The time of exposure can only be ascertained by practice, no rules can be laid down, and I am unacquainted with any royal road but that of experience, leading to constant success in this most important section of photography.
The plate being taken from the camera and placed upon a levelled stand, develope immediately the latent image with the following solution:
Pyrogallic acid.....3 grains.
Distilled water.....1 ounce.
Acetic acid (glacial) ... 1 drachm.
Take one part of this solution and two parts of distilled water for use. The pyrogallic solution made with proper acetic acid and of the above strength, may be kept for a month or more in a cool place away from the light.
When the image is sufficiently intense, wash freely with common filtered water; then pour on a saturated solution of hyposulphite of soda, which should immediately remove the iodide of silver, wash again well with water; allow as much as the plate will hold to soak in for at least half an hour, to remove all traces of hyposulphite of soda; lastly, stand up to dry, and if required, varnish with amber varnish.
Having, by the foregoing means, obtained and fixed a negative photographic image on glass, and which is capable of producing positives upon paper by the ordinary photographic means, it is as well, previous to obtaining these, to render the tender film of collodion less liable to injury.
This can be accomplished by means of a varnish, of which there are different kinds that may be used.
By far the best kind of varnish which can be employed is one for which we are indebted to Dr. Diamond. This is made by powdering some amber and putting it into chloroform ; by the application of a gentle heat under pressure, with care, a perfect solution takes place. This varnish flows readily over the plate, and dries in a few minutes, leaving a beautifully transparent hard glaze upon the picture.
It was shown by Mr. Horne in the early days of collodion, that the negative images could be converted into positive ones by mixing with the pyrogallic solution a very small quantity of nitric acid; but it has since been shown by Mr. Fry, and others, that a better result may be obtained by the use of proto-sulphate and proto-nitrate of iron.
The former salt is readily obtained, and in a very pure form. It should be used as follows :—
Proto-sulphate of iron. . . . 10 grains.
Distilled water......1 oz.
Nitric acid ....... 2 drops.
To develope the image pour the above over the plate, taking care not to carry the development too far.
The proto-nitrate may be obtained by double decomposition, as recommended by Dr. Diamond : 600 grains of proto-sulphate of iron are dissolved in one ounce of water, and the same quantity of nitrate of baryta in six ounces of water ; these being mixed together, proto-nitrate of iron and sulphate of baryta are formed by double decomposition ; also by dissolving sulphuret of iron in dilute nitric acid, as recommended by Mr. Ellis, who proceeds as follows :—
To one ounce of nitric acid and seven of water, add a small quantity of sulphuret of iron broken into fragments. Place the vessel aside, that the sulphuretted hydrogen may escape, and the acid become saturated with iron. Pour off the liquid, and filter. Then boil in a florence flask, to get rid of the sulphur, and again filter, when a dark green liquid will be obtained, which is the proto-nitrate of iron. This should be kept in well-stopped bottles, and protected from the air as much as possible, to prevent its changing into a pernitrate, in which state it is quite useless as a photographic agent.
To develope the picture mix one part of the above proto-nitrate with three of water, and apply it to the plate in the ordinary way, when a most beautiful clear image can be obtained.
The negative image being developed, a mixture of pyro-gallic and hypo-sulphite of soda, which has undergone partial decomposition, is poured over the plate, and then it is gently warmed. Upon this the darkened parts are rendered brilliantly white by the formation of metallic silver. This picture being backed up with black velvet assumes the air of a fine daguerreotype, without any of the disadvantages arising from the reflection of light from the polished silver surface. For this beautiful result photography is indebted to Dr. Diamond, who is still pursuing the subject with much zeal. We have also seen similar effects produced by Mr. Fry and Mr. Berger, by the use of the proto-sulphate of iron solution and pyrogallic acid. The image is first developed by the iron and the solution poured off; immediately another of pyrogallic acid is poured on, and the effect is produced.
The pictures are fixed with the hyposulphite in the usual method.
A peculiar whitening process was introduced by Mr. Archer, which is as follows :—
The picture being thoroughly washed in plenty of water, after fixing with hypo-sulphite of soda, is treated in the following manner:—
Prepare a saturated solution of bi-chloride of mercury in muriatic acid. Add one part of this solution to six of water. Pour a small quantity of it over the picture at one corner, and allow it to run evenly over the glass. It will be found immediately to deepen the tones of the picture considerably, and the positive image will almost disappear ; presently, a peculiar whitening will come over it, and in a short time a beautifully delicate white picture will be brought out.
The negative character of the drawing will be entirely destroyed, the white positive alone remaining. This picture, after being well washed and dried, can be varnished and preserved as a positive ; but nevertheless, even after this bleaching, it can be changed into a deep-toned negative, many shades darker than it was originally, by immersing it, after a thorough washing, in a weak solution of hypo-sulphite of soda, or a weak solution of ammonia. The white picture will vanish, and a black negative will be the result.