CHAPTER 9 CHEMICAL MIXING When  light-sensitive  emulsions  are  used,  photog- raphy is essentially a chemical process. You depend upon the chemical process to produce visible and permanent  images.  An  important  requirement  for optimum  photographic  processing  is  the  careful  and correct  preparation  of  photographic  solutions.  Improper mixing  of  chemicals  or  contamination  during  mixing can have far-reaching effects on operations, quality, production, and mission accomplishment in the imaging facilities of the Navy. It is often difficult to determine the cause of poor quality when improper chemical mixing is at fault, and the need for discarding incorrectly prepared or contaminated solutions cuts down on production  and  wastes  money. The main function of the darkroom portion of the photographic process is to develop film and produce prints,  and  this  requires  photographic  chemistry.  It  may be your job to ensure that all chemicals needed are mixed  and  checked  for  quality.  “This  is  a  responsibility that you cannot take lightly.” A solution that is mixed improperly may cause an entire mission to be lost. You must use the utmost precautions when mixing, checking, or analyzing the photographic solutions used in your lab. PHOTOGRAPHIC CHEMICAL AND SOLUTION STORAGE When you receive chemicals in your imaging facility, the cartons, packages, or containers should be dated to show either the date received or the date shipped. This helps provide proper stock rotation and systematic  control  of  chemical  usage.  Chemicals  should be issued from the storeroom on a first-in-first-out (FIFO)  basis. Unmixed  chemicals  should  be  stored  in  their original,   unopened   containers   in   a   cool,   dry, well-ventilated storage area where the temperature is maintained at or about 75°F with a relative humidity of about  40  percent. Prepared solutions, like dry chemicals, also must be protected  from  adverse  conditions,  especially  oxidation and   contamination.   When   the   following recommendations  are  adhered  to,  most  “unused” solutions stay in good condition for a reasonable period of time: Small amounts of replenisher and stock solutions are best kept in stoppered or screw-cap bottles. Glass bottles are best for developer and developer replenisher. Screw  caps  must  be  free  of  corrosion,  foreign  particles, cardboard inserts, and be airtight. Never interchange bottle tops from one bottle to another. A cap-to-bottle color or number code is suggested. When large bottles are used to store solutions, the air space in the bottle is increased each time the solution is  removed.  Since  this  increases  the  chance  for oxidation, store solutions in small bottles instead. The entire contents of a small bottle can then be used at one time. However, a small air space should be left even in small bottles. This allows for varying solution volume due to temperature changes and keeps the cap from loosening  or  the  bottle  from  bursting. When tanks are used for the storage of large volumes of solutions, they should have floating lids to protect the solutions from aerial oxidation Dust covers also should be used to cover the top of the tank. The tank, the lid, and the cap should be coded in such a way that they are reassembled with the correct parts. Always   follow   the   storage   and   capacity recommendations  of  the  manufacturer.  They  are packaged with the chemicals. Do not use chemicals that have been in storage too long. Before you use any solution, no matter how long it   has   been   mixed   or   in   storage,   check   it   for discoloration. Each solution has its own “signature” or characteristic  appearance;  and  any  change  from  normal may be a sign that it will produce unsatisfactory results. Check  both  sides  and  the  bottom  of  the  tank  for precipitates. If there are any, carefully stir the solution to redissolve them. When you are unsure of the quality of the solution, discard it. Most photo-processing chemical formulations are based  on  both  their  photographic  qualities  and  their chemical stability or keeping qualities, both on the shelf before  mixing  and  as  prepared  solutions.  After long-term storage, chemicals may lose some of their chemical  activity. 9-1