To separate the two metals contained in the above solution from each other, put some bright copper coins into the solution and set it aside in a warm place for three or four days, occasionally giving it a circular motion. The separated laminae are pure silver, which is to be digested with ammonia until it ceases to be colored blue. The silver, after being washed and dried, is again dissolved in nitric acid, and the liquid, diluted with water, is kept as solution, of silver.
Either of the above solutions (the one of oxide of silver and copper, and the pure silver solution) may be prepared for use by putting them in a bottle, with a quant.i.ty of water, and adding common fine salt, you obtain a white curdy precipitate of chloride of silver. No matter how much salt is used, provided enough be added to throw down all the chloride of silver. This solution should be well agitated and then allowed to stand for a few minutes; thus the white precipitate is in the bottom of the bottle. When the water has become clear, pour it off with care, leaving the sediment behind, then add a fresh quant.i.ty of clean water, shake, let settle, and pour off as before. Repeat the same for several times, and the excess of salt will disappear, leaving the white precipitate, which may be drained of the water and dried in the dark, and kept free from light and air.
CYANIDE OF POTa.s.sIUM.
Cyanide of Pota.s.sium.--This important article is worthy the undivided attention of every Daguerreotypist. I here give Mr. Smee"s process for its preparation. This is from that author"s work ent.i.tled, "Electro Metallurgy," American edition:
"The cyanide of pota.s.sium, so often alluded to while treating of the metallo-cyanides, may be formed in several ways. It may be obtained by heating to a dull redness the yellow ferrocyanate of potash, in a covered vessel, filtering and rapidly evaporating it. The objection to this method, however, is that without great care the whole of the ferrocyanate is not decomposed, a circ.u.mstance which much reduces its value for electro-metallurgy. By boiling, however, the ignited residue with spirits of wine this difficulty is said to be overcome, as the ferrocyanate is absolutely insoluble in that menstruum, while the cyanuret, at that heat, freely dissolves, and is as easily re-deposited on cooling.
"There is, however, a much better process by which this salt may be formed, viz. by simply transmitting hydrocyanic acid through pota.s.sium. Although the modes of making this acid are very numerous, there is but one which is likely to be employed on a very large scale, and that is its formation from the yellow ferrocyanate by means of sulphuric acid. This process is performed as follows: any given weight of the yellow salt is taken and dissolved in about five times its weight of water; this is placed in a retort, or some such a.n.a.logous vessel, to which is then added a quant.i.ty of strong sulphuric acid, twice the weight of the salt, and diluted with three or four times its quant.i.ty of water. A pipe is carried from the neck of the retort to the receiving bottle, which should be kept as cool as possible.
"For small operations, those invaluable vessels, Florence flasks, answer well: a bent tube being connected at one end to its month, the other pa.s.sing into the second vessel; heat should be cautiously applied by means of an Argand lamp, a little vessel of sand being placed under the flask, which helps the acid to decompose the salt. Prussic acid is then generated and pa.s.ses through the tube to the recipient vessel, which is to be charged with liquor pota.s.sae.
"When the potash is saturated, the operation is completed. The Germans recommend a strong, alcoholic solution of pota.s.sa to be used in the second vessel, for in this case, the hydrocyanic or prussic acid combines with the pota.s.sa, forming a hydrocyanate of pota.s.sa, or, the water being abstracted, the cyanuret of pota.s.sium, which spontaneously precipitates, on the saturation of the fluid, the cyanuret, being insoluble in strong alcohol. The ferrocyanate of potash may be considered as containing three equivalents of hydrocyanic acid, two of potash and one of iron; but, unfortunately, we can only obtain half the acid from the salt, owing to the formation of a compound during its decomposition which resists the action of the acid. The decomposition of this salt taking 2 equivalents or 426 grains (to avoid fractions) would afford 3 equivalents or 81 grains of hydrocyanic, or prussic acid, capable of forming 198 grains of cyanuret of pota.s.sium, while in the retort there would remain 384 grains or 3 equivalents of bisulphate of potash, and 1 equivalent or 174 grains of a peculiar compound, said to contain 3 equivalents of cyanogen, 1 of pota.s.sium, and one of iron (Pereira). It is manifest that, but for this later compound, we might double the quant.i.ty of hydrocyanic acid from the yellow salt."
The decomposition just described is the one usually received; but too much reliance must not be placed on its accuracy, for the a.n.a.lysis of the several compounds is too difficult for the results to be fully admitted. The residue left in the retort speedily turns to one of the blues, identical with, or allied to, Prussian blue. This is at best a disagreeable process to conduct, for the hydrocyanic acid formed adheres so strongly to the gla.s.s, that, instead of being freely given off, bubbles are evolved suddenly with such explosive violence as occasionally to crack the vessel. This may be remedied as far as possible by the insertion of plenty of waste pieces of platinum--if platinized, so much the better, as that facilitates the escape of the gas. The heat should be applied to every part of the vessel, and the flame should not be allowed to play upon one single part alone. Large commercial operations are performed in green gla.s.s or stone-ware retorts.
"Now for one word of advice to the tyro: Remember that you are working with prussic acid; therefore, never conduct the process in a room, the fumes being quite as poisonous as the solution of the acid itself; moreover, have always a bottle of ammonia or chlorine by your side, that should you have chanced to inhale more than is pleasant, it will be instantly at hand to counteract any bad effects. It is stated by Pereira, that a little sulphuric acid or hydroferrocyanic acid pa.s.ses to the outer vessel, but probably the amount would be of no consequence for electro-metallurgy, otherwise, it might be as well to use a Woulfe"s apparatus, and discard the salt formed in the first vessel.
To the large manufacturer it may be worth considering whether some other metallo-cyanuret, formed in a similar manner to the ferrocyanuret, might not be more advantageously employed, because the residue of the process last described contains a large quant.i.ty of cyanogen which the acid is unable to set free.
"There are other modes of procuring prussic acid, besides the one which has been so tediously described; but these are found to be more expensive. The only one which I shall now notice is the process by which it is obtained from bicyanide of mercury. The bicyanide of mercury itself is formed when peroxide of mercury is digested with Prussian blue, the peroxide of mercury abstracting the whole of the cyanogen from the blue, and leaving the oxides of iron at the bottom of the vessel. The solution may be evaporated to dryness, and one part of the salt dissolved in six of water; one part of muriatic acid, sp. gr.
1.15, is then added, and the solution distilled, when the whole of the hydrocyanic acid pa.s.ses over, and by being conducted into a solution of pota.s.sa, as in the former process, forms cyanuret of pota.s.sium. This process, though easier than the first described, is rather given as a resource under peculiar circ.u.mstances than as one to be adopted by the large manufacturer. The expense is the only objection, but in a small quant.i.ty this cannot be a consideration.
"In giving this very rough outline of the general mode of forming salts, the minutiae necessary for chemical work have altogether been avoided, and those parts alone are entered upon which are more immediately necessary for the electro metallurgist to know and practice for himself. This will account for the long description of the cyanuret of pota.s.sium, while the preparation of the equally important and even more used acids, the sulphuric, muriatic, etc., commonly found in commerce, are altogether neglected.
"In using solutions of cyanide of pota.s.sium, the workman should not immerse his arms into them, otherwise it occasionally happens that the solution produces very troublesome eruptions over the skin."
HYPOSULPHITE OF SODA.
Hyposulphite of Soda.--This salt forms one of the important chemicals for the Daguerreotype operator. Its application to this art is of an interesting nature. It is used to dissolve the sensitive salt of silver which remains unchanged during the exposure in the camera. It has the property of readily dissolving the chloride, bromide and iodide of silver. It should be pure and free from sulphuret of sodium; should this last be present, it will cause brown spots of sulphurated silver upon the Daguerreotype impression. This annoyance is a great source of complaint from many operators, and ever will be, so long as it is prepared by men who have no reputation to lose, and whose eyes are blinded by the "Almighty Dollar."
A good article may be prepared as follows:
"Mix one pound of finely pulverized carbonate of soda with ten ounces of flowers of sulphur, and heat the mixture slowly in a porcelain dish till the sulphur melts. Stir the fused ma.s.s, so as to expose all its parts freely to the atmosphere, whereby it pa.s.ses from the state of a sulphuret, by the absorption of atmospheric oxygen, into that of a sulphite, with the phenomenon of very slight incandescence. Dissolve in water, filter the solution, and boil it immediately along with flowers of sulphur. The filtered concentrated saline liquid will afford, on cooling, a large quant.i.ty of pure and beautiful crystals of hyposulphite of soda."
Hyposulphite of Gold.--This compound salt is by a few considered preferable to the chloride of gold, but our experience has induced us to use the latter, believing we are enabled to produce a more brilliant and warm-toned impression with it. When the hyposulphite of gold is used in gilding, it requires less heat and a longer application, as there is some danger of producing a glossy sc.u.m over some parts of the surface of the plate. I prepare this salt as follows:
Dissolve one part chloride of gold and four parts hyposulphite of soda in equal quant.i.ties of distilled water: pour the gold into the hyposulphite solution, in the same manner as in mixing the gilding solution; let it stand until it becomes limpid; filter and evaporate to dryness. Re-dissolve and add a few grains of burnt alum.
After standing a few hours, filter and evaporate again. If not sufficiently pure, repeat the crystallization until it is so. For gilding, dissolve in water and use in the same manner as the common gilding solution.
N.B.--The four following mixtures were employed in Neipce"s process in his earliest experiments:
Aqueous Solution of Bichloride of Mercury.--Eight grains of bichloride of mercury in 10,000 grains of distilled water.
Solution of Cyanide of Mercury.--A flask of distilled water is saturated with cyanide of mercury, and a certain quant.i.ty is decanted, which is diluted with an equal quant.i.ty of distilled water.
Acidulated White Oil of Petroleum.--This oil is acidulated by mixing with it one tenth of pure nitric acid, leaving it for at least 48 hours, occasionally agitating the flask. The oil, which is acidulated, and which then powerfully reddens litmus paper, is decanted. It is also a little colored, but remains very limpid.
Solution of Chloride of Gold and Platinum.--In order not to multiply the solutions, take the ordinary chloride of gold, used for fixing the impressions, and which is composed of 1 gramme of chloride of gold and 50 grains of hyposulphate of soda, to a quart of distilled water.
With respect to chloride of platinum, 4 grains must be dissolved in 3 quarts of distilled water; these two solutions are mixed in equal quant.i.ties.
Acids.--I shall not go into the preparations of the various acids employed in the Daguerreotype. This would be useless to the operator, as there are few, if any, that it would be advisable to prepare. It is only necessary for the experimenter to be made acquainted with their properties, and this in order to prevent any haphazard experiments, which are too common among operators. Any person who may be desirous to try an experiment, should first study the agents he wishes to employ. By so doing much time and money will be saved; while the searcher after new discoveries would rarely become vexed on account of his own ignorance, or be obliged to avail himself of the experience of others in any department of science.
Nitric Acid--Exists in combination with the bases, potash, soda, lime, magnesia, in both the mineral and vegetable kingdoms, and is never found insoluble. It has the same const.i.tuents as common air, but in different proportions. The strongest nitric acid contains in every pound, two and a quarter ounces of water. Pure nitric acid is colorless, with a specific gravity of 1.5, and boiling at 248 deg.. It is a most powerful oxidizing agent, and is decomposed with more or less rapidity, by almost all the metals, to which it yields a portion of its oxygen.
The nitric acid of commerce, is generally the article used by the Daguerreotypist. This usually contains some chlorine and sulphuric acid. It is obtained by the distillation of saltpetre with sulphuric acid. It is employed in the Daguerreotype process for dissolving silver, preparing chloride or oxide, nitrate of silver, [the former used in galvanizing,] and in combination with muriatic acid for preparing chloride of gold, used in gilding. It is also used by some for preparing the plate.
Acidulated Solution.--This solution is used for cleaning the surface of the Daguerreotype plate. It has the property of softening the silver, and bringing it to a state in which it is very susceptible of being either oxidized or iodized, hence it contributes to increase the sensibility of the plate. The proportions are to one drop of acid add from 15 to 20 drops of water, or make the solution about like sharp vinegar to the taste.
Nitro-Muriatic Acid.--Aqua Regia is a compound menstruum invented by the alchemists for dissolving gold. It is composed of colorless nitric acid (aqua-fortis) and ordinary muriatic acid; the mixture is yellow, and acquires the power of dissolving gold and platinum. These materials are not properly oxidized; it nearly causes their combination with chlorine, which is in the Muriatic acid.
Hydrochloric Acid (Muriatic Acid).--This acid forms a valuable addition to the chemicals employed by the practical Daguerreotypist. This acid is formed by acting upon common salt (which is chloride of sodium) by concentrated sulphuric acid. The water of the acid is decomposed, and its hydrogen combines with the chloride of the salt to form muriatic acid, and this unites with the sulphuric acid to form sulphate of soda; 60 parts of common salt and 49 parts of concentrated sulphuric acid, afford, by this mutual action, 37 parts of muriatic acid and 72 parts of sulphate of soda. The muriatic acid of commerce has usually a yellowish tinge, but when chemically pure it is colorless. The former is commonly contaminated with sulphurous acid, sulphuric acid, chlorine, iron, and sometimes with a.r.s.enic.
Muriatic acid, from the fact of the presence of the chlorine, is used in the Daguerreotype process for dissolving gold, and in combination with various accelerators. Its presence can be detected by ammonia. A strip of paper dipped in this and waved to and fro will emit a thick white smoke if the acid vapor be in the atmosphere. The ammonia neutralizes the acid fumes. By reversing the experiment we can determine whether vapor of ammonia be in the air, and also deprive these suffocating and dangerous gases of their injurious properties, and remove them from the air. Every Daguerreotype operator should be furnished with, at least, a six ounce bottle of aqua ammonia. Its operation is very nearly the same on bromine and iodine vapor.
Hydrofluoric Acid (Fluorohydric Acid).--This acid is used to form some of the most volatile and sensitive compounds employed in the Daguerreotype. It is one of the most dangerous bodies to experiment with: it is volatile and corrosive, giving off dense white fumes in the air. It combines with water with great heat. At 32 deg. it condenses into a colorless fluid, with a density 1.069. It is obtained from decomposition of fluorspar by strong sulphuric acid. It readily dissolves the silica in gla.s.s, and consequently cannot be kept in a vessel of that material. It is prepared and kept in lead. It is employed in accelerators on account of its fluorine.
One small drop on the tongue of a dog causes death. The operator who wishes to use it should pour some of the liquid for which he intends it into a graduate, or other vessel, and then add the desired quant.i.ty of acid. If by accident any of the spray should fall upon the skin, it should at once be copiously drenched with water.
Sulphuric Acid.--There are two sorts of this acid: one is an oily, fuming liquid; this is made in Nordhausen, in Saxony, and is commonly called "Nordhausen sulphuric acid," or oil of vitriol. The other which is the kind used in connection with the Daguerreotype, is common sulphuric acid. It is somewhat thinner, and when undiluted is not fuming. This acid may be obtained in a solid and dry state, called anhydrous sulphuric acid.
The common sulphuric acid is made by burning sulphur, which forms sulphurous acid. To convert this into sulphuric acid and gain more oxygen, nitric acid, which is rich in that body, is added. It forms a limpid, colorless fluid, of a specific gravity of 1.8. It boils at 620 deg.; it freezes at 15 deg. It is acrid and caustic, and intensely acid in all its characters, even when largely diluted.
Its attraction for basis is such that it separates or expels all other acids, more or less perfectly, from their combinations. Its affinity for water is such that it rapidly absorbs it from the atmosphere, and when mixed with water much heat is evolved. It acts energetically upon animal and vegetable substances, and is a poisonous, dangerous substance to get on the skin. It is a powerful oxidizing agent; hence its use in the galvanic battery, for which purpose it is mostly used by the Daguerreotypist. The fumes of this being so much more offensive than nitric acid, the latter is sometimes used. It is also employed in some of the more sensitive accelerators.
ACCELERATING SUBSTANCES.
Remarks on the Accelerating substances Used in the Daguerreotype.--I have now arrived at a point in this work, where the eye of the Daguerreotype public will intently search for something new. This search will prove in vain, at least so far as regards those who have enjoyed and embraced the opportunities for studying the principles of our art. Every experienced operator has in a degree become familiar with the mechanical uses of all the agents employed, while I fear but few understand the properties, and laws governing those properties, which are so indispensable to produce an image impressed upon the silver surface.
There are three substances which form the bases for producing a Daguerreotype; silver, iodine and bromine. Each forms a separate body which is indispensable to the operators success as the art is now practiced in America. With these three, compounds of great variety are formed.
The silver surface is first thoroughly cleaned and freed from all organic matter, then exposed to vapor of iodine, producing an iodide of silver. The plate upon which is this salt, is again exposed to the vapor of bromine, forming a bromo-iodide of silver, a salt also.
As most of the various accelerators are compounds of bromine, with either chlorine or fluorine combination, they partake somewhat of the nature of these latter, giving results which can be detected by the experienced operator. Thus muriatic acid is added for its chlorine, which can generally be detected by the impression produced, being of a light, soft, mellow tone, and in most cases presenting a brilliant black to that colored drapery. Those who wish to experiment with agents for accelerating substances, should first study to well understand their peculiar nature and properties; as well, also, to endeavor to find out what will be the probable changes they undergo in combination as an accelerator. This should be done before making the experiments. From the foregoing it will be seen that numerous compounds are formed from the same basis, and, consequently, it would be a waste of time and a useless appropriation to devote more of our s.p.a.ce than is necessary to give the princ.i.p.al and most reliable combination.
In America, the words "Quick" and "Quick Stuff," are more generally used for and instead of the more proper names, "Sensitives," or "Accelerators," etc. As it has by use become common, I frequently use it in this work.
Liquid Accelerator, No. 1.--This mixture was used by me in 1849, and is given as it appeared in my "System of Photography," published at the above date:
Take pure rain or distilled water, one quart, filter through paper into a ground stopper bottle, and add, for warm weather, one and a half ounce chloride of iodine; or for cold, one ounce; then add one ounce bromine, and shake well. Now with care not to allow the vapor to escape, add drop by drop, thirty drops of aqua ammonia, shaking well at each drop. Care must be taken not to add more at a time, as it evokes too much heat. This mixed, in equal proportions with John Roach"s quick, forms an excellent chemical combination. For this purpose, take one and a half ounce of each, to which add ten ounces water, for warm weather, or from six to seven for cold. Pour the whole into a large box, and it will work from two to four months. I am now using (1849) one charged as above which has been in constant use for three months, and works uniformly well. The above is right for half or full size boxes, but half of it would be sufficient for a quarter size box.
Coat to the first shade of rose over iodine, change to a deep rosy red over quick, and black about one tenth the first.
I would not now recommend the addition of "John Roach"s quick," as I believe equally good results can be produced without it. This liquid is now used by many, and is very good for taking views.