dust or fumes can ruin these materials. There should be adequate ventilation, a complete air change every 3 minutes, and an exhaust fan to the outside atmosphere in the area where chemicals are mixed. CONTAINERS Containers  for  photographic  solutions  should  be made of a material that does not affect or is not affected by chemicals. Glass is the best material. Stainless steel is a highly suitable material, provided it is of the proper composition. Hard rubber and glazed earthenware may also  be  used  satisfactorily.  Acid  and  alkali-resistant plastic  containers  are  acceptable. Containers,  graduates,  sinks,  and  every  utensil  used in the photographic lab should always be clean. As soon as work is finished with an item of equipment, it should be cleaned and returned to its proper place. When chemicals are spilled, clean them up as soon as possible. Chemical solutions and chemical dust corrode and cause pitting of most materials, including stainless steel, when allowed to remain for any length of time. ACCURACY Photographic quality suffers when the chemicals are improperly mixed. You must be certain that the amount of chemical you put into a solution is the amount specified. The mixing of processing solutions has been greatly simplified  over  the  years  by  the  introduction  of packaged  photo-processing  chemicals.  Packaged chemicals come in convenient sizes for most needs. They   offer   standardized   quality,   economy,   and convenience. Packaged  chemicals  include  film  and  paper developers and fixing solutions of various types that are manufactured under tightly controlled conditions. These packaged chemicals are available in either liquid or powder form. Processing solutions can be mixed easier, faster,  and  more  accurately  with  packaged  chemicals than  with  bulk  chemicals. When mixing packaged chemicals, you should always mix the entire package. Packaged chemicals usually contain more than one ingredient. During shipping and handling, these ingredients may separate with the heavier elements settling to the bottom of the package. When only part of the package is mixed, some of the ingredients that have separated or settled may not be put into the solution and the result of the process is not  predictable. MIXING Always add chemicals to the water or solution Dry chemicals should be poured slowly into the water while it is being stirred. When preparing a developer, be careful while you are stirring so air is not beaten into the solution. When water is poured on dry chemicals, they will cake and form hard lumps that are difficult to dissolve. Lumps or hard particles should be ground up, or crushed, with the stirring rod or with a pestle. Never add another chemical to a solution before the previous part has been completely dissolved Sometimes there is a residue that will not dissolve. The residue may be sand in the water supply, impurities in the chemicals, or other matter that found its way into the water; however, when the solution is allowed to stand for awhile, these particles usually settle and the clear liquid can be poured off. To remove sludge or dust particles that may not settle, pour the solution through a funnel containing three  or  four  layers  of  cheesecloth  or  absorbent  cotton. Many chemicals are very sensitive to heat, and even moderate temperatures seriously affect their chemical properties. However, the rate of chemical reaction increases with an increase in temperature, and all chemicals dissolve more readily in warm water than in cool   water;   consequently,   many   formulas   and instructions recommend that water as hot as 125°F be used to prepare the solution that must then be cooled to the  correct  processing  temperature.  You  should  always try to mix solutions at the minimum temperature recommended by the manufacturer. Solutions oxidize faster at higher temperatures because of increased chemical  activity  at  these  temperatures. When all crystals are dissolved, the solution should be practically colorless. Sometimes a solution appears cloudy or milky for a short time after it is mixed. This appearance may be caused by air taken into the solution by the dry chemicals. Air taken into a solution is distributed through the solution as tiny bubbles that cannot escape while the solution is being stirred. When the presence of bubbles has caused the discoloration, the solution will clear up when it is allowed to stand for a while. The bubbles rise to the surface of the solution and escape into the air. Always add acid to the water. This is as easy to remember  as  AAA  (Always  Add  Acid).  It  is  dangerous to pour water into an acid. Some acids generate heat 9-8