (1 of acid to 60 of water). As soon as it ceases to give off gas add more acid until the zinc is entirely dissolved. Should any chloride remain add more zinc and acid. The gray powder is metallic silver, and may be run into an ingot in a crucible by mixing with twice its weight of carbonate of soda or borax.
Boil for several hours with a suspended bar of zinc, filter out the precipitate, and fuse with carbonate of soda or borax. The button contains silver and gold if the solution has been used to tone and fix prints.
Add a solution of sulphate of iron, filter out and wash the precipitate; digest with diluted nitric acid for several hours; filter out the residue, which is metallic gold.
All waste solutions, containing silver, in the laboratory should be run into a large vessel, and acidulated with muriatic acid every evening, and the clear liquid siphoned off in the morning; the precipitates will be principally chloride of silver, and when enough has accumulated it should be removed from the vessel, and reduced, as advised for baths of nitrate of silver.
The clippings of sensitized paper should be preserved, and when a quantity has accumulated burned to ashes, the ashes to be in a crucible mixed with twice their weight of a mixture of carbonate of soda and borax. If carefully brought to a full red heat, and allowed to cool, the silver will be found collected into an ingot at the bottom of the crucible. It is best to trim the prints before washing or toning, but the whole of the cuttings are worth saving.
1. By rubbing with a moistened lump of cyanide of potassium, and washing freely with water. This mode is dangerous, on account of the highly poisonous nature of the cyanide.
2. By rubbing the spot with moistened iodide of potassium, then with diluted nitric acid, and then with hyposulphite of soda, and washing with water.
3. Apply a paste of chloride of lime for a few minutes, wash thoroughly with water; rub with moistened iodide of potassium, and dissolve out the iodide of silver formed by hyposulphite of soda.
4. For Linen or other Fabrics. - Rub with solution: Cyanide of potassium, 100 grs.; iodine, 10 grs.; water, 1 oz.; (very poisonous;) and wash with large amount of water.
From a negative on glass or paper a positive impression is taken on paper prepared in the following manner: Take sized paper, very smooth and even in texture, which coat with a solution of gum Arabic in pure water, saturated with bichromate of potash; it is known that bichromate of potash, in combination with an organic substance, such as gum, gelatine and starch, becomes insoluble in water after a certain exposure to light. The paper, prepared as above, is then exposed to light behind a negative, and when the photographic image is sufficiently developed, such parts of the gum impregnated with bichromate of potash as receive the rays of light become insoluble, or partly so, exactly according to the gradation of tone in the negative employed. The sheet of prepared paper, with the photographic image thus printed, is placed face downwards on a lithographic stone, grained very fine, or polished according to the nature of the image to be reproduced, and previously arranged in a percussion-press (it can be done in a lithographic press, but the result is uncertain). Place several sheets of damped paper upon the stone over the photographic proof, and apply a heavy pressure; the water contained in the damp paper is pressed through the photographic proof and dissolves the parts of gum remaining at liberty; the dissolved gum attaches itself to the surface of the stone. When the stone has remained a certain length of time in the press sufficient to allow the small quantity of soluble gum in the dark shadows to attach itself to the surface of the stone remove the pressure, and withdraw the photographic proof carefully from the stone; a negative image is then visible in gum on the stone, with all the gradations of tone. Dry the stone, either spontaneously or by gently warming it; when well dried, cover the whole surface of the stone with greasy ink, which may be applied with a roller or otherwise; the greasy ink is thus brought into contact with all the parts of the stone untouched by gum; the coating of ink is then removed by passing through the lithographic press, by spirits of turpentine or otherwise, and all the gum removed from the surface by washing. The stone is then rolled in with ordinary printingink, and the positive image appears in black; it is then printed as every lithographic drawing, but has the great peculiarity of requiring no etching, the gum having so far penetrated by pressure into the substance of the stone as to allow of a great number of impressions being taken off. Gum Arabic is preferable, but similar substances can be used instead such as gelatine, dextrine, and mucilaginous solutions.
In photozincography proceed as above described, substituting a zinc plate for a lithographic stone.
In photographic engraving on copper, steel or other metal plates, the preliminary operations are the same as those employed for stone or zinc, excepting that a positive image on glass or paper should, in the first instance, be used instead of a negative. When the plate is withdrawn from the press, and the photographic proof detached from it it should be well dried for the stone. The plate is then covered with a thin coating of varnish, and when dry may be soaked in water to remove the gum, or immediately plunged in a weak solution of acid, and etched in the usual way of etching on steel or copper.
Wherever the plate is protected from the varnish by the gum it will be attacked by the acid, and an engraving is produced, which is then printed as an ordinary etching on metal plate.
Let us suppose that a map has been compiled and drawn with great care, and that it is desired to multiply copies of this original in the lithographic process. The first step in the process is to obtain a negative; for which purpose the map is placed upright upon a plane-board, and the camera opposite to it at such a distance as to give the desired ratio between original and copy. A negative is now taken on glass coated with collodion in the usual way, observing the greatest care to avoid distortion of all kinds and to produce a negative of the highest excellence, success in which depends entirely upon the knowledge, judgment and experience of the operator. A sheet of plain, positive photographic paper is now coated on one side with a mixture, consisting of gelatine, softened and dissolved in water, to which a quantity of bichromate of potash and albumen has been added. The paper, evenly covered with this fluid, is dried in the dark, when it will be found possessed of a smooth glassy surface, and a bright yellow color. This surface is still further improved by passing it through the press in contact with a polished plate.
A suitable piece of positive photolithographic paper thus manufactured is now to be exposed to the action of the light under the negative of the map already described. This is accomplished in an ordinary pressure-frame, the time required varying from 10 to 15 seconds, or several minutes, according to the brightness of the weather. The positive thus obtained presents itself to the eye as a brown drawing upon the clear yellow of the sheet.
The exposed photographic copy of the original is covered all over, while dry, with transfer-ink, which is accomplished by running it through the press with its face in contact with a stone which has already received a coating of such ink. After it is separated from the blackened stone it will be found to have brought away with it an evenly distributed film of inky matter, forced by the pressure into intimate contact with the unexposed, as well as the exposed portion of the surface. This operation is known as "blacking" the positive print, that now to be described is called "coagulation," its object being to effect a change of that nature upon the albumen contained in the coating of the organic matter. For this purpose moisture and heat are necessary, and both are applied very simply, by letting the blackened photographic copy swim upon the surface of boiling water with its inky side upwards, for it is important not to wet that with hot water. After the lapse of a certain period, determined by the experience of the operator, he proceeds to the next step in the process, that of "washing off." For this purpose the print is laid upon a smooth surface, such as a plate of glass or porcelain, and friction with a wet sponge, or other suitable material, is applied to the black inky coating, under which the photographic image still exists, and to develop which is now the object in view. The operator soon becomes aware that the moisture which percolated through the paper from the back has exerted a softened or gelatinizing influence upon the gelatine in the sensitive coating; it has caused it to swell, and to let go its hold upon the ink. But this change does not extend to those parts of the coating which were acted on by light; in other words, to those places which were unprotected by the opacity of the negative, they remain intact, uninfluenced by the solvent or moistening effect of the water. Accordingly the operator finds a facsimile of the original map gradually develop under his hand as he continues the friction. This process is proceeded with until all traces of ink are removed, save those required to form the picture, which must be clear and distinct in all its details. Abundance of hot water is then poured over it, so as to remove every particle of soluble matter, and it is then finally dried, which completes its preparation
A stone to which a fine smooth surface has been imparted, is now slightly warmed, and put in the lithographic press; upon this is placed (inverted) the positive print, after it has been damped by lying between moist paper, and the whole is then passed repeatedly through the press. On examination the paper will now be found to have attached itself firmly to the stone, so that some force is required to separate the two. When the former is removed it brings with it its albuminous coating, which gives to it while damp a parchment-like appearance; but the ink is gone: it has left the paper for the stone, and on the latter we find a reversed drawing of the map, one which, after it has been properly "prepared," will print as well as if it had been drawn by hand.
Take a plate of glass, and spread on it ordinary glue, to which bichromate of potash and a small quantity of nitrate of silver has been added. For instance, take 2 or 3 solutions of glue, into one of which put a little nitrate of silver, into another bichromate of potash, and into another iodide of potassium. The silver and the iodide are for the purpose of producing a little iodide of silver on the sensitive film, so as to produce on the picture that grain which is necessary for holding the ink in the process of printing. Take the photographic picture obtained by any of the customary processes, and place it on the sensitive plate thus prepared and exposed to the action of the light. In the course of a short time (all those parts which are dark in the photograph, protecting the plate from change, and all those which are white, allowing the sunlight freely to pass through and the change to take place), we have a combination of bichromate of potash and gelatine in two different states, one soluble and the other insoluble. Consequently, when the plate is then put into water all the parts which remain soluble are dissolved out, whilst the other parts remain as they were, and we have the picture produced not only in different lights and shades, but also in different depths, the solution being eaten into by the process. When the plate is prepared to this point, there is poured upon it a preparation of gutta percha, which, being kept under pressure for a short time, receives the reverse image of the photographic picture. This is now prepared for the voltaic battery by being simply rubbed over with fine black lead, and it being placed in connection with the trough, copper is precipitated on the plate, which receives an image the reverse of the mould. Then by the ordinary electrotype process another plate may be obtained, from which copies may be primed.
|
|