[29]The writer has seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute Rheumatism. This acid is probably Lactic Acid!
[29]The writer has seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute Rheumatism. This acid is probably Lactic Acid!
c.Moisture as a cause of fading.—Although. Photographs properly printed are not readily injured by damp air (p. 153), yet as there areimpuritiesof various kinds constantly floating in the atmosphere, a state of comparative dryness may be said to be essential to the preservation of all Photographs. In collecting evidence upon the subject, "wet" and "damp" are frequently alleged as having been causes of fading—the prints were hung against a damp wall during frosty weather, in a room without a fire: or the rain had been allowed to penetrate the frame! No pictures will long survive such treatment, and Photographs, like engravings and water-colour paintings, require common care to be exercised in their preservation.
d.The modes of Mounting the Proof.—This subject has been alluded to in the abstract of the Author's papers atp. 155. All cements which are of an acid nature, or which are liable to becomesourby acetous fermentation, should be avoided. Flour paste is especially injurious, and many cases of fading have been traced to this cause. The addition of Bichloride of Mercury, which is often made to prevent the paste from becoming mouldy, would still more unfit it for Photographic use (p. 151). Starch is not much preferable. No substance appears better than Gelatine, which does not readily decompose, and shows no tendency to absorb atmospheric moisture. Thedeliquescentnature of many bodies is a point which should be borne in mind in mounting Photographs, and hence the use of a salt likeCarbonate of Potash, which the writer has known to be added to paste to prevent the formation of acid, would be unadvisable.
e.The effect of Imperfect Fixation.—The earlier Photographers did not always succeed in properly fixing their prints, since old Photographs are often found thickly studded with spots and blotches in the tissue of the paper. These prints however are not invariably faded upon the surface, and hence it cannot be said that imperfect fixation will certainly end in the total destruction of the picture.Still a notice of the subject may properly be introduced in this place, and the attention of the reader be once more drawn to the importance of washing the print in water on removing it from the printing frame; a decomposition invariably occurring when paper Positivessaturated with free Nitrate of Silverare plunged in a dilute solution of Hyposulphite of Soda, containing an insufficient quantity of the salt to dissolve away the Hyposulphite of Silver before it begins to undergo spontaneous change.
f.Exposure to an impure Atmosphere as a cause of Fading.—The five causes of fading which precede, have mostly reference to an intrinsically faulty condition of the print. This, the sixth, explains the mode in which a Photograph carefully prepared may yet suffer injury from deleterious matters often present in the atmosphere. The air of large cities, and particularly that emanating from sewers and drains, contains Sulphuretted Hydrogen, and hence articles of silver-plate become tarnished unless placed beneath glass. The injury which a print sustains by exposure to air contaminated with Sulphuretted Hydrogen, is less than the tarnish produced upon the bright surface of a silver plate (seep. 148); but it is recommended as a precautionary measure, that Photographic Pictures be protected by glass or kept in a portfolio, and that they be not exposed too freely to the air.
The products of the combustion of coal-gas are probably more likely than the cause last named, to be a source of injury to Photographs suspended without any covering. The sulphur compounds in gas burn into Sulphurous and Sulphuric Acids, the latter of which, in combination with Ammonia, produces the sparkling crystals often observed upon the shop windows.
The question as to the manner in which the Photographic Image may best be protected from these extraneous causes of fading has been mooted, and many plans of coating prints with some impervious material have been devised. If the pictures are to be glazed or kept in a portfolio, thisof itself will be sufficient, but in other cases it may perhaps be useful to apply a layer of spirit or gutta-percha varnish. The use of wax, resin, and such bodies is likely, by introducing impurities, to act injuriously rather than otherwise.
g.Decomposition of Pyroxyline a source of Injury to Collodion Photographs.—Collodion Positives and Negatives are usually esteemed permanent; but some have been exhibited which, having been put away in a damp place, gradually became pale and indistinct. The change commences at rough edges and isolated points, leaving the centre, as a rule, the last affected. On examination, numerous cracks are often visible, thus seeming to indicate that the Collodion film has undergone decomposition. The result of this would be the liberation of corrosive Oxides of Nitrogen, which destroy the image. Substitution compounds containing Peroxide of Nitrogen are known to be liable to spontaneous change. The bitter resin produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept perfectly dry, will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept; the solution of the resin has then a strong acid reaction, and rapidly fades an ordinary Positive Print.
These facts are interesting, and indicate that Collodion Pictures, containing in themselves the elements of their destruction, should be protected from moisture by a coating of varnish.
Comparative Permanence of Photographic Prints.—There is every reason to think that the Photographic Image, however formed, is permanent, if certain injurious conditions are avoided;—in other words, that prints do not necessarily fade, in the same manner as fugitive colours, by a simple exposure to light and air. But supposing a case, which is the common one, of injurious influences which cannot altogether be removed, it may be useful to inquire what mode of printing gives the greatest amount of stability.
Positives produced by a short exposure to light andsubsequent development with Gallic Acid, may be expected to be more permanent than ordinary sun-prints; not that there is any reason to suppose the chemical composition of a developed image to be peculiar, but that the use of the Gallic Acid enables us to increase the intensity of the red picture first formed, and to add to its stability by precipitating fresh Silver upon it. This point has not always been attended to. It has been recommended to remove the print from the developing solution whilst in theredand early stage of development, and to produce the dark tones subsequently by means of gold; but this plan, although giving very good results as regards colour and gradation of tone, appears to lessen the advantage which would otherwise accrue from the adoption of a Negative process, and to leave the picture, as regards permanency, much in the condition of an ordinary print obtained by direct action of light.
The original Talbotype process, in which the latent image is formed upon Iodide of Silver, produces, next to Collodion, the most stable image; but the difficulty of obtaining bright and warm tints on Iodide of Silver, will stand in the way of its adoption.
Thetoningof Paper Positives is the part of the process which is likely to injure their stability; inasmuch as the finest results cannot easily be obtained without incurringsulphuration, and the action of Sulphur, if carried to any extent, has been shown to be detrimental. The point to be kept in view, is to alter the original structure of the image as little as possible in toning; and it is best to use Gold in preference to Sulphur as the colouring agent. On theoretical grounds, toning by an alkaline solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best process; but the employment of Sel d'or, which gives a more agreeable colour and has not been found practically to injure the image, will be generally preferred. In usinga single fixing and toning Baththe same object of working by Gold rather than by Sulphur may be best attained bymaintaining the activity of the Bath by constant additions of Chloride of Gold.
The prints which areleast stableare such as have been toned inacid Hyposulphite Baths, without Gold;and the difficulty of preserving such pictures from becoming yellow in the half-tones is very great. Possibly a portion of the Sulphuretted Acid may unite with the Suboxide of Silver and cannot be removed by washing (seep. 158); but even if this be not the case, it is certain that no ordinary amount of care will obviate the occasional occurrence of fading, unless the Hyposulphite Bath be keptneutral to test-paper. And all those plans of toning in which Acetic or Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive immersed whilst the liquid is in a milky state from precipitation of Sulphur, ought studiously to be avoided.
It will be well also to avoid pushing the action of the fixing and toning Bath to its utmost limits, since practice and theory both teach us that the Positives which have been long in the Hyposulphite, and consequently show a tendency to yellowness in the light parts, are most liable to lose their half-tones on keeping. Photographic Prints are found often todarkenslightly in the course of years; and therefore by suspending the toning action at an earlier stage a margin is left for what some have termed "an improvement by time."
The use ofAlbuminizedin preference to plain paper gives an advantage in protecting the image from oxidation; but if constantly exposed to moisture, a putrefactive decomposition of the animal matter may occur. The proper colour of the Albumen image being apale red, the black tones should not be sought for on that variety of paper: their production, if Hyposulphite of Soda were used in toning, would probably imply an amount of Sulphuration which would more than counterbalance any advantage otherwise derivable from the Albumen.
Permanent Positives of a black colour may easily be obtained by sensitizing plain paper, free from animal matters,with Oxide of Silver in place of Nitrate. The simply fixed image being in that case of asepia tint, requires a less amount of toning to change it to black. An impression was at one time prevalent that Ammonio-Nitrate prints were unstable; but so far from such being the case, they are proved to withstand the action of all destructive tests better than pictures prepared upon the same kind of paper sensitized with plain Nitrate of Silver.
Mode of testing the permanence of Positives.—The tests for Hyposulphite of Soda are not sufficiently delicate to indicate with certainty when the process of washing has been properly performed. The quantity of that salt left in the paper is usually so small and so much mixed up with organic matter, that the application of Protonitrate of Mercury or of Nitrate of Silver to the liquid which drains from the corner of the print, would probably mislead the operator.
A dilute solution of Permanganate of Potash, prepared by dissolving from half a grain or two grains of the salt, according to its purity, in one gallon of distilled water, affords a convenient mode of testing Positives as regards their power of resisting oxidation; and to an experienced eye it will prove the presence or absence of Hyposulphite of Soda, the smallest trace of which is sufficient to remove the pink colour of the Permanganate.
The most available and simple plan of testing permanence is to enclose the pictures in a stoppered glass bottle with a small quantity of water. If they retain their half-tones after a course of three months of this treatment, and do not become mouldy, the mode of printing followed is satisfactory.
Boiling water will also be found useful in distinguishing the unstable colours produced by Sulphur from those following the judicious employment of Gold; in all cases the image will at first be reddened by the hot water, but if toned without Sulphur it will, as a rule, recover much of its dark colour on drying.
The characteristic appearance of prints which have been I much sulphuretted in the toning Bath, and are very liable to fade, should be known. A yellow colour in the lights is a bad sign; and if the half-tones are at all faint and indistinct, with an aspect of commencing yellowness, it is almost certain that the Positive will not last for any considerable length of time.
ON THE THEORY OF THE DAGUERREOTYPE AND TALBOTYPE PROCESSES, ETC.
SECTION I.
The Daguerreotype.
Itwas not the original intention of the Author to include a description of the Daguerreotype Process within the limits of the present Work. The Daguerreotype is a branch of the Photographic Art so distinct from the others, that, in manipulatory details, it bears very little analogy to them; a slight sketch of the theory of the process may not however be unacceptable.
All necessary remarks will fall under three heads:—The preparation of the Daguerreotype film;—the means by which the latent image is developed;—and the strengthening of the image by Hyposulphite of Gold.
The Preparation of the Daguerreotype Film.—The sensitive film of the Daguerreotypist is in many respects different from that of the Calotype or Collodiotype. The latter may be termed wet processes, in contradistinction to the former, where aqueous solutions are not employed. The Daguerreotype film is a pure and isolated Iodide of Silver, formed by the direct action of Iodine upon the metal. Hence it lacks one element of sensitiveness possessed by the others, viz. the presence of soluble Nitrate of Silver in contact with the particles of Iodide of Silver.
It is important to remember that the Iodide of Silver prepared by acting with vapour of Iodine upon metallic Silver, is different in its Photographic action from the yellow salt obtained by double decomposition between Iodide of Potassium and Nitrate of Silver. A Daguerreotype film, when exposed to a bright light, first darkens to an ash-grey colour and then becomes nearly white; the solubility in Hyposulphite of Soda being at the same time lessened. A Collodion film, on the other hand, if the excess of Nitrate of Silver be washed off, although it is still capable of receiving the radiant impression in the Camera, does not alter either in colour or in solubility, by exposure even to the sun's rays.
Details of the process for preparing a Daguerreotype Plate.—A copper plate of moderate thickness is coated upon the surface with a layer of pure Silver, either by the electrotype or in any other convenient manner. It is then polished with great care, until the surface assumes a brilliant metallic lustre. This preliminary operation of polishing is one of great practical importance, and the troublesome details attending it constitute one of the main difficulties to be overcome.
After the polishing is complete, the plate is ready to receive the sensitive coating. This part of the process is conducted in a peculiar manner. A simple piece of cardboard or a thin sheet of wood, previously soaked in solution of Iodine, evolves enough of the vapour to attack the silver plate; which being placed immediately above, and allowed to remain for a short time, acquires a pale violet hue, due to the formation ofan excessively delicate layerof Iodide of Silver. By prolonging the action of the Iodine the violet tint disappears and a variety of prismatic colours are produced, much in the same way as when light is decomposed by thin plates of mica or the surface of mother-of-pearl. From violet the plate becomes of a straw-yellow, then rose-colour, and afterwards steel-grey. By continuing the exposure, the same sequence of tints isrepeated; the steel-grey disappears, and the yellow and rose-colours recur. The deposit of Iodide of Silver gradually increases in thickness during these changes; but to the end it remains excessively thin and delicate. In this respect it contrasts strongly with the dense and creamy layer often employed in the Collodion process, and shows that a large proportion of the Iodide of Silver must in such a case be superfluous, as far as any influence produced by the light is concerned. An inspection of a sensitive Daguerreotype plate reveals the microscopic nature of the actinic changes involved in the Photographic Art, and teaches a useful lesson.
Increase of sensibility obtained by combining the joint action of Bromine and Iodine.—The original process of Daguerre was conducted with the vapour of Iodine only; but in the year 1840 it was discovered by Mr. John Goddard that the sensibility of the plate was greatly promoted by exposing it to the vapours of Iodine and Bromine in succession,—the proper time for each being regulated by the tints assumed.
The composition of this Bromo-Iodide of Silver, so called, is uncertain, and has not been proved to bear any analogy to that of the mixed salt obtained by decomposing a solution of Iodide and Bromide of Potassium with Nitrate of Silver. Observe also that the Bromo-Iodide of Silver is more sensitive than the simple Iodideonly token the vapour of Mercury is employed as a developer. M. Claudet proves that if the image be formed by the direct action of light alone (seepage 174), the usual condition is reversed, and that the use of Bromine under such circumstances retards the effect.
The Development and Properties of the Image.—The latent image of the Daguerreotype is developed in a manner different from that of the humid processes generally,—viz. by the action of Mercurial vapour. Mercury, or Quicksilver, is a metallic fluid which boils at 662° Fahrenheit. We are not however to suppose that the iodized plate issubjected to the vapour of Mercury at a temperature at all approaching to 662°. The cup containing the Quicksilver is previously heated by means of a spirit-lamp to about 140°, a temperature easily borne by the hand, in most cases, without inconvenience. The amount of Mercurial vapour evolved at 140° is very small, but it is sufficient for the purpose, and after continuing the action for a short time the image is perfectly developed.
There are few questions which have given rise to greater discussion amongst chemists than the nature of the Daguerreotype image. Unfortunately, the quantity of material to be operated on is so small, that it becomes almost impossible to ascertain its composition by direct analysis. Some suppose it to consist of Mercury alone. Others have thought that the Mercury is in combination with metallic Silver. The presence of the former metal is certain, since M. Claudet shows that, by the application of a strong heat, it can actually be volatilized from the image in sufficient quantity to develope a second impression immediately superimposed.
It is a remarkable fact that an image more or less resembling that developed by Mercury can be obtained bythe prolonged actionof light alone upon the iodized plate. The substance so formed is a white powder, insoluble in solution of Hyposulphite of Soda; amorphous to the eye, but presenting the appearance of minute reflecting crystals when highly magnified. Its composition is uncertain.
For all practical purposes the production of the Daguerreotype image by light alone is useless, on account of the length of time required to effect it. This was alluded to in the third Chapter, where it was shown that in the case of the Bromo-Iodide of Silver an intensity of light 3000 times greater is required, if the use of the Mercurial vapour be omitted.
M. Ed. Becquerel's discovery of the continuing action of rays of yellow light.—Pure homogeneous yellow light has no action upon the Daguerreotype plate; but if the iodizedsurface be first exposed to white light for a sufficient time to impress a latent image, and thenafterwardsto the yellow light, the action already commenced iscontinued, and even to the extent of forming the peculiar white deposit, insoluble in Hyposulphite of Soda, already alluded to.
Yellow light may therefore in this sense be spoken of as adevelopingagent, since it produces the same effect as the Mercurial vapour in bringing out to view the latent image.
A singular anomaly however requires notice, viz. that if the plate be prepared with the mixed vapours of Bromine and Iodine, in place of Iodine alone, then the yellow light cannot be made to develope the image. In fact, the same coloured ray which continues the action of white light upon a surface ofIodideof Silver, actuallydestroysit, and restores the particles to their original condition, with a surface ofBromo-Iodide of Silver.
These facts, although not of great practical importance, are interesting in illustration of the delicate and complex nature of the chemical changes produced by light.
The Strengthening of the Daguerreotype Image by means of Hyposulphite of Gold.—The use of the Hyposulphite of Gold to whiten the Daguerreotype image, and render it more lasting and indestructible, was introduced by M. Fizeau, subsequent to the original discovery of the process.
After removal of the unaltered Iodide of Silver by means of Hyposulphite of Soda, the plate is placed upon a levelling stand and covered with a solution of Hyposulphite of Gold, containing about one part of the salt dissolved in 500 parts of water. The flame of a spirit-lamp is then applied until the liquid begins to boil. Shortly a change is seen to take place in the appearance of the image; it becomes whiter than before, and acquires great force. This fact seems to prove conclusively that metallic Mercury enters into its composition, since a surface of Silver—such, for instance, as that of the Collodion image—isdarkenedby Hyposulphite of Gold.
The difference in the action of the gilding solution upon the image and the pure Silver surrounding it illustrates the same fact. This Silver, which appears of a dark colour, and forms the shadows of the image, is rendered still darker; a very delicate crust of metallic Goldgraduallyforming upon it, whereas with the image the whitening effect is immediate and striking.
SECTION II.
Theory of the Talbotype and Albumen Processes.
The Talbotype or Calotype.—This process, as practiced by many at the present time, is almost identical with that originally described by Mr. Fox Talbot. The object is to obtain an even and finely divided layer of Iodide of Silver upon the surface of a sheet of paper; the particles of the Iodide being left in contact with an excess of Nitrate of Silver, and usually with a small proportion of Gallic Acid, to heighten, still further, the sensibility to light.
The English papers sized with Gelatine are commonly used for the Calotype process; they retain the film more perfectly at the surface, and the Gelatine in all probability assists in forming the image. With a foreign starch-paper, unless it be re-sized with some organic substance, the solutions sink in too deeply, and the picture is wanting in clearness and definition.
There are two modes of iodizing and sensitizing the sheets: first, by floating alternately upon Iodide of Potassium and Nitrate of Silver, in the same manner as in the preparation of papers for Positive Printing; and second, by what is termed "the single wash," which is thought by many to give superior results as regards sensitiveness and intensity of image. To iodize by this mode, the yellow Iodide of Silver, prepared by mixing solutions of Iodide of Potassium and Nitrate of Silver, is dissolved in a strong solution of Iodide of Potassium; the sheets are floated foran instant upon this liquid and dried; they are then removed to a dish of water, by the action of which the Iodide of Silver is precipitated upon the surface of the paper in a finely divided state.
The properties of a solution of Iodide of Silver in Iodide of Potassium, or of the double Iodide of Potassium and Silver, are described atpage 43, a reference to which will show that the double salt is decomposed by a large quantity of water, with precipitation of the Iodide of Silver, this substance being insoluble in a dilute solution of Iodide of Potassium, although soluble in a strong solution.
Paper coated with Iodide of Silver by this mode, after proper washing in water to remove soluble salts (which if allowed to remain would attract damp), will keep good for a long time. The layer of Iodide appears of a pale primrose colour, and is perfectly insensitive to light. Even exposure to the sun's rays produces no change, thus indicating that an excess of Nitrate of Silver is essential to the visible darkening of Iodide of Silver by light. The paper is also insensitive to the reception of an invisible image, differing in this respect from the washed Collodion plate, which receives an impression in the Camera, although apparently freed from Nitrate of Silver.
To render Calotype paper sensitive to light, it is brushed with a solution of Nitrate of Silver containing both Acetic and Gallic Acids, termed "Aceto-Nitrate" and "Gallo-Nitrate" solution. The Gallic Acid lessens the keeping qualities of the paper, but increases the sensitiveness. The Acetic Acid prevents the paper from blackening all over during the development, and preserves the clearness of the white parts; its employment is indispensable.
The paper is commonly excited upon the morning of the day upon which it is intended to be used; and the longer it is kept, the less active and certain it becomes. An exposure of five to eight minutes in the Camera is the average time with an ordinary view lens.
The picture is developed with a saturated solution ofGallic Acid, to which a portion of Aceto-Nitrate of Silver is added to heighten the intensity. Both Sulphate of Iron, and Pyrogallic Acid have also been used, but they are unnecessarily strong, the invisible image being more easily developed upon paper than upon Collodion (seepage 143).
After fixing the Negative by removing the unaltered Iodide of Silver with Hyposulphite of Soda, it is well washed and dried. White wax is then melted in with a hot iron, so as to render the paper transparent, and to facilitate the after-process of printing.
The Calotype cannot be compared with the Collodion process for sensitiveness and delicacy of detail, but it possesses advantages for tourists and those who do not wish to be encumbered with large glass plates. The principal difficulty appears to be in obtaining a uniformly good paper, many samples giving a speckled appearance in the black parts of the Negative.
The Waxed Paper process of Le Grey.—This is a useful modification of the Talbotype introduced by M. Le Grey. The paper is waxed before iodizing, by which, without involving any additional operation, a very fine surface layer of Iodide of Silver can be obtained. The Waxed Paper Process is well adapted for tourists, from its extreme simplicity and the length of time which the film may be kept in a sensitive condition.
Both English and foreign papers are employed: but the former take the wax with difficulty. Mr. Crookes, who has devoted his attention to this process, gives clear directions for waxing paper; it is essential that pure white wax should be obtained direct from the bleachers, since the flat cakes sold in the shops are commonly adulterated. Thetemperaturemust also be carefully kept below that point at which decomposition of the wax takes place; the use of too hot an iron being a common source of failure (see 'Photographic Journal,' vol. ii. p. 231).
The sheets of paper, having been properly waxed, are soaked fortwo hoursin a solution containing Iodide andBromide of Potassium, with, enough free Iodine to tinge the liquid of a port-wine colour. The greasy nature of wax impedes the entry of liquids, and hence a long immersion is required. The iodizing formulæ of the French Photographers have been encumbered by the addition of a variety of substances which appear to introduce complications without giving proportional advantage, and Mr. Townshend has done the art a service by proving that the Iodide and Bromide of Potassium, with free Iodine, are sufficient. This latter ingredient was first used by Mr. Crookes; it seems to add to the clearness and sharpness of the Negatives; and as the papers arecolouredby the Iodine, air-bubbles cannot escape detection. The process of exciting with Nitrate of Silver is also rendered more certain by the employment of free Iodine, the action of the Bath being continued until the purple colour gives place to the characteristic yellow tint of the Iodide of Silver.
Waxed Paper is rendered sensitive by immersion in a Bath of Nitrate of Silver containing Acetic Acid; the quantity of which latter ingredient should be increased when the papers are to be long kept. As the excess of Nitrate is subsequently removed, the solution may be used weaker than in the Calotype or Collodion process.
After exciting, the papers are washed with water to reduce the amount of free Nitrate of Silver to a minimum. This lessens the sensitiveness, but greatly increases the keeping qualities, and the paper will often remain good for ten days or longer.
It is a very important point, in operating with Waxed Paper, to keep the developing dishes clean. The development is conducted by immersion in a Bath of Gallic Acid containing Acetic Acid and Nitrate of Silver; and being retarded by the superficial coating of wax, there is always a tendency to an irregular reduction of Silver upon the white portions of the Negative. When the developer becomes brown and discoloured, this is almost sure to happen; and it is well known to chemists that the lengthof time during which Gallic Acid and Nitrate of Silver may remain mixed without decomposing, is much lessened by using vessels which are dirty from having been before employed for a similar purpose. The black deposit of Silver exercises acatalytic(καταλυσις, decomposition by contact) action upon the freshly-mixed portion, and hastens its discoloration.
The Waxed Paper process is exceedingly simple and inexpensive,—very suitable for tourists, as requiring but little experience, and a minimum of apparatus. It is however slow and tedious in all its stages, the sensitive papers frequently taking an exposure of twenty minutes in the Camera, and the development extending over an hour or an hour and a half. Several Negatives however may be developed at the same time; and as the removal of the free Nitrate of Silver gives the process a great advantage during hot weather, it will in all probability continue to be extensively followed. The prints which have been sent to the Exhibition of the Photographic Society, show that waxed paper in the hands of a skilful operator may be made to delineate architectural subjects with great fidelity, and also to give the details of foliage and landscape Photography with distinctness.
The Albumen process upon Glass.—The process with Albumen originated in a desire to obtain a more even surface layer of Iodide of Silver than the coarse structure of the tissue of paper will allow. It is conducted with simple Albumen, or "white of eggs," diluted with a convenient quantity of water. In this glutinous liquid Iodide of Potassium is dissolved; and the solution, having been thoroughly shaken, is set aside, the upper portion being drawn off for use, in the same manner as in the preparation of Albuminized paper for printing.
The glasses are coated with the Iodized Albumen, and are then placed horizontally in a box to dry. This part of the process is considered the most troublesome, the moist Albumen easily attracting particles of dust, and being aptto blister and separate from the glass. If an even layer of the dried and Iodized material can be obtained, the chief difficulty of the process has been overcome.
The plates are rendered sensitive by immersion in a Bath of Nitrate of Silver with Acetic Acid added, and are then washed in water and dried. They may be kept for a long time in an excited state.
The exposure in the Camera must be unusually long; the free Nitrate of Silver having been removed by washing, and the Albumen exercising a direct retarding influence upon the sensitiveness of Iodide of Silver.
The development is conducted in the ordinary way by a mixture of Gallic Acid and Nitrate of Silver, with Acetic Acid added to preserve the clearness of the lights. It usually requires one hour or more, but may be accelerated by the gentle application of heat.
Albumen pictures are remarkable for elaborate distinctness in the shadows and minor details, and are admirably adapted for viewing in the Stereoscope; but they do not often possess the peculiar and characteristicsoftnessof the Photograph upon Collodion. The process is well adapted for hot climates, being very little prone to the cloudiness and irregular reduction of Silver which are often complained of with moist Collodion under such circumstances.
M. Taupenot's Collodio-Albumen process.—This is a recent discovery which seems to involve a new principle in the Art, and gives promise of great utility.
One of the greatest objections to the Albumen process has been its want of sensitiveness; but M. Taupenot found that this was obviated to a great extent by pouring the Albumen upon a platepreviously coated with Iodide of Silver. In this way two layers of that sensitive salt are formed, and the sensibility of the surface layer, which alone receives the image, is promoted by its resting upon a substratum of Iodide rather than upon the inert surface of the glass. In this view, if the theory be correct, the lowerparticle of Iodide of Silver promotes the molecular disturbance of the upper, itself remaining unchanged.
Other experimenters, pursuing the subject further, have asserted that a successful result may be obtained by coating the plate with plain Collodion and subsequently with Iodized Albumen. If this observation should prove correct, the process will be simplified and its utility increased.
In the sixth Chapter of Part II. the practical details of the Collodio-Albumen process will be described.
END OF PART I.
PRACTICAL DETAILS OF THE COLLODION PROCESS.
bar diamond
PREPARATION OF COLLODION.
Thisincludes—the soluble Paper;—the Alcohol and Ether;—and the iodizing compounds.
The formulæ for Negative and Positive Collodion, and for the Nitrate Bath and developing fluids, are given in the second Chapter.
THE SOLUBLE PAPER.
Pyroxyline may be prepared either from cotton wool or from Swedish Filtering-paper. Most operators prefer the latter, from its giving a product of constant solubility, and yielding a fluid solution.[30]The Cotton Wool however is better adapted for use with the Sulphuric Acid and Nitre, since the Paper, from its closeness of texture, requires a longer immersion in the mixture.
[30]Swedish filtering-paper may be procured at the operative chemists', at about five shillings the quire. Each half-sheet has the water-mark "J. H. Munktell."
[30]Swedish filtering-paper may be procured at the operative chemists', at about five shillings the quire. Each half-sheet has the water-mark "J. H. Munktell."
Preparation of a Nitro-Sulphuric Acid of the proper strength.—There are two modes of preparing the Nitro-Sulphuric Acid: first, by mixing the acids; second, by the Oil of Vitriol and Nitre Process. The former is thebest in cases where large quantities of the material are operated on, but the amateur is recommended to begin by trying the Nitre Process (p. 190) as the most simple.
PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS.
The operator may proceed in either of two ways; first, by taking the strength of each sample of acid, and mixing according to fixed rule; second, by a more ready plan, which may be used when the exact strength of the acids is not known. Each of these will be described in succession.
a.Directions for mixing according to fixed rule.—This process is given from Mr. Hadow's original paper in the 'Quarterly Journal of the Chemical Society.' It is certain in its results if the strength of both acids be accurately determined.
A very perfect process for taking the strength of Nitric Acid is by means of powdered Marble or Carbonate of Lime, as described in various works on practical Chemistry. Sulphuric Acid may be estimated by precipitating with Nitrate of Baryta, and weighing the insoluble Sulphate with the proper precautions.
The specific gravity is not a criterion of strength to be perfectly relied on, but if it be adopted as a test, the following points must be attended to.
1st. That the temperature of the acid be at or near 60° Fahrenheit; the density of Sulphuric Acid especially is, from its small specific heat, greatly influenced a change of temperature.
2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or only slightly coloured by it. This substance, when present, increases the specific gravity of the acid without adding to its available properties. A yellow sample of Nitric Acid will therefore be somewhat weaker than is indicated by the specific gravity.
3rd. The Oil of Vitriol should yield no solid residue onevaporation. Sulphate of Lead and Bisulphate of Potash are often found in the commercial acid, and add much to its density. Oil of Vitriol containing Sulphate of Lead becomes milky on dilution.
The formula for a definite Nitro-Sulphuric Acid, of the proper strength for making the soluble Pyroxyline, may be stated thus:—
HO NO5, 2 (HO SO3) + 3½ HO
or
Having found the percentage of real acid which is present,[31]the following calculation will give the relative weights of the ingredients required to produce the formula:—
{
[31]Tables are given in the Appendix for calculation by specific gravity; but direct analysis of the acids is the most certain.
[31]Tables are given in the Appendix for calculation by specific gravity; but direct analysis of the acids is the most certain.
Observe that the numbers in the calculation correspond to the atomic weights recently given; and that the amount of water is derived from thetotal atomic weight, viz. 192,minusthe sum of the weights of both acids.
Hence if the samples of acid employed are too weak for the purpose, the formula for the water gives a negative quantity.
The weight of mixed acids produced by the formula is 192 grains, which would measure somewhere about two fluid drachms. Ten times this quantity forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper may be immersed.
In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small glass may be counterbalanced in the scale-pan, and the acid poured in carefully. If too much is added, the excess can be removed by a glass rod, or by "the pipette" commonly employed for such a purpose.
The following example of a calculation similar to the above may be given:—
Multiplying these weights ten times, we have
Having prepared the acid mixture of a definite strength by the above formula, the paper must be immersed according to directions given atpage 191.
b.Process for mixing Nitro-Sulphuric Acid, the strength of the two acids not having been previously determined.—Take a strong sample of Nitric Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with Oil of Vitriol as follows:—
Now immerse a thermometer and note the temperature;[32]it should be from 130° Fahr. to 150°. If it sinks below 120°, place the mixture in a capsule, and float upon boiling water for a few minutes.