r88
PHOTOGRAPHIC
EMULSIO
TECHNIQUE
have a small, uniform grain and possess the necessary qualities for both soun-d and picture reproduction. A fine-grained, longscale emulsion is needed for both positive and negative sound records. While commercial positive gamma is usually in the neighborhood of two, it must be remembered that the sound is ordinarily restricted to a part of it only; negative gamma ranges from 0.5 to 0.9, and the exposure is so regulated as to keep the maximum value below the shoulder of the characteristic curve. Fig. 58 shows a smear of commercial positive motionpicture emulsion at X rooo magnification and indicates the extremely uniform grain, which is nevertheless coupled with a long straight-line portion of the curve. A formula for a positive emulsion is as follows: A. Gelatin Distilled
B. Potassium Potassium Water
.. water
_
: _. . ..
bromide. iodide
_. : __
_
. -. .. __
C. Silver nitrate '" Distilled water . -
D. Gelatin
__ _
__
_. - - - - - .. "
Water Ammonia
(s.g.
400 2500
0_910)
.......••..........
__
400
g cc g
_ _. _. __ ..
IS g 1500 cc
__ . . .. . . ..
1500
_. __ " _. _ _
1000
g cc
45
cc
500
350
g cc
This is a mixed-jet type, so that Band C are poured through five- to six-millimeter nozzles, at temperatures of 1400 F. 0 (60 C.) and 770 F. (250 C.) respectively, into the crock containing the emulsifying gelatin A, which should be at about 0 120 F. (500 C.). The emulsion is digested at 1200 F. for thirty to forty-five minutes, and is then poured into D, which has been previously heated to the same temperature. The emulsion may be further digested after washing if the tests show it to be necessary.
FILMS,
N EGA TIVE AND POSITIVE
An all-ammonia positive film emulsion may be made as below: A. Water
.
Ammoniu-m Ammonium Soft gelatin
B_ Water Silver
nitrate
Ammonia
. bromide ,, iodi le .. , . , , .. , . , ,
,,
__
1200
cc g
, , . _. _ , , .. _, ,. ,.
150
120
g
,. _, , . , . , . , , __ , .. , _.. _. , _, . , , , . , . , , , .. __
100
cc g
4g
200
to redissolve.
Half the silver is poured at 770 F. through a nozzle or jet into the crock containing the A solution (at 1050 F.); the other half is then" flopped" in without stirring, and 200 grams of dry, high-viscosity gelatin is then added and stirred in until completely dissolved. The emulsion is maintained at 1050 F. or just under, then set in ice water. It is very thoroughly washed in order to get rid of all the ammonia, and made up with spirit and finals to 4000 cubic centimeters. Coating weight should be 90 to lIO milligrams per square decimeter. In view of the increasing interest in the making of color prints by the wash-off relief process, and by superimposition of three dyed gelatin images on film base, some description of the materials used should be given. The original idea of using gelatin reliefs as matrices for producing the elements of a subtractive transparency must be ascribed to ,E. Sanger Shepherd, to whom the Progress Medal of the Royal Photographic Society was awarded for this work nearly forty years ago. A wash-off relief film can be used in two ways. One way is by sensitizing a slow gelatine-bromide emulsion with dichromate, exposing it through the back of the film and developing it in hot water; the washed-ou t relief image is fixed and washed, when it can be used to imbibe the necessary dye, which can be transferred to paper or left in the film if the latter is to be made one element of a three-color transparency. The other way is to expose the film under a negative and after developing it to