The Cine Technician (1935-1937)

38 The Journal of the Association of Cine-Technicians August, 1935 Cine Perfection Demands Purity in Chemicals By John j. curtis It is somewhat surprising to those who are connected with the supply of chemicals how very often they are confronted with carelessness or indifference on the part of laboratory technicians. A manufacturer of chemicals knows full well that if he is to obtain the pure form of any particular product, he must use only pure ingredients. The presence of any impurity is fairly certain to upset his calulations and most probably the result will differ from the specification. If this is a rule attending the manufacture, it would appear to be only right or logical to apply the same to the use of the chemicals in the laboratory. In cine work, many of the chemicals used belong to the group termed "fine chemicals," and the very nature of their work demands a high state of purity, and it is interesting to recount what work these have to perform. The emulsion of the films contains a silver salt in the form of silver halides, which, on exposure, are acted upon by light ; as soon as the developing solution reaches these a very delicate reaction occurs, converting the halides into metallic silver ; this is purely a chemical change — at first it is a solution change — and then precipitation takes place, forming thereby an image with varying densities according to the light action. So much for developing. Now consider the second important work in connection with the production of a finished cine film, namely. Fixing ; there are still some silver halides left in the emulsion after developing, which have to be dispersed and disposed of. These are not soluble in water, and another chemical, namely sodium thiosulphate, com monly known as hypo, is used. This must be in excess in order to transform the sodium thiosulphate into silvermonosodium-thiosulphate, which, although soluble in water, is soluble in hypo solution, in which it becomes converted into a silver-disodium-thiosulphate. It is this double salt which is soluble in water ; hence the most important process of finally washing. Where such definite chemical reactions have to take place, it does not require a very strong imagination to realise how very necessary and important it is to have the right quality of chemicals to start with. A slight impuritv may retard the reaction, or even upset it altogether, and where developing or fixing is done by time, any slowing up of the solutions would tend to spoil the work. How can the cine worker be sure of his chemicals ? It would require a very fully and well-equipped chemical laboratory to test for impurities, for these can be very numerous and, after all, the manufacturer, if he be one who is justly proud of his productions and values his trade, should surely be the one to find any impurity long before the chemical has reached its final stage of packing. Therefore, the answer to the (juery is to obtain your chemicals from a thoroughly reliable source. As an example of what a slight impurity will do, a few years ago the following instance came to the notice of the writer. Several films on leaving the washing water had small comet-like specks, which were almost impossible to remove ; after unsuccessfully testing everything to find the (Continued on page 39) Johnsons the hAonufoaurers of Pure Chemicals for Cinematography and all Photographic Processes, including AMIDOL METOL PYRO HYDROQUINONE & GLYCIN JOHNSON & SONS, MANUFACTURING CHEMISTS LTD.. HENDON, LONDON MANCHESTER OFFICE: 12. QUEEN STREET. DEANSGATE I,