(5) Underlying the whole system of photographing upon wood is this principle - nothing must remain on the surface which is capable of clogging the point of the graver, hence the vehicle in which the composition of the photographic image is applied must be of the most attenuated nature possible. Again, it is easy to imbue the surface of the wood itself with the chemicals by which the image is formed, but difficult to prevent the wood that has been subjected to such treatment from becoming rotten or friable, by which, fine delicate lines crumble and give way under their construction, owing to the pulverulence of the surface. Firmness and closeness of texture are essential requisites in the surface that has to be operated on by the engraver.

Collodion, gelatine, starch, and other media have all been employed as vehicles, combined with silver salts of various kinds, and chromates. Those processes known generically as "dusting on" bave also conduced to the successful application of photography to wood, together with the simpler system of rubbing over the surface with a sensitive powder, made to adhere with sufficient tenacity to ensure its not becoming dislodged by any after treatment - such powder being capable of being impressed by an image under the negative. This enumeration, incomplete as it intentionally is, serves to show how much ramified has become the important art of photographing on wood.

The first process to describe is one involving the rendering of the wood sensitive with silver chloride, without any rottenness arising. The surface of the wood is first of all whitened by being well rubbed with a paste composed of finely powdered white-lead and a little water. This procedure is sometimes varied by the use of alcohol. When dry, the surface receives a coating of an exceedingly weak solution of mastic and guttapercha in benzole. The following strength will suffice: -

Guttapercha .. .. 3 gr.

Mastic ...... 3 gr.

Benzole ...... 1 oz.

This does not leave any film on the wood, but serves merely to fix the white pigment.

The next operation is an unpleasant one, as it necessitates the working with albumen which, before being used, must have passed into a state of putridity. Beat into a froth the whites of as many eggs as may be found desirable, and for each egg employed add 4 gr. sodium chloride and 18 m. strong ammonia. Keep this standing in a warm place for about a month, and add water to make up the loss from evaporation. When putrid, filter and apply to the surface of the wood by means of a brush. After being dried, apply, by the same means, a 45-gr. silver solution. The block, thus sensitised, is exposed under a reversed negative until printed sufficiently deep, after which it is washed by means of a broad camel-hair brush, and toned and fixed in the usual way.

The negative may be used in a reversed position. Of course, when the negative is taken expressly for the purpose of being employed in connection with engraving, the photographer will take care that it be reversed or non-reversed to suit his special purpose; but in the case of pre-existing collodion negatives the case is different.

Let us suppose that a collodion negative several years old, and well varnished, is required to produce a reversed print. The first operation is to remove the varnish. This is best effected by pouring over the surface a little of the following mixture: -

Caustic potash .. 2 parts Alcohol .. .. .. 2 „

Water...... 20 "

This must be poured off and on until the varnish is dissolved, when the surface is well washed with water and allowed to become dry. The plate is now placed on a levelling stand and coated with a very thin solution of rubber in benzole, followed, after being dried, by a coating of transfer collodion composed of alcohol and ether, 2 pints of the former to 1 of the latter, in which is dissolved 1 oz. each of castor oil and guncotton. The object of the coating of rubber is to prevent the transfer collodion from acting upon the collodion that contains the image.

When the transfer collodion is quite dry, which may take one or even several hours, it is gently warmed to dispel any milkiness, should such exist. A knife is next run round the margin so as to cut through the film, and the negative is placed face upward in a flat vessel of water. Ere long the film will be seen to become loosened on the glass, and in a short time it will become altogether detached. It must then be placed between 2 sheets of blotting-paper to be dried, after which it is kept in a folio for use.

In printing from a pellicular negative prepared as described, the picture may be either reversed or non-reversed according to the side placed next to the paper, and both classes of prints will be equally sharp.

The wet collodion process is applied to this purpose by a transfer system which does not injure the surface of the wood by the action of chemicals. Its practical nature may be deduced from the fact that it was by this agency the large portrait of Prof. Huxley, together with similar portraits of other eminent men of science, which appeared some time ago in a leading London illustrated newspaper, were placed upon the wood ready for the engraver. The process is mainly that of Grime, subjected to such modifications as were found necessary in getting it brought to a successful state of working.

It is first of all necessary, by means of a copying camera, to produce from the negative a transparency the exact size which the engraving is required to be. The knowledge required to produce a transparency, together with a suitable camera for the purpose, is assumed as being in the possession of the operator. The collodion must be prepared with soluble cotton, made at a low temperature to ensure its being tough or skinny, and it is so far fortunate that cotton of this class can be readily obtained. The mechanical characteristic of the collodion is that it shall be tough; its photographic peculiarity being that it shall not yield a dense image, but one that is very soft and transparent in even the deep blacks. The following is a formula which, we were informed by a professional photographer on wood, who is also a practical engraver, invariably gave the best results.