Film Chemistry: Overview and Terms
In reality, photography and baking bread are very similar. Both are largely a chemistry experiment. Ingredients are mixed together, used in a particular sequence, to get some desired results. Although both are subject to interpretation, inspiration, and experimentation, the base remains consistent.
In real, sensible terms, most black-and-white photographic chemistry is pretty safe with occasional exposure. However, the more you do it, the wiser it is to protect yourself. If the safety section was avoided, please go back and refer to it.
When talking about chemistry (or breadmaking) you need to define some basic terms, if just for clarification.
- Basically a prepackaged stock solution. Kodak's Indicator Stop Bath comes this way. The concentrate is diluted 1:62 (2 oz. concentrate for a gallon of water) to get the working solution.
- Stock Solution
- This is the liquid form of a chemical or formula. It usually is in a concentrated form, requiring dilution. Developers are usually mixed into a stock solution, and diluted for use, but not always. The film developer D-76 comes in a powder, which is mixed into a stock, which in turn can be used at full stregth, but is most often diluted. Some developers, Kodak HC-11 for example, come in the liquid, stock solution. Stock solutions have a longer shelf life than diluted, and for this reason are kept that way (concentrated) until use.
- In photography, dilution most often refers to mixing a stock solution with water to a working solution. Dilutions are referred to as ratios, such as 1:1 or 1:3, with the stock solution first, then the dilutant. 1:3 usually means one part chemistry to three parts water. Using this example, if a tank holds 8 ounces, you would use 2 ounces of chemistry and 6 ounces water for the 8 ounce total.
- Working Solution
- This is the formula or chemical diluted (or not) for direct use. D-76 diluted 1:1 is a common working solution for film development.
As a chemistry experiment, photography has a pretty set amount of variables. The ability to produce good work lies in major part in how consistent you are with these variables. Consistency is largely as or more important than accuracy. Don't be horrified. Consider: the thermometer you use for film processing reads 1 degree lower than correct. When you measure the temperature of your developer, 69º reads as 68º. As long as this stays the same, you can alter the processing time or agitation to compensate. So consistency counts, and can save you if you aren't making other grievious errors.
Time is one of the more important variables. With film, development controls contrast (while exposure controls density). So the time you develop becomes pretty important. Learn to get chemistry in and out of tanks rapidly and smoothly. Use the same timer. Here again, if a timer is a second slow per minute, in 15 minutes you are adding 15 seconds to the development. This could be significant.
Time, as with dilution, can be used to control contrast in the development step: the more time, the more contrast, and conversely, the less time, the less contrast. There are minimums and maximums, however. About 80% of normal development time is the minimum, and about 140% of normal is the maximum.
Development times are given (by manufacturers of both film and developers) as recommendations, or starting points. Ideally, one would test a film/developer combination for a definitive ASA for the film and development time for the developer, to get the ideal density/contrast combination.
For black and white film: 68° Farenheit. Enough said.
In reality, these temperatures are critical for development, and less so for the rest of the processing, with some footnotes. You want to stay whithin about a 5-degree range for everything. It is far better when processing film to keep all the chemistry, including the wash water, at 68°, but if the stop and fix are a couple of degrees warmer or cooler, you usually won't have much trouble.
Cooler is a bigger concern than warmer. At about 62-63°F, fixer slows down so much as to not work. This is bad. Those minimum times mentioned above are for the proper temperature, and cooler slows reactions. Warmer speeds them up, which is why a couple of degrees warmer is not such a concern.
Another factor is reticulation, an effect, usually with film, where there is a big, sudden change in temperature. If film is subjected to a radical temperature shift, the emulsion can crack, just like an ice cube does when you toss it into warm water. This is due to different rates of expansion and contraction of the film and the substrate. To make this happen, you normally need a quite large jolt, 50 degrees or better, and it is actually pretty hard to do on purpose, much less inadvertently. However, if in the middle of summer your cold water is at 90° and there is no practical way to cool it to do the washing, you can gradually warm the film by using stop at, say, 72°, then the fix at 76°, to keep the shift to a minimum.
Dilution is fairly simple to control. Just learn how and measure carefully. Read graduates (measuring cups) at eye level. Be as precise as you can, which means use the smallest graduates that will hold the amount you need. A gallon bucket with some vague markings inside will usually be less accurate than a quart graduate.
Another issue is water quality. High mineral content in the watter may require the use of distilled water for developers and other chemistry. If the tap water in your area is not drinkable, you probably shouldn't use it for developers.
Agitation serves to remove the very thin layer of solution that is working on the emulsion of the film or paper, thereby being used up, and replaces it with fresh(er) solution, keeping the process moving along. Without agitation, things will rapidly come to a halt, chemically.
Proper agitation is often overlooked. It is, again, more important with film development, as film is more sensitive to overagitation, which will produce excessive contrast. Agitate using two hands, and try for three inversions in five seconds. Steady and complete is the key. Stop and fix can be more vigorous.
With paper, it is very difficult to over-agitate, and very easy to under-agitate. Always put the paper in the chemistry emulsion-side down, curving the paper so that no bubbles of air are trappped. I lift the paper out of the solution about every 30 seconds, and reinsert it. This helps drain the used off, and makes it easier for the fresher to get to the emulsion.