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Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter XVIII.

INTENSIFIERS are substances which, when applied in solution to the developed image, increase the opacity of the shadows and middle tints, rendering them more impermeable to light in direct positive printing. With a proper adjustment of light and developer, and especially in ordinary landscape-photography, an intensifier is seldom needed; but many artists prefer the use of the intensifier on every occasion; they maintain that a negative can always be preserved as clear and transparent in the lights as a positive lay this process, and yet the density of the shadows may be increased to any extent without any fear of fogging. The intensifying process becomes, therefore, a fixed part in the preparation of a negative. The operation is partly physical and partly chemical; physical, because whatever may have been the action of the light on those parts in which the, image is now apparent, they seem still to be endowed with properties of attraction of an intensity in proportion to the development produced, just as they were at the commencement of reduction; but the nitrate of silver, iodide; or bromide of silver, having been exhausted, the application of any developer, however sensitive or intense, could produce no more opacity on the shadows for want of material to be reduced--but, mark it well, the physical condition is there to institute this reduction the moment material is supplied.

From my preceding remarks it is supposed that the developed image consists of reduced silver, or in altered salt of silver very different from any with which we are acquainted; there is no more iodide or nitrate of silver; these have been removed in the fining and washing Now in order to restore the partially developed image to the chemical condition requisite for the recommencement of the development, a solution of iodine in iodide or potassium, or a dilute solution of tincture of iodine, is flowed over the plate, and kept in motion over the image in order to preserve uniformity of action. The iodine thus coming in contact with the silver shallows enters into combination with this metal, and forms a new and thicker deposit of iodide of silver with all the gradations of opacity of the image, and not a uniform film of deposit. The solution of iodine on the collodion loses color all the while; but the collodion film assumes at first a grayish and then a yellowish-gray hue. Even at this stage there is much more opacity in the shadows of the picture than before, and the negative by this proceeding may probably ire dense enough; if not, proceed to the second stage. The first stage is the depositing stage; the second, the reducing or developing stage proper; and yet this deposit of the first stage is a chemical combination of iodine and silver which is now soluble in the fixing solutions, and before it was not. By this process of depositing and fixing, and by regulating the quantity of the iodine solution, a negative which is too opaque may be rendered more transparent and less dente ad libitum. Osborne has availed himself of this property to clarify his negatives for the photolithographic process; I would recommend it also in the preparation of clear and sharp negatives for obtaining enlarged positives in the solar camera. As soon as the depositing stage is complete, and the film leas been washed, the collodion film is ready for the reception of the next operation.

The second stage consists in communicating to the iodized image a minute quantity of nitrate of silver, either alone and diluted, or in connection with the developer; it is, in fact, a mere repetition of the original process of development; the surface of the collodion is in the same condition as at the commencement when it left the camera; there are present iodide of silver, nitrate of silver, iodide of potassium, the peculiar and unknown physical attraction existing in the formed image where before the image as yet was unformed, and the developing solution either of sulphate of iron or pyrogallic acid. The second stage is then a system or process of redevelopment. By this operation the intensity may be increased to any extent; the shadows can be made quite opaque and utterly impermeable to the actinic influence. The intensifying part of the collodion process is very much in the power of the artist; success, therefore, will depend principally on the artistic condition of what I denominate the Foundation Negative If the foundation negative, however thin the shadows may be, contain light, shades, and middle tones in perfect detail, then the artist has it in his power to raise these three conditions gradually and uniformly higher, until the shadows become endowed with a proper opacity. At the end of this stage fixing solutions have but little effect, which seems to demonstrate that the yellowish-gray iodide has been converted into an insoluble metallic film or an unknown insoluble silver salt. It is not necessary to use the fixing solution. All that is required is to wash the image well before it is dried and varnished.

Other deposits and other metals may be introduced in the intensifying operations, which will be found described below.

From the recent experiments and observations of Blanquart Evrard,* it appears that a negative may be intensified by a second exposure to light before fixing. Thus, supposing a negative be developed as far as it seems possible to carry on the reduction, in this condition let it be exposed for a short time to diffused light. This physical force, it is said, again acts actinically, but now only upon the parts which contain the image, communicating to these new vigor, and a fresh impulse, which, on the application of the developer, again will assist in the formation of further reduction.

As soon as the image has been fixed, as in the first example, it is sometimes flowed with a saturated solution of bichloride of mercury, by which probably the bichloride is reduced to the protochloride, and the liberated chlorine goes over to the silver, and forms chloride of silver. This application communicates a whiteness to the image, and thickens the deposit. When the negative has been washed, it is flowed with an iodizing solution, containing five per cent of iodide of ammonium in water. In this way the image becomes converted into a double iodide of silver and mercury, which, when washed, is treated with the iron or pyrogallic developer, containing a few drops of nitrate of silver, as before. It frequently happens in this, as in the preceding case, that the film at the end of the first stage is opaque enough. In this case it maybe rendered black by flowing it with ammonia, hyposulphite of soda, or cyanide of potassium.

A third method of strengthening the dark parts of a negative takes advantage of the alkaline sulphides, which convert the developed film into a sulphide. By this operation, however, the film as a rule is not increased in thickness, its color alone being changed, which is frequently more agreeable to look at, and apparently more dense, because it is black, or bluish-black. These alkaline sulphides may be used with advantage at the end of the first stage or deposit, in order to blacken this deposit; but by this mode of intensifying there is a great liability to unequal action, to decomposition after the negative is varnished, to contraction of the collodion film, and its separation from the glass; besides this, sulphur seems to be precipitated sometimes in very irregular patches, giving a speckled appearance to the negative.

Preparation of Bichloride of Mercury--Corrosive Sublimate.

Symbol, Hg. Cl. Combining Proportion, 136.9. Spec. grav., 5.4.

Dissolve red oxide of mercury in hydrochloric acid; evaporate and crystallize; or sublime a mixture of equal weights of sulphate of mercury and common salt in a stoneware retort by heating to redness in a sand-bath. The bichloride, being volatile, passes out, whilst sulphate of soda remains behind in the retort. This substance melts at 509°, and boils at 563°; it dissolves in twenty parts of cold water, in two parts of boiling water, in two and one third of cold alcohol, and in three of cold ether. When hydrosulphuric acid is passed through a solution of this salt, a brownish precipitate is first formed, which eventually becomes quite white. This is a chlorosulphide.

Preparation of Sulphide of Potassium.--Hepar Sulphuris.

Symbol, K S3.

Fuse together, at a low red heat, one part of sulphur, and two of carbonate of potash, as long as effervescence takes place; then pour on to a marble slab, and when cool, break up the mass, and keep it in well-closed bottles. This sulphide has a liver-brown appearance. By the addition of an acid to a solution of the sulphide, hydrosulphuric acid is liberated, a soluble salt formed, and sulphur precipitated of a milk-white color. The alkaline sulphides have the same reaction on metallic salts as hydrosulphuric acid, forming precipitates of different colors, by which frequently the metals can be recognized, as, for instance, antimony, cadmium, etc.

Preparation of Sulphide o f Ammonium.

Symbol, N H4 S. H S.

Let a current of hydrosulphuric acid pass through concentrated ammonia to saturation; then add an equal bulk of ammonia. This is one of the most important reagents in chemistry. Hydrosulphuric acid produces precipitates in metallic salts, Some of which are soluble in sulphide of ammonium and others not; from this fact we can distinguish one metal from another, thus the sulphide of arsenic is yellow, and so is that of cadmium; but the former is soluble in sulphide of ammonium, the latter is insoluble. The alkaline sulphides precipitate silver black from its solutions; thus nitrate of silver, as a dye for the hair, is turned of an intense black if followed up with sulphide of ammonium.


* Vide Humphrey's Journal. Vol. XV. No. 1.

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