GRAIN IN NEGATIVES In the early days of rapid plates a grain was perceptible in the negative, it being coarser as the speed of the plate increased ; but the defect has now almost vanished, although it may still be produced under certain conditions. " Grainy " negatives are more frequent in hot weather than in cold ; in the summer, and particularly when development is forced and the plate happens to be under-exposed, excessive coarseness of grain may often be seen. The temperature of the atmosphere during the drying of the negative affects the grain, and the more quickly a nega tive is dried the coarser will be the grain ; there fore, when a negative is to be used for printing upon glossy paper, or lantern slides are to be made from it, the cooler the atmosphere employed for drying the better. One theory (there are several) is that when negatives dry slowly on a hot day, the gelatine becomes partly decom posed, allowing the particles of the silver bromide to come together ; these particles have an affinity for each other, and are enabled to come together when the gelatine which keeps them apart has been to some extent destroyed. Thus
the particles form coarse particles, and impart to the negative a " grainy " or woolly appearance. Excess of alkali also increases grain ; therefore, in order to produce a negative as grainless as possible, let the exposure be full, use a normal developer at no higher temperature than 65° P. (i8° C.), and dry as quickly as possible in a cool, clean current of air.
In process work, it is a disadvantage to use a plate that gives too granular an image. Hence, process dry plates are relatively slow in order to secure a fine-grained emulsion. In collodion emulsion work also, the grain must be kept fine.