When placing the plate in the silver bath, the dish should be tilted, so that the solution flows to one end. Place the plate in the other end of the dish, and immediately lower the dish to let the bath flow in an even wave over the plate. If the flow is checked, a streak across the image will result on development. The cover of the dish is now replaced, and the door of the dark room can be opened for a minute or so.
Sensitising begins directly the plate is inserted in the bath, and is complete in about two and a half minutes. The plate is ready to be removed from the bath when the film presents a creamy appearance ; but, as a rule, leaving the plate in the bath for two or three minutes will suffice. Of course, if it is desired to look at the plate while in the bath the door of the dark-room must be dosed.
If ordinary dry-plate slides are to be used the rebates for the glass must first be covered with strips of blotting - paper. If the wet plate touches the woodwork of the slide, scum will form over the plate and the picture will be spoilt. If the strips of blotting-paper are damped before use they can easily be fixed in the rebate. Wet-plate slides are provided with silver wires with the object of supporting the plate, but even these slides require blotting-paper at the bottom and top of the plate. After remaining in the bath for two or three minutes the plate is removed by being raised with the handle of a silver spoon, or with a lifter made of horn or vulcanite. The fingers must not be dipped into the silver bath, and neither wood nor metal, other than silver, must be used. The collodion of the sensitised plate has a creamy, opalescent appearance, owing to the formation of silver iodide in the film.
The plate is allowed to drain for a few seconds over the bath, and the moisture is then removed from the back with a piece of blotting-paper, the plate meanwhile resting on its edge upon a sheet of dean paper.
Next, the plate is inserted in the dark-slide, care being taken that the collodion film is not in contact with either wood or metal. The expo sure for wet plates is from ten to twenty times longer than is required for an ordinary dry plate ; fresh collodion requires less exposure than stale, and in cold weather the sensitiveness of the film is considerably diminished. In a weak light or
in a slightly yellow one a wet collodion plate is far less effective than a gelatine film.
An acid, instead of an alkaline, developer is necessary for wet collodion plates. The fol lowing is a formula for a pyro developer stock solution :— Pyrogallic acid . . . 24 grs.
Glacial acetic acid . . 2 oz.
This solution keeps well. For use, add 6 parts of water tor part of the stock solution.
The above is quite reliable, but some workers prefer ferric sulphate, as in the following : Ferric protosulphate . . r oz.
Glacial acetic acid • • r >, Water Methylated spirit should be added to the developer after the bath has been in use some time, to ensure even flowing of the solution over the plate. A shorter exposure is required for iron development. Developing and fixing dishes are not required. The plate is removed from the dark-slide, and the developer is poured quickly and evenly over the film. The plate is kept mov ing during development, in order to keep the film covered with solution. Fresh developer must be used for each plate. The image develops steadily, and usually begins to appear in about ten seconds ; but in cold weather the time may be considerably longer. Development is stopped when all the details are visible. The plate is washed for a few seconds under the tap and is then fixed with potassium cyanide, which should be kept in a saturated solution, and for use diluted with double its volume of water.
" Hypo " can be used instead of cyanide ; but it does not work so quickly, and takes longer to wash out of the film. After fixing with potassium cyanide, the plate is washed for a minute or so under the tap ; if " hypo " is the fixing agent, five minutes' washing is necessary. If the pic ture is satisfactory, the plate can be dried in front of the fire. The collodion image, when fixed, should be bright and dear, without a trace of fog or stain. The best reducer, should one be found necessary, is cyanide and iodine, made by dissolving a few crystals of iodine in methylated spirit and adding a saturated solution of potas sium cyanide until the red colour of the iodine has disappeared.