DRY COLLODION PROCESS.—See Collodion Emulsion Process.
DRY DISTILLATION.—See Destruct ive Distillation.
DRY-PLATES.—Plates of glass of dif ferent sizes, coated with a film of gelatine in which a sensitive silver salt is emulsified. Dry plates are now manufactured in such large quantities that it is rarely beneficial to either amateur or professional photographer to manufacture his own, unless for special purposes. The competition in price of com mercial dry plates has resulted in the man ufacture of many kinds which are weak in emulsion and These should be avoided, as the slight difference in price is by no means economical.
Under the heading Emulsion will be found the necessary instructions for making the bromide of silver, or other gelatine emul sion ready for coating the glass plates. We shall first describe the method of preparing dry plates by hand, and afterward by Ma chinery.
The emulsion ready filtered is placed in a convenient flask for pouring. Some glass plates of the required size are then thoroughly cleaned by immersion in nitric acid, and then rubbed with caustic soda and a little alcohol, and set up on end on blotting paper to dry.
With some kinds of emulsion it is necessary, to prevent frilling, to first coat the plates with a substratum, which may be made up from either of the two following formulm. The first is Mr. Forrest's and the second Dr. Vogel's.
White of egg t ounce Water zo ounces Methylated spirit z ounce Carbolic acid 20 drops The carbolic acid is added to the spirit, stirring the mixture the while, when it is poured into the albumen and water previously mixed together.
Take 2 j4 parts of No. 1, one part of No. z, and add them to 70 parts of methylated spirit. Filter and coat the plates, then clean and dry as with collodion.
The next point to be considered is the leveling stand, or shelf, upon which the plates are to rest after coating. An ordinary triangular leveling stand will answer the purpose. The following contrivance is, however, simpler (see Fig. 169). A piece of thick plate glass about three feet long and two feet broad is laid upon the table, and three mahogany wedges placed under it in the position shown.
A small spirit level D is placed across the plate as shown, and the two wedges C and B are altered until the bubble in the level is in the centre. It is then turned lengthways across the plate, and the bubble again ad justed to the centre by altering the position of wedge A, not disturbing the other two in the least. The plate should now be perfectly level.
The emulsion should be kept at a temperature of about 90 deg. Fahr. by a hot water bath. The plate is laid hold of by a pneumatic holder, or by the tips of the fingers, and the necessary quantity of the emulsion poured on. For a whole plate 84- by 6f inches about three drachms are required, and other sizes in proportion. This is poured on as shown in Fig. r71, care being taken that no air-bubbles are formed. Ar rangements are now made for pouring emulsion on to plates. They are some what in the shape of teapots, with an inner vessel into which the emulsion is poured. The emulsion that is poured out comes from the bottom of the bulk, so that bubbles on the surface are avoided.
When the emulsion is poured on to the plate the latter is slightly tilted, and the emulsion made to run into the right-hand top corner, and from there to the left-hand top corner, left-hand bottom corner, and finally to the right-hand bottom corner. To facilitate the coating a larger amount than required may be poured on, and after flow ing over the plate a portion may be returned to the bottle, leaving only the required quantity on the plate.
The temperature of the room should be between 55 degrees and 65 degrees Fahrenheit.
The plate after coating is slowly rocked about until the emulsion is evenly spread, when it is placed upon the leveling stand, on a level tank filled with ice water to cool the emulsion and to cause it to set as rapidly as possible. Another plate is then coated in the same manner, and if the leveled glass plate be full, those that have set may be removed to the drying box or cupboard.