Photo Chemistry - Black and White Film
Photography uses three basic chemicals in the processing of light sensitive materials. The first chemical is called Developer. The developer detects changes in the silver salt in the emulsion of the film or paper and turns those that have been struck by light into metallic silver. The second chemical is stop bath and is used to stop the developer from working any longer. Many schools use plain water for this step. The final chemical is fixer. It fixes the film or paper so that it is no longer sensitive to light. It will dissolve the unexposed silver salt from the light sensitive emulsion while leaving the metalic silver in tact. Fixer will, however, bleach the metalic silver if left in contact for a long enough period of time. Finally we wash the emulsion to remove the dissolved unexposed silver salt as well as all the fixer residue.
The paper developer used in most labs is called Dektol. It is made by Kodak. This developer will mix 50-50 or equal parts of water and stock solution. Some photographers will mix this differently, but in our lab we have found photographs turn out better with this concentration. We usually use 10 oz of Dektol and 10 oz of water when we mix. The chemicals can absorb into the plastic of the mixing container so they should be rinsed thoroughly when finished. Developer can cause skin irritation to some people so avoid contact and wash your hands often if you get the chemical on you. This chemical will also stain your clothes when it drys. You should wash your clothes in COLD water the same day as contamination. Keep the lid tightly on this bottle because the developer will mix with the oxygen in the air and turn brown which reduces its ability to make a good contrasty print. To mix Dektol we take a clean brown bottle and fill it half way with hot water that is around 100 degrees. We open the package of developer and carefully pour the powder into the bottle. AVOID breathing the powder fumes. After pouring the developer in put on the cap and SHAKE the bottle very hard in all directions to mix the chemicals. After shaking add cold water to fill the remainder of the bottle. Developer will be used at 68 degrees when mixed for use in the darkroom.
One of the more popular film developers is Kodak HC-110. This comes in as a thick liquid concentrate. The liquid is poured into a half full bottle of room temperature water. Water is poured into the HC110 bottle that the thick concentrate just came from. The bottle is shaken vigorously and the resulting mixture is added to the main bottle of developer. Additional water to make a gallon is added and the mixture shaken vigorously. When used this developer is diluted 8:1 or. We use one and one half ounce with 10 oz of water to make a total of 11 to 12 oz of mixture for processing. When used in a dilution like this the original bottle is called a "stock" solution. NEVER pour anything into the stock bottle. Even a small amount of chemical in the bottom of the mixing container could destroy or KILL the entire stock solution. Developer for paper and developer for film are NOT the same and will not work when interchanged. Film developer will produce an image on paper but it will be very low in contrast and will take a long time to process.
The ideal temperature for photography is 68 degrees. When the temperature is hotter the chemicals work faster, but sometime too fast causing lower contrast. If film or photo paper is not left in the developer long enough it will not allow all of the possible silver to be created resulting in a light negative or low contrast print. The photographer must be careful to keep the temperature between 60 and 75 degrees and to adjust the time of development to keep the contrast good. Once the thermometer has been placed in a solution it is important to rinse it off after to avoid contamination. CONTAMINATION is the biggest problem in the new photo students darkroom. It can destroy a roll of film, lower contrast and make poor results.
Specialty developers exist that allow the film to respond to a lower light level and thus increasing its ASA. These films often trade off a lower contrast in the finished print. Other developers with a great deal of water in their dilution and special chemicals added to them allow the photographer to be way off in exposure and still get a good range of black and white shades on the finished print.
The other chemical used often is called STOP BATH. This is acetic acid which is found in orange juice. This acid usually has a colored indicator dye placed in it that is yellow when the acid is strong and useful and purple when exhausted and no good. In our lab for most classes we use just plain water to rinse off the developer for either film or paper. The stop bath or rinse is needed to remove the developer from the film or paper and keep it from wearing out the fixer as fast. It will allow more pictures to be processed in our chemicals. The Stop Bath is mixed from a strong concentrate that is ALWAYS added to a full container of water. Never pour a small amount of water into a strong acid as it can create air pollution and can splash in the eyes. For most of our work at Santa Cruz High we will not use real stop bath but will replace it with a good rinse of water.
The last chemical we use is called FIXER. This is sodium thiosulphate in chemical terms. This chemical can react with developer to create AMMONIA which can cause a major smell in the darkroom. This chemical when spilled on the floor will create a white stain. The powder that is created on this stain can float into the air and create a smell in the air. The solution to this pollution problem is to rinse fixer off of paper or film before moving it around in the lab or room. At the end of every class or lab use the photographer should rinse down the sink and counter top. The fixer does not react with air like the developer does so it can be left in the sink tray for a few days at a time. The fixer is not thrown out after being used for film or paper.....it can be used over and over until it is exhausted. The lab technician will test it with a test kit to know when to throw it out. With this chemical three drops into the fixer should not turn cloudy. If it does it is no longer good fixer.
In the darkroom a photographer may use print tongs to pick up paper in the developer trays. One tong should be used in the developer and a second tong used in the stop bath and fixer. NEVER put a tong or hand that has been in fixer back into the developer because the fixer will neutralize the developer.
A darkroom needs to be kept dry on the counter top and your hands need to be dry when touching photo paper. IF you touch photo paper with wet hands you might get a white spot or finger prints on the print. Spills on the counter top can cause spots on your finished print. Chemicals spilled on the floor or counter can also get on clothes and cause stains. Developer on clothes will make a dark brown stain if not washed away with COLD water in 24 hours before it crystallizes. Developer or fixer on your hands is usually not a problem except to the very sensitive skinned person. Exposure to developer and fixer for just a few hours a day has no long term health effects. However, the chemical can cause nausia and stomach problems if you get it in your mouth by putting your hands in your mouth. Photographers should wash their hands with soap and water after working in the lab before eating food.
MSDS Material Safety Guidelines
Every manufacturer by law must put out a MSDS or Material Safety Data Sheet for all of their products. The school is expected to keep this form on file. The chemicals used in our high school program are all produced by Kodak. MSDS forms are posted on the wall in the photo lab or they can be found at Kodak MSDS web site.
In general the primary safety concern is for strong concentrations of photo chemicals. For Dektol, HC 110 and Fixer the most dangerous exposure to the chemicals comes while mixing from concentrates or powder. Care to keep from making contact with the concentrates includes use of eye glasses, gloves and care to do mixing in a well ventilated room.
The concentrates are all diluted to a major degree for working solutions which reduces the exposure to potentially harmful substances. Danger from ingestion on any of the chemicals is considered to be a low hazard. All solutions can be irritating to eyes and care should be taken in handling of chemical trays to avoid splashing. The main problem found in our lab has been allergic skin reactions from a few students with sensitive skin. Otherwise, in all our years of work no health effects have been reported.
Environmental hazards are not considered a problem when solutions are flushed with large amounts of water and are emptied into a waste treatment facility. Old fixer will create sulfur dioxide and some minor ammonia and chloramine which will smell up the darkroom which is why good ventilation is required. For some asthmatics or sulfite sensitive individuals exposure to fixer may cause wheezing and stomach upset. As long as students use caution photography presents no health threat. Care should be taken in washing hands after contacting chemicals, although the concentration of potentially toxic chemicals is very, very low it is just good practice to wash hands after working with chemicals and to be cautious to avoid splashing into eyes or open cuts on your hands. Good ventilation in the darkroom is also necessary. All chemicals in the photo lab at SCHS are flushed down the sink with large amounts of water. In general this amounts to only 2 gallons per week of material which presents no environmental hazard according to our research. Large photo labs with more chemical work than our lab should consider using silver recovery equipment to wash the fixer clear of silver.