CHAPTER IX.. On the Theory of the Daguerreotype and Talbotype Processes, Etc.

ON THE THEORY OF THE DAGUERREOTYPE AND TALBOTYPE PROCESSES, ETC.

SECTION I.

The Daguerreotype.

It was 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 of an excessively delicate layer of 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 is repeated; 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 Iodide only 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 (see page 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 is subjected 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 by the prolonged action of 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 iodized surface be first exposed to white light for a sufficient time to impress a latent image, and then afterwards to the yellow light, the action already commenced is continued, 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 a developing agent, 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 of Iodide of Silver, actually destroys it, and restores the particles to their original condition, with a surface of Bromo-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--is darkened by 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 Gold gradually forming upon it, whereas with the image the whitening effect is immediate and striking.