CONTROL OF CONTRAST BY DEVELOPMENT.
Within certain narrow limits the con trasts of a platinotype print may be con trolled by modification of the developer. For example, if a solution of potassium oxalate only is used, development takes place on the shadows and on the lights more equally, resulting in less contrast. On the other hand, if a. solution of potas sium phosphate only is used, only the deeper shadows are acted upon, with a onscquent increase of contrast. It is therefore evident that the addition of a certain quantity of potassium phosphate to the normal developer is useful in print ing from flat negatives. Besides the physical restrainer already referred to —namely, glycerine—there are the chemi cal restrainers, potassium nitrate and po tassium chloride. The addition of an equal part of a 10-per-cent. solution of potassium chloride, or of 1 gr. to every 10 oz. of potas sium nitrate, will cruise considerable re straining action. The addition of a small quantity of potassium biehromate solution to the developer will also result in in creased contrasts, or produce a hard result if over-done, whilst the citrates tend to give a softer image. These modifications,
however, are much more marked with the hot bath process than they appear to be with the newer cold bath. The use of a stronger developing solution will give deeper blacks than a weak one, as the platinum is reduced more readily, although there does not appear to be much differ ence if a solution of moderate strength is allowed longer to act. It is a good plan to filter the developer after use. It will pro bably soon be found to deposit white and green crystals by evaporation ; the former are crystals of potassium oxalate, and the latter potassium ferric oxalate. In com mercial work it is most essential that a negative of proper contrast with well balanced shadows be produced, as con stant alteration of contrast leads to uneven results.