The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

Home | About | Photography | Contact

place the thermometer in the gelatine solution, and make the same register 400 C. (= 1040 F.). Should the emulsion be too cool, raise the temperature of the water bath ; if too hot, place the vessel in cold water for a little time till the emulsion reaches the desired temperature.
Silver nitrate... ............ 30 parts.
Distilled water ......... ... 250 ..
When dissolved, add cautiously liq. ammonia, *88o, till the brown precipitate first formed is redissolved. So far these operations may be performed in daylight, but it is now necessary to enter the dark-room, and the silver solution should be added to the bromised gelatine in small quantities at a time, vigorously shaking between each addition. The total time of mixing must not be long, as otherwise the temperature sinks too low. Eder states that the silver solution should be used at the ordinary temperature and not heated ; but I have obtained equally good results by using hot water for dissolving the silver, and thus using the solution warm. As soon as the emulsion is mixed it should be placed in the water bath, the temperature of which should be 45o C. (= 1130 F.). Eder now recommends wrapping the vessel in blankets, flannels, etc., to retain as far as possible an even temperature. By using a very faint spirit flame or gas flame, however, it is quite possible to keep the temperature the whole time at 450, or just above. It will be noted in the above formula that the quantity of iodide solution is not definite - viz., 3 to 8 parts. According to Eder, increase of the iodide up to 8 parts gives rather more sensitive emulsions which are less liable to halation in landscape work. Many operators prefer to use, for portraiture, an emulsion which contains little or no iodide, and, therefore, the amateur plate-maker can take his choice. The duration of the digestion of the emulsion in the water bath has the most important influence on the final sensitiveness of the emulsion. When the above emulsion is digested for about 15 minutes, a slow landscape emulsion registering about 150 W. and working with great clearness and vigour will be obtained ; with 30 minutes' digestion the sensitiveness will be about 17 to 19o W.; with 45 minutes' digestion about 22 to 240 W. During the digestion the flask must be shaken two or three times so as