| The Tannin Process | |||||||||||||||||||||||||
| This dry process derives its name from the use of tannin-a bitter principle obtained from gall-nuts a preservative agent. To Major Russell is due the credit of having introduced it. The glass to be prepared should be cleaned with great care, particularly from any greasy substances. This is conveniently done with a mixture of Tripoli powder, spirits of wine, and solution of ammonia. A tuft of cotton is dipped into this mixture and rubbed over its surface for a minute or so; then well rinsed in water and rubbed dry with a clean cloth. The glass, just before being used, should be wiped with a perfectly dry and warm cloth, and then coated with the following solution:
Dissolve and filter; this solution will keep good for a considerable time. This gelatine solution is applied to the glass in the same way as ordinary collodion, taking care that the whole of the surface is covered, and that the back of the plate be net soiled. The superfluous liquid is received back into the bottle, and the plate set to dry, as shown at Fig. 69; when well drained, remove the accumulation of fluid very carefully from the lower edge of the plate by a piece of blotting-paper drawn along it. When the surface is dry, warm gently by the fire, and retain for use in a grooved box. As plates thus coated will keep good any length of time, any required number may be prepared, taking care that the backs of them are quite free from stains of gelatine. The gelatinised glass is now coated with old iodised collodion in the usual manner, taking particular care that the whole surface of the plate be covered; it is then immersed in the silver-bath employed for the wet collodion process, and allowed to remain in it from three to five minutes. Remove the sensitive plate from the bath, and wash it freely under a water-tap for about a minute, it will then be ready to receive the preservative solution, composed as follows:
Filter through paper, and measure out two separate portions according to the size of the plate to be prepared, allowing about two drachms in each quantity for a stereoscopic plate. The first portion of tannin solution is poured over the washed coating of the sensitised plate two or three times, so as to remove the water adherent to it, then the other quantity is poured on and off, and the plate placed on end on a piece of blotting-paper, and allowed to dry in a perfectly dark and warm place. After exposure in the camera, which averages from one to three minutes on a favourable day, and from four to eight minutes in dull weather, the picture is to be developed, for which purpose the following solutions are required:
Dissolve and keep in a stoppered bottle.
Dissolve and filter should any white or other precipitate be formed. To three ounces of distilled water add half a drachm of No. 1, and if the plate to be developed be a stereoscopic size, take three drachms of this solution and add to it from ten to twenty minims of No. 2; this forms the developing fluid. The exposed plate is first moistened with distilled water, which must be done quickly and evenly, otherwise stains are produced, and then the developing fluid poured over its surface and kept slightly in motion. The development must be carefully watched, and if' in a short time the sky comes out strongly, but, is not followed by the other details of the object, the plate was not long enough exposed, and the developing fluid must be poured back into the measure, and say ten minims of No. 1 added, so as to increase the quantity of pyrogallic acid. If the whole of the picture, however, appears to come out at once, a few drops of No. 2 is to be added, so as to increase the density of the sky. When the picture is properly developed, it is fixed with a solution of hyposulphite of soda, washed and varnished as described for the wet collodion process. By van Monckhoven, Désiré van |
|||||||||||||||||||||||||
|
Information provided by: http://albumen.stanford.edu/library/monographs/monckh/chap12.html |