Gelatin Silver processing chemistry
From: George T Eaton, Photographic Chemistry
Arnold Gassan, Handbook for Contemporary Photography.
Almost all manufactured black and white photographic paper is composed of a silver based chemistry, with the final image being rendered by tiny grains of metallic silver suspended in a layer of hardened gelatin on the paper surface, which may also have an undercoating of whitening agent. Consequently, almost all such photographic papers require a similar processing sequence in order to make the image appear after exposure to light.
There are four basic steps in this sequence:
Developer where the image actually appears as exposed silver halides are reduced to produce a visible metallic deposit.
Stop Bath neutralizes the alkalinity of the developer thus abruptly stopping developer action and preventing staining.
Fixer removes all residual silver halide that has not been reduced to metallic silver in development, thus fixing the image so it will not discolor in light.
Washing where any of the processing chemicals are removed almost entirely. Both the residual silver compounds and fixing agent will eventually cause discoloration and damage to the print if not removed.
The basic components of the developer are:
Reducing agent: This is the ingredient that actually causes the exposed silver to be reduced from a salt to its metallic state
Preservative: The main purpose of this is to remove free oxygen from the developing solution: free oxygen destroys the effectiveness of the reducing agent. The reducing agent also adds some alkalinity.
Accelerator: The reducing agent by itself is relatively weak and performs more vigorously if activated by an alkaline accelerator.
Restrainer: The reducing agent eventually begins to develop silver in the photographic emulsion, whether it has been exposed or not. Only by restraining this tendency can one avoid an overall gray appearance to the print (more accurately referred to as chemical fog). Restrainers actually inhibit, and in extreme cases, prevent development, but make controlled development possible in the same way that automobile brakes permit greater control of a car. Restrainers also change the color and contrast of the silver image.
Solvent: Water is the most common solvent for developers.
This stops development action by neutralizing the alkalinity of the process. In doing so, the stop bath reduces the possibility of print staining and helps prevent the contamination of the fixer bath with alkaline developer solution.
An acetic acid solution, usually with an indicator dye, is the most commonly used stop bath. An acid stop bath can tolerate a certain amount of alkali carried into it by the developed films and prints before its effectiveness is lost. This can be checked by the use of indicator dyes that change color (usually from orange to blue) when the acidity of the bath has been decreased beyond the point of effectiveness. Generally, a fresh stop bath has a pH of about 3.5; a used or exhausted bath, a pH of about 5.5.
The fixing bath removes all the unreduced (and unexposed) silver halide from the print, thus preventing further darkening of the image by the action of light.
One of the most effective fixing agents is sodium thiosulfate (Na2S2O3.5H2O). It is also referred to as hypo. In addition to this, fixing baths contain a number of other ingredients which increase both their effectiveness and life.
Thorough washing of the prints is essential if they are to be preserved from discoloration and deterioration, both of which can be caused by residual fixer and silver compounds left in the prints during the fixing stage.
Good washing of the photographic material is dependent on the water supply, the design of the washing apparatus, and the manner in which the apparatus is used. Ideally, there should be a complete change in the volume of water once every five minutes. Temperature is critical too, the most effective washing being accomplished when the water is between 18-22 °C.
PRINT PROCESSING: getting started
Darkroom: [white light ON]
1 Assemble the following
Photographic trays: 5 8x10" or 11x14"
Contact Printing frame
Kodak Dektol developer
Kodak Stop Bath
Printing paper: Ilford Glossy Multigrade RC
Darkroom: WET AREA [white light ON]
2 Prepare your working solutions:
NOTE: Temperatures of all the working solutions should be kept within a range of 18-22 oC. The developer temperature is particularly important.
Always wear gloves when preparing you working solutions. All concentrated chemicals are hazardous and should be handled with great caution.
Quantities of working solution should be about 1 L for an 8x10" tray
For Kodak Dektol developer: dilute the stock solution 1+3 and pour into tray.
EXHAUSTION: replace after about 20 8x10" prints
For Kodak Indicator Stop Bath dilute the stock solution according to the manufacturers instructions and pour into tray.
EXHAUSTION: replace after about 20 8x10" prints or when the stop bath begins to look dark under red safelight conditions
For Kodak Rapid Fixer dilute the stock solution 1+7 and pour into tray.
EXHAUSTION: replace after about 20 8x10" prints
3 Arrange trays in order: developer, stop, fix, rinse, wash.
4 Arrange tongs in trays.
Use each tong for one solution only: i.e. the developer tong must not be used for any solution other than the developer etc. This reduces the chance of cross contaminating the solutions.
5 Assemble your negatives and printing paper in your printing area.
Note that the printing area is a dry area.
You are now ready to print: refer to "Print processing sequence: Resin Coated Paper".
PRINT PROCESSING SEQUENCE: Resin Coated Paper
(determine initial exposure by making a test strip - refer to "Printing: making a test strip")
2 Develop 2.0 minutes
AGITATION: continuous for first 30 sec, then for 5 sec per 30 sec
3 Stop 30 sec
4 Fix 2.0 minutes
AGITATION: continuous for first 30 sec, then for 15 sec every other 15 sec.
5 Rinse (fresh water) 30 sec
6 wash in running water (~20 °C) [total time = ______] ____ minutes
Blot or squeegee and:
dry face up on clean, lint free surface;
or hang up to dry;
or dry in heated cabinet at ~40 °C.
Do not attempt to dry a print that has not received the complete wash cycle.
Printing: Making a test strip
Refer to 'Printing: Getting Started'
Red safelight conditions
1 Load your negative and printing paper (emulsion side to emulsion side) into the contact printing frame. Make sure that you have closed the box of paper!
2 Place the frame under the printing light.
3 Adjust the f stop setting to 5.6 or 8, as directed.
4 Adjust the height of the enlarger light source so that the circle of light is even across your 4x5 inch negative, i.e, the diameter of the circle of light will be a little larger than the negative.
5 Set the timer to expose in 5 sec intervals.
6 Using an opaque sheet of cardboard (with a black underside) expose the paper in sections, starting at 5 seconds and proceeding in multiples of 5 seconds. Start exposing by covering all but 1/5 of the paper. At the end of five seconds, move the cardboard so that it uncovers another 1/5 paper and expose for another 5 seconds. Proceed in this manner until the whole print has been exposed in five sections ranging from 5 to 25 seconds.
Refer to hand-out Print processing sequence: Resin Coated paper
Keep a careful record of your test exposures and development times for each test sheet.
A useful test print straddles the optimum exposure. In other words, it should appear too dark at one end (left hand side according to the diagram above) and too light at the other (right...). After examining this first test print, choose the section that concurs with what you feel the print should look like.
At this point you may need to make another test print, but with a narrower range of exposures. e.g. if the 15 sec. exposure approximates the final appearance of the print, but is not quite there, make another test print, starting exposures at 10 seconds with subsequent increments at 2 second intervals. Be sure to introduce only one variable at a time - do not alter the development time, for instance, while processing the second test strip.
A correct exposure is determined by personal taste.
Details for Lab
You will have eight 4x5 inch sheets for testing and one 8x5 sheet for your final product.
As you obtain each piece of paper, use a pencil to write your initials and a sequence number [1, 2, ...] on the back (non-shiny side). This is VERY important - to distinguish your work from that of others and so that you know which sheet goes with which step!
As you take each piece of paper, be sure the bag is closed back up.
In your laboratory book, record observations and details of procedure for each test sheet, perhaps something like this - spread across two facing pages or horizontally:
Station/enlarger # __________ Enlarger at vertical height ____________
Developer and dilution
Comments & observations
What was learned
Test for exposure
Refine exposure time or f stop
Possible 3rd refinement
5x8 sheet final
Steps 3-8 should/might include:
Develop and stop bath, but no fixer. Wash. Leave in light awhile. How long? and then fix a portion of the sheet to 'freeze' what further exposure has taken place on that portion. Perhaps make a test strip to include several re-exposing times, and then do the fixing on half each of the strips, vertical exposure strips and horizontal fixing of half.
A much shorter developing time at best exposure setting
A much longer developing time at best exposure setting
Develop in another developer rather than in Dektol.
Effects of filters
Best conditions on a 4x5 sheet just before making the 5x8 sheet.
Extra pieces of the 4x5 are available if you want to do more than the 8 preliminary steps.
Enhancement: Step 9 - best conditions. 5x8 sheet. Talk to the lab assistant about necessary changes since the area of this print is twice what you've been using. Use a "mat" to form a border around your negative. We want the edge of the finished product to be white, not black.
The writeup should include all your samples and details of both what you learned as you were doing each sample AND what you learned between lab and this writeup. Point out features of each sample - experimental steps and the result on the pictures.
Enhancement: Look at your best 4x5 sample and mark three to four areas that represent steps from your darkest dark to your whitest white. Mark these same spots on several of your other samples. Use the Colorimeter to measure the L*a*b* values of each spot. Make a table. Comment on the results.