CHAPTER VII. A DRY COLLODION PROCESS.
DRY collodion plates are very rarely, if ever, now used for making negative~>, but, as the process naturally follows the wet collodion process, we propose to insert here a dry collodion process, which we have used extensively and successfully for lantern-slides, and which may be used, if desired, for negativemaking. The formula is mainly due to Mr. W. B. Bolton. Dry collodion emulsion is called "washed" 9r "unwashed," according to the stage at which it is washed, for washed it always is, at one stage or another. Instead of using a bath of. silver nitrate solution, and immersing a coated plate therein, we add the silver nitrate to the liquid salted collodion, thereby producing an "emulsion" of silver haloids in collodion, and that emulsion, sensitive to light, we pour on plates which we thereafter dry. But in the process of "double decomposition," by which the sensitive salts are formed, there are formed other compounds, or "bye-products," which, if left to dry on the emulsion, or on .the film, would crystallize, and spoil all our plates. In the "washed" emulsion process these bye-products are washed out of the bulk of emulsion before the plates are coated. In the "unwashed" emulsion proce"s the bye-products are washed out of the film of each plate after it is coated. PxOCEss. The zinc bromide must be dry, or dried by heat on a clean surface. The pyroxyline is that made at, and known as, ''high temperature." UNWASHED CoLLODION EMULSION
Sulphuric ether, •720 ......••••••••••.•••. 3)4: fluid ounce~. Alcohol, .820 ............................ 2 fluid ounces. Pyroxyline, (H.T.) ...................... 36 grains. Zinc bromide ..............•.••••.•••.... 59 grains.
Mix in this order, and let stand one day, at least, to settle.