This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Small Transparent Spots, Triangular In Shape And Irregular In Size. These are caused by particles of dust adhering to the gelatine surface of the plate at the time of exposure in the camera, or by dirt in your developer or hypo. Remedy: Filter all solutions, dust the camera and plate-holder and plate, and no spots of this kind will appear.
Large Transparent Spots, Triangular In Form And Irregular In Size. These are generally caused by a scum which forms on the surface of old developer, and sometimes on new developer, if after it has been mixed it is left exposed for some time before using, in the developing tray, or an open vessel, such as a graduate. When the developer is then distributed this scum breaks up into small triangular particles, and it adheres to the surface of the plate, thus preventing the action of the developing solution. Remedy: Filter such developers immediately before using.
Small Transparent Spots Resembling Little Particles Of Lint And Dust. These are usually caused by brushing the surface of the plate hurriedly before placing in the holder. The gelatine film becomes electrified by the friction and attracts the dust and lint floating in the surrounding atmosphere. Remedy: Always dust carefully and slowly; do not press on with your brush too hard; dust only in one direction.
Purple Or Dark Opaque Spots, Regular In Size But Comet Or Irregular In Shape. These are generally caused by small particles of dry pyro coming in contact with the plate either before or during development, or by adding dry pyrogallic acid to the developing solution just before or while developing. These little particles of undissolved pyro when coming in contact with the plate will cause opaque spots. Remedy: Never add dry pyro to your developer. Never weigh your pyro in your developing room, especially just before developing. Sediment in your hypo bath often causes opaque spots. Iron or rust in the water used will cause opaque spots. Remedy: Make a new bath. If the water contains iron or rust, the pyro attacks the rust and is immediately coated with it, and when this touches the film it will leave an opaque spot. Remedy: Filter water through two thicknesses of muslin, tying the muslin over the tap. These spots at times can be removed by soaking the plate in water to which has been added a few drops of nitric acid, being careful not to use too much acid.
Yellow Negatives. Negatives will some times turn yellow in the final washing. This is attributable to the water. If the yellow is produced by weak or decomposed sulphite, or decomposed pyro, it will show just as soon as the plate is fixed. In either case the negatives should be immersed in a clearing bath. ( For formula, see Chapter X (Varying Water Conditions. Their Effects Upon The Manipulation Of Sensitized Papers), on Negative Reducing.)