COLLODIO-ALBUMEN PROCESS.
This is a process, invented by Dr. Taupenot, for obtaining negatives on glass, which bids fair to outrival all others, being easy of manipulation, and giving results of the most exquisite minutiæ and beauty. Glass plates, when prepared and excited by this process, may be kept at least a fortnight before being developed, and these plates when exposed in the frame may be developed immediately, or kept for days before commencing this operation. Indeed it is quite possible to prepare and excite a number of plates before leaving home to go on a tour of twelve or fourteen days; to expose the plates at any time or place during the journey, and bring them home to be developed.
The manipulation may be said to consist of nine distinct operations.
1. Cleaning the plate. 2. Coating with iodized collodion. 3. Exciting the collodion film. 4. Coating with albumen. 5. Exciting the albumen coating. 6. Exposure in the camera. 7. Developing the image. 8. Fixing the image. 9. Varnishing the plate.
Before describing these operations, I propose to give clear directions for preparing the necessary solutions, merely promising that, where I have deviated from the inventor's plan, it has been after performing careful experiments, to test the merits of the two modes of proceeding.
The necessary solutions for this process are:
Collodion bath solution. Iodized collodion. Iodized albumen. Albumen bath solution. Pyrogallic solution. Silver developing solution. Fixing solution.
Collodion Bath Solution.
Dissolve the ounce of nitrate of silver in two ounces of the distilled water, and the two grains of iodide of potassium in one drachm of distilled water; mix the two solutions and shake well together until the precipitate which is first thrown down isre-dissolved; when this takes place, add the remaining fourteen ounces of distilled water, and the two drachms of alcohol. On the addition of the water a turbidness ensues, which must be removed by the solution being very carefully filtered through filtering paper; and the filtered liquid should be clear and transparent, free from any deposit or floating particles, and must possess a slightly acid reaction of test-paper.
In order to ascertain if the solution thus prepared possesses the necessary amount of free acid without superabundance, proceed to test and to correct it, if necessary.
Iodized Collodion,
The collodion to be used in this process must be one yielding goodnegativepictures—that supplied by Home & Thornthwaite under the name of negative collodion answers admirably. This is supplied either ready iodized, or the collodion and iodizing in separate bottles. As this collodion becomes less sensitive after being iodized a fortnight, it is advisable to iodize no more than will be used in that time—therefore, obtain the collodion and the iodizing solution separate, as the mode of iodizing this collodion is very simple. Half an ounce of the iodizing solution is mixed with one ounce and a half of collodion, and the mixture allowed to settle twelve hours before being used; and it is even advisable to pour off the clear solution into a perfectly clean bottle, in order to get rid of any insoluble matter which may fall to the bottom.
Iodized Albumen.
Mix these together in a tall glass jar, or wide-mouthed bottle of at least one pint capacity; tie a piece of paper, pierce with small holes over the top to keep out dust; then place the whole near a fire or other warm situation, where the temperature is not lower than seventy degrees, or higher than ninety degrees. In a few hours fermentation commences, which is evident by the formation of bubbles of gas, rising through the liquid. This action continues for five or six days; when it ceases, pour the whole on a paper-filter contained in a funnel, underneath which must be placed a bottle to receive the liquid as it passes through. The fluid being of a viscid nature filters slowly, generally occupying twelve hours.
The filtered liquid is the "iodized albumen," which is said byDr. Taupenot to keep good for years. It must be carefully preserved from dust or contact with any substance, as the success of the picture depends materially on the condition of this albumen.
Albumen Bath Solution.
Dissolve the nitrate of silver in the distilled water, then add the acetic acid and animal charcoal, and keep in a closely stoppered bottle for use.
Pyrogallic Solution.
Dissolve the pyrogallic acid in the distilled water, and then add the acetic acid and alcohol.
Silver Developing Solution.
Dissolve the nitrate of silver in the distilled water, and then add the acetic acid.
Fixing Solution.
Varnish.
The varnish best adapted for this purpose is that supplied by Home & Thornthwaite, and termed negative varnish.[H]
[H]In this country, Humphrey's Collodion Gilding is the article in almost universal use.
[H]In this country, Humphrey's Collodion Gilding is the article in almost universal use.
Cleaning the Plate.
The plates must be cleaned in the usual way, merely premising that extra care must be observed to remove every impurity, as cleanliness in photography is an absolute necessity.
In order to hold large plates whilst being cleaned, the "screw plate-holder" is exceedingly useful. This is made in three sizes, and adapts itself to all sized plates.
The small size is useful for plates up to 7 inches by 6.
The second size is for plates up to 10 inches by 8.
And the third size for plates up to 14 inches by 10.
Coating with Iodized Collodion.
The plate having been thoroughly cleaned, and received its final polish by the use of a prepared chamois leather, is coated with negative collodion, which has been iodized at least twelve hours, and allowed to settle.
Exciting the Collodion Film.[I]
[I]This and subsequent operations (except exposure in the camera) must be performed in a dark room.
[I]This and subsequent operations (except exposure in the camera) must be performed in a dark room.
After the ether has evaporated, and the surface of the collodion appears set, the plate must be laid, collodion side upwards, on a glass dipper, and plunged withone downward movementinto a bath filled to within an inch of the top with collodion bath solution, made as described atpage 190, which must be carefully filtered through filtering paper before being used. After the plate has been allowed to remain in the bath one minute, it is lifted out three or four times, in order to facilitate the removal of the oily appearance the plate now presents. When the surface appears wetted uniformly, on being drawn out of the solution the plate is removed from the dipper, and the excess of solution drained off, and is then placed collodion side upwards, on a fixing stand, and distilled or filtered rain water poured over the surface, so as to remove as much as possible of the bath solution from the surface. The plate is now removed from the fixing stand; the back well washed with water, and then placed nearly upright on blotting paper, with the face against a wall forone minuteto drain.
Coating with Albumen.
Having allowed the plate to drain one minute, place it again on a levelling stand, with the film upwards, and pour over it as much of the iodized albumen as the plate will hold, from a glass measure containing not more than enough of the albumen to coat two plates with, pour off the excess into the measure, and again cover the plate with albumen three separate times; ultimately drain off as much as possible of the excess of albumen, and place the plate nearly upright against the wall,with the coated side inwards, to dry, which takes place in an ordinary temperature in about one hour.
In coating with albumen, the presence of air-bubbles or dust must be guarded against. The former can be easily done by taking care, in pouring the albumen into the measure and on the plate, not to pour so as to generate air-bubbles in the liquid. But should any be detected, hold the plate horizontally and give it another coating of albumen, then incline the plate so that the bulk of the liquid shall pass over and carry off the bubbles with the running stream. Dust on the plate must be prevented by operating in a room as free from this photographic enemy as possible.
In order to render the coating of albumen as uniform as possible, the plate must stand to dry on two or three layers of filtering paper and the upper surface must touch the wall atone point onlyand not to be allowed to rest against it along its entire upper edge.
When the albumen coating isthoroughly dry(and not till then), the plate is ready to be excited, but if more have been prepared than are likely to be used for taking pictures on during the next ten days or fortnight, they may be stowed away in a plate box, ready to receive the sensitive coating at any time. The author's experience has led him to believe that these albumenized plates will keep good any length of time, as plates which had been coated a month, when excited, exposed, and developed, appeared to possess all the properties of recently prepared plates.
Exciting the Albumen Coating.
Prior to the plates being excited they must bethoroughly dryand free from any particles of loose dust on the surface, back, or edge. Sufficient of the albumen bath solution,page 192, must be filtered through filtering paper to fill a dipping bath of the required size, so that the plate can be immersed in it.
The careful filtering of the fluid is very necessary in order to free it from any floating particles, and to separate the animal charcoal.
The plate is now taken and laid, albumen side upwards, on the dipper, and then lowered into the bath with one steady downward movement, where it is allowed to remain one minute; it is then taken out, the excess of liquid drained off, and placed on the fixing stand, with the albumen surface uppermost, and a stream of water poured over it for at least one minute, so as to remove every particle of the bath solution. This complete washing is very necessary, in order to prevent stains in the after development, which invariably takes place around the edges, if not thoroughly washed. The plate having been thoroughly washed, is leaned against a wall to dry, or if required for immediate exposure, may be dried on a plate of heated metal or foot warmer, but in no case must the exposure in the camera take place until the surface is thoroughly dry.
Exposure in the Camera.[J]
[J]Remarks as to the selection of the view, &c., are not given, as this can be effected by the individual taste of the operator, but care must be taken that direct rays from the sun shall not fall on the lens or enter the camera during the exposure of a plate.
[J]Remarks as to the selection of the view, &c., are not given, as this can be effected by the individual taste of the operator, but care must be taken that direct rays from the sun shall not fall on the lens or enter the camera during the exposure of a plate.
As has been before stated, this operation may take place immediately the plate is thoroughly dry after being excited, or a fortnight may intervene between the excitement and exposure, provided the plate is kept very carefully excluded from light and any chemical or sulphurous vapors, in a plate-box adapted for that purpose, with the sensitive surface towards the back of the box. When the exposure is about to take place, or at any time previously, the camera-backs may each have a plate placed in them ready for exposure; to do this, the camera-back must be taken into the operating room and the door closed, so as to exclude all white light. The hinged flap of the camera back is opened, and the prepared plate laid, with its sensitive surface downwards, or next the sliding flap, so that its corners may rest on the silver wire corners of the plate frame previously placed within the camera back ready to receive it. The hinged flap is now closed and kept from opening by turning the flap button over it; the sliding flap is examined to see that it is pushed closely down so as to guard any access of light, and it is then ready to be placed in the camera, and may be taken into the open air with impunity. Should the exposure not take place immediately, or, should the camera back have to be carried any distance, it is advisable either to wrap it up in black cloth, or secure the flaps from the chance of coming open during transit, by a stout string being tied around the back.
The focussing is conducted in the usual way and the cap replaced on the lens; the focussing glass is now removed and the camera back fitted into the same aperture, with the sliding flap next the lens. The sliding flap is pulled up to its fullest extent, placing the hand on the camera back to prevent it rising out of the camera with this action. The cap of the lens is then removed, so that the light may be admitted into the camera, and to fall on the sensitive surface of the plate. After the necessary time of exposure has taken place, the cap is replaced on the lens, the sliding flap is pushed down, and the camera back withdrawn from the camera; the plate can then be taken into the operating room to be developed, or this operation may be deferred for days or even a week, or more if convenient. The time of exposure in the camera varies according to the intensity of the light and the aperture and focal length of the lens; therefore, to give the exact time of exposure would be impossible, still it may assist the amateur if I give the time required in summer with full sunshine, andmerely state that this time may be increased to double in winter or dull weather.
In the ordinary sunshine of a summer's day the time of exposure will be:
30 seconds with a lens of 4-inch focus and ½-inch stop.21 seconds with a lens of 4-inch focus and5/8-inch stop.5 seconds with a lens of 4-inch focus and 1¼ inch aperture with no stop.1½ minute with a lens of 6-inch focus and ½-inch stop.4½ seconds with a lens of 6-inch focus and 2¼-inch aperture with no stop.2 minutes with a lens of 8-inch focus and ½-inch stop.1¼ minute with a lens of 8-inch focus and5/8-inch stop.3¼ minutes with a lens of 10-inch focus and ½-inch stop.2 minutes with a lens of 10-inch focus and5/8-inch stop.5 seconds with a lens of 10-inch focus, 3¼-inch aperture, with no stop.6¼ minutes with a lens of 14-inch focus and ½-inch stop.4 minutes with a lens of 14-inch focus and5/8-inch stop.2¼ minutes with a lens of 14-inch focus and ¾-inch stop.8¼ minutes with a lens of 16-inch focus and ½-inch stop.5¼ minutes with a lens of 16-inch focus and5/8-inch stop.2¼ minutes with a lens of 16-inch focus and ¾-inch stop.
Developing the Image.
The camera back is taken into the operating room, from which all white light is carefully excluded, the plate removed from the camera back, and laid, albumen side upwards, on the fixing stand; as much distilled water is now poured on it as the surface will hold, taking care that every part of the sensitive surface is covered with the liquid; allow the water to remain on the surface for one minute, then pour off and drain slightly; replace the plate on the stand, and pour over the surface so as thoroughly to cover every part, the pyrogallic solution (made as described atpage 192, and carefully filtered just before being used); allow this to remain on the plate for one minute, then drain off into a perfectly clean measure, and add to it an equal bulk of silver developing solution,page 192; mix these thoroughly together with a glass rod, and then pour the mixed liquids over the plate; allow them to rest until the picture begins to appear, which generally takes about from three to five minutes; then pour off and on repeatedly, until the developing fluid becomes opaque, which then contains floating particles, and these, if allowed to do so, would settle on the plate, to the injury of the picture; but this may be prevented by brushing the surface with a camel's-hair brush frequently during the development. When this opacity ofthe developing fluid takes place, drain all the fluid off the plate, and thoroughly wash with water; then mix another quantity of pyrogallic and silver developing solution in the same proportions as before, and pour this on and off the plate as before, until the picture appears sufficiently intense, and the middle shades well brought out; when this takes place drain off, and wash with water, so as to clean the surface thoroughly, and the plate is then ready for the next step, "fixing the image."
Should the picture begin to develope in less than three minutes after the application of the mixed developing fluids, thoroughly drain the plate, and wash well with water, then continue the development with a solution of three parts pyrogallic solution and one part silver developing solution; but should the picture not begin to appear in five minutes, the addition of half a drachm of the albumen bath solution to each ounce of mixed developing solution will be necessary, in order to obtain the middle shades and the required intensity. It may be stated, as a guide, that the best negatives which the author has produced occupied from ten to twelve minutes in developing.
Fixing the Image.
The plate, having been thoroughly freed from the developing fluid by careful washing, is now placed on the fixing stand, and the surface covered by the fixing solution, made as described at page 192, being poured over it. In a few seconds the yellow opalescent color of the film will begin to disappear, and its complete removal may be hastened by blowing gently on the plate, so as to disturb the fluid.
When every particle of yellowness has disappeared, the fixing solution is drained off, and the surfacethoroughlywashed, and it is then leaned against the wall to drain and dry.
Varnishing the Plate.
The plate, being thoroughly dry, is ready to receive a coating of transparent varnish.[K]in order to protect the albumen surface from injury during the printing process. To do this effectually the plate must be held before a fire, or over a lamp, until it is slightly warm all over; then pour over its surface the negative varnish, in the same manner as collodion is applied; allow the superfluous varnish to drain back into the bottle; hold the plate again before the fire until the whole of the spirit is evaporated; and, when cold, the plate is ready to be printed from, so as to produce any number of positive pictures on paper.
[K]Humphrey's Collodion Gilding is the best for this purpose.
[K]Humphrey's Collodion Gilding is the best for this purpose.
It will be observed, that in describing this process, theoperator has been supposed to be so situated, that in case a second view of the same spot were required, he could return to his operating room, remove the plate which had been exposed, from the camera back to the plate box, and place another in the camera back, ready for taking another view. But, unfortunately, this is not at all times practicable. We, therefore, require some means of removing the plates, after being exposed, from the camera back into the plate box, and substituting others in their stead, whilst we are in the open air.
In order to effect this, the "field plate box" has been devised by the author, by the aid of which the plates may be removed from the box, exposed in the camera, and again returned into the box, without any possibility of access of light falling on it.
This box is but a trifle larger than the ordinary one, and is furnished with two sliding bottoms, working in grooves, one over the other; the lower bottom has a grooved channel, into which the side of the camera back slides; the camera back has an aperture through the side, closed by a narrow slide, and the lower bottom of the field box has a corresponding one. We now suppose the field box to have been previously filled with excited glass plates, having their sensitive sides towards the back of the box, and the box lid closed. The bottom slide is now pushed on until the aperture is in a line with any particular groove of the field box (which position is indicated by a numbered scale and index point). The camera back is then slid on to its place on the field box, so that the hinged flap is towards the front of the box, and its narrow slide drawn out. The upper slide is then withdrawn, and the box inclined, so that the plate in that groove opposite the aperture in the lower slide, may pass through into the camera back. When this has taken place, push in the narrow slide of the camera back, invert the box, and push in the inner slide; then withdraw the camera back from its channel, and expose the plate in the camera. When this is done, slide the back again into its channel, draw out the inner box slide, then the narrow camera back slide, invert the box, and the plate will then leave the camera back and pass into the field box, occupying the same groove as before.
In order to get out another plate, slide the lower bottom, so that the index points to the number on the scale, as that of the groove in which the required plate is situate. Then proceed as before directed.
On a mode of Printing enlarged and reduced Positives, etc., from Collodion Negatives.
To explain the manner in which a photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made atpage 20, on theconjugate fociof lenses.
If a collodion negative be placed at a certain distance in front of a camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the negative, a reduced image will be formed upon the ground glass; but if the negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original negative; the focus becoming more and more distant from the lens, orreceding, as the negative is brought nearer.
Again, if a negative portrait be placed in the camera slide, and if the instrument be carried into a dark room, a hole be cut in the window-shutter so as to admit light through the negative, the luminous rays, after refraction by the lens, will form an image of the exact size of life upon a white screen placed in the position originally occupied by the sitter. These two planes, in fact, that of the object and of the image, are strictlyconjugate foci, and, as regards the result, it is immaterial from which of the two, anterior or posterior, the rays of light proceed.
Therefore in order to obtain a reduced or enlarged copy of a negative, it is necessary only to form an image of the size required, and to project the image upon a sensitive surface either of collodion or paper.
A good arrangement for this purpose may be made by taking an ordinary portrait camera, and prolonging it in front by a deal box blackened inside and with a double body, to admit of being lengthened out as required; or, more simply, by adding a framework of wood covered in with black cloth. A groove in front carries the negative, or receives the slide containing the sensitive layer, as the case may be.
In reducing photographs, the negative is placed in front of the lens, in the position ordinarily occupied by the object; but in making an enlarged copy it must be fixed behind the lens, or, which is equivalent, the lens must be turned round so that the rays of light,transmitted by the negative, enter the back glass of the combination, and pass out at the front. This point should be attended to in order to avoid indistinctness of image from spherical aberration.
A portrait combination of lenses of 2½ or 3¼ inches is the best form to use, and the actinic and luminous foci should accurately correspond, as any difference between them would be increased by enlarging. A stop of an inch or an inch and a half aperture placed between the lenses obviates to some extent the loss of sharp outline usually following enlargement of the image.
The light may be admitted through the negative by pointing the camera towards the sky; or direct sunlight may be used, thrown upon the negative by a plane reflector. A common swing looking-glass, if clear and free from specks, does very well; it should be so placed that the centre on which it turns is on a level with the axis of the lens.
The best negatives for printing enlarged positives are those which are distinct and clear; and it is important to use a small negative, which strains the lens less and gives better results than one of larger size. In printing by 2¼ lens for instance, prepare the negative upon a plate about two inches square and afterwards enlarge it four diameters.
Paper containing chloride of silver is not sufficiently sensitive to receive the image, and the print should be formed upon collodion, or on iodized paper developed by gallic acid.
The exposure required will vary not only with the intensity of the light and the sensibility of the surface used, but also with the degree of reduction or enlargement of the image.
In printing upon collodion the resulting picture is positive by transmitted light; it should be backed up with white varnish, and then becomes positive by reflected light. The tone of the blacks is improved by treating the plate first with bichloride of mercury, and then with ammonia.
Mr. Wenham, who has written a most practical paper on the mode of obtaining positives of the life size, operates in the following way:—he places the camera, with the slide containing the negative in a dark room, and reflects the sunlight in through a hole in the shutter, so as to pass first through the negative and then through the lens; the image is received upon iodized paper, and developed by gallic acid.
On Printing Collodion Transparencies for the Stereoscope.—This may be done by using the camera to form an image of the negative in the mode described at the last page; but more simply by the following process:—Coat the glass, upon which the print is to be formed, with collodio-iodide of silver in the usual way, then lay it upon a piece of black cloth, collodion side uppermost, and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad, along the two opposite edges, to preventthe negative being soiled by contact with the film. Both glasses must be perfectly flat, and even then it may happen that the negative is unavoidably wetted; if so, wash it immediately with water, and if it be properly varnished no harm will result.
A little ingenuity will suggest a simple framework of wood, on which the negative and sensitive plate are retained, separated only by the thickness of a sheet of paper; and the use of this will be better than holding the combination in the hand.
The printing is conducted by the light of gas or of a camphine or moderator lamp, diffused daylight would be too powerful.
The employment of a concave reflector, which maybe purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until anevenly illuminated circleis thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame—that the light falls most powerfully upon the central part; and less so upon the edges, of the negative.
(From Humphrey's Journal, No. 17, Vol. 8.)
On the Use of Alcohol for Sensitizing Paper.
TO SENSITIZE PAPER.
I have practised for some time the following simple method, which appears to me to be very superior for cleanliness and celerity in working, for depth of tone, and especially for purity of white in its results. By means of it T have produced very satisfactory results upon paper which was otherwise nearly worthless.
To your sensitizing solution (which should be not less than 60 grains to the ounce), whether simple nitrate or ammonio-nitrate, add 50 per cent, of alcohol. Float the paper upon the solution for 40 seconds.
This method answers equally well for albumenized or plain paper. You will find that the solution penetrates the paper which flattensinstantaneouslyupon it. It becomes as transparent as though it were oiled, and every minute air-bubble or defect in the paper is rendered visible. Remove the air-bubbles by pressing upon the paper about an inch from the bubble, and thus driving it out under the paper. In doing this, if the solution flows partially over the back of the paper, shake it until the paper is whollyimmersed, which will prevent any unevenness in printing. The paper reassumes its transparency in the toning bath, but it will dry a pure white. The sensitising solution will not become materially discolored even after frequent applications of albumenized paper. Should it become so much discolored as to give a dark hue to the paper, shake it in a bottle with two drachms Of animal charcoal and leave it a night to settle. It will filter clear. A very small portion of your solution may be made available in sensitizing a sheet of paper by pouring it upon a clean glass, the size of the paper or a little larger, which is carefully levelled and nicely laying down the paper upon it. This is useful when your solution is too small to float in your trays. The alcohol causes it to flow and be absorbed with perfect evenness.
To remove the papers from the solution and dry them:—Provide a dozen or more clothes-pins, of the kind that are supplied with a ring of india-rubber for a spring. Into the top drive a pin firmly and bend it to a hook. Lift a corner of the paper by passing under it the point of a quill tooth-pick, and attach to it one of the clothes-pins; lift the edge out by this, and attach another to the other corner. You may thus carry the sheet by the pins and hang it upon a line to dry without touching it with the fingers, a matter of some importance to Amateurs of the art, who must have unstained hands for their day'sbusiness.
It may be worth while here to add the following simple and economical method of printing, which I have found to surpass in convenience and afford all the advantages of the most expensive printing frames. Four common clothes-pins, such as work with awirespring supply pressure enough for a4/4-plate. Lay your prepared paper upon the negative, and next to it about twentyseparate leavesof thin common wrapping paper cut to the size of the negative; next a sheet of tolerably stiff and smooth writing paper, and lastly, a piece of glass as a back to the whole. Let the glass back be pushed from the lower edge of the papers about1/20of an inch, or just so far as to enable you to pinch the negative andpapers with the thumb-nail and forefinger. Attach a pin to each corner and your negative is prepared for exposure. Now, to examine your picture without endangering its displacement:—remove the pins from one end, and place it, face downwards, on a table, the other end with pins attached projecting an inch beyond the edge. Hold down the back glass with the left hand, while with the right you remove the pins and pinch the papers and negative together between the forefinger and thumb-nail. Upon the smooth sheet of paper you can easily slide the back glass an inch from the edge. Hold it there, and on the uncovered margin attachthreeof the pins with as deep a bite as they will take. You may now examine your picture to within an inch of its margin, as you would turn over the leaves of a book. To replace the back, lay it again on the table and slide the back glass up to the pins before you remove them. The rest of the process is obvious.
The minutiæ of my communication may excite a smile with some, but I shall always act upon the principle, that nothing is more out of place than an apology for minuteness in describing manipulations.
G. B. C.
Recovery of Silver from waste Solutions,—from the black Deposit of Hypo Baths, etc.
RECOVERY OF WASTE SILVER, ETC.
The manner for separating metallic silver from waste solutions varies according to the presence or absence of alkaline hyposulphite and cyanides.
a.Separation of metallic Silver from old Nitrate Baths.—The silver contained in solutions of the nitrate, acetate, etc.; may easily be precipitated by suspending a strip of sheet copper in the liquid; the action is completed in two or three days, the whole of the nitric acid and oxygen passing to the copper, and forming a blue solution of the nitrate of copper. The metallic silver however separated in this manner, always contains a portion of copper, and gives a blue solution when dissolved in nitric acid.
A better process is to commence by precipitating the silver entirely in the form ofchloride of silver, by adding common salt until no further milkiness can be produced. If the liquid is well stirred, the chloride of silver sinks to the bottom, and may be washed by repeatedly filling the vessel with common water, and pouring off the upper clear portion when the clots have again settled down. The chloride of silver thus formed may afterwards be reduced to metallic silver by a process which will presently be described.
b.Separation of Silver from solutions containing alkaline Hyposulphites, Cyanides or Iodides.—In this case the silver cannot be precipitated by adding chloride of sodium, since the chloride of silver issolublein such liquids. Therefore it is necessary to use the sulphuretted hydrogen, or the hydrosulphate of ammonia, and to separate the silver in the form ofsulphuret.
Sulphuretted hydrogen gas is readily prepared, by fitting a cork and flexible tubing to the neck of a pint bottle, and having introducedsulphuret of iron(sold by operative chemists for the purpose), about as much as will stand in the palm of the hand, pouring upon it 1½ fluid ounces of oil of vitriol diluted with 10 ounces of water. The gas is generated gradually without the application of heat; and must be allowed to bubble up through the liquid from which the silver is to be separated. The smell of sulphuretted hydrogen being offensive, and highly poisonous if inhaled in a concentrated form, the operation must be carried on in the open air, or in a place where the fumes may escape without doing injury.
When the liquid begins to acquire a strong and persistent odor of sulphuretted hydrogen, the precipitation of sulphuret is completed. The black mass must therefore be collected upon a filter, and washed by pouring water over it, until the liquid which runs through gives little or no precipitation with a drop of nitrate of silver.
The silver may also be separated in the form of sulphuret from old hypo baths, by adding oil of vitriol in quantity sufficient to decomposethe hyposulphite of soda; and burning off the free sulphur from the brown deposit.
Conversion of Sulphuret of Silver into Metallic Silver.—The black sulphuret of silver may be reduced to the state of metal by roasting and subsequent fusion with carbonate of soda; but it is more convenient, in operating on a small scale, to proceed in the following manner:—first convert the sulphuret into nitrate of silver, by boiling with nitric acid diluted with two parts of water; when all evolution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists principally of sulphur, but also contains a mixture of chloride and sulphuret of silver, unless the nitric acid employed was free from chlorine; this precipitate may be heated in order to volatilize the sulphur, and then digested with hyposulphite of soda, or added to the hypo bath.
The solution of nitrate of silver obtained by dissolving sulphuret of silver is always strongly acid with nitric acid, and also containssulphateof silver. It may be crystallized by evaporation; but unless the quantity of material operated on is large, it will be better to precipitate the silver in the form of chloride, by adding common salt, as already recommended.
On the Use of Test Papers.
ON THE USE OF TEST PAPERS.
The nature of the coloring matter which is employed in the preparation of litmus-paper has already been described atpage 98.
In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however decidedly less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as oxide of silver.
In using test papers observe the following precautions:—theyshould be kept in a dark place, and protected from the action of the air, or they soon become purple from carbonic acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of liquor potassæ in four ounces, the blue color is restored.
Test-papers prepared with porous paper show the red color better than those upon glazed or strongly sized paper. If the quantity of acid present however is small, it is not sufficient in any case simply to dip the paper in the liquid; a small strip should be thrown in, and allowed to remain for ten minutes or a quarter of an hour.
If the paper, on immersion, assumes awine-redor purple tint, in place of a decided red, it is probably caused by carbonic aid gas: in that case the blue color returns when the paper is washed and held to the fire.
Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with sulphuric acid, one drop to half a pint.
The Salting and Albumenizing Paper.
SALTING PAPER, ETC.
Take of
If distilled water cannot be procured, rain water or even common spring water[L]will answer the purpose. To obtain the albumen, use new-laid eggs, and be careful that in opening the shell the yelk is not broken; each egg will yield about one fluid ounce of albumen.