Processing Silver Microfilm
For the average user the photographic process contains many mysteries. For most, the biggest mystery occurs when the exposed film enters the processor. Every mystery contains an element of fear. In the case of silver film our fears may be based on not being able to observe what is going on in the processor or perhaps past nightmares with our processor. Whatever the case may be the aim here is to de-mystify the processing of silver film. To accomplish this aim we will discuss the role of processing chemistry, processing conditions and the processor itself. Each of these topics will be covered as it relates to development, fixing, washing and drying. Before we get into the intricacies of chemical processing we must have a clear understanding of film construction and of what happens during image exposure.
Silver film consists of a photosensitive layer coated onto a support (base), composed of polyester film. The photo sensitive layer is comprised of light-sensitive silver salts (usually a combination of silver bromide and silver iodide) in the form of microscopic crystals evenly dispersed in a gelatin support (binder). For those curious souls, a salt is the result of the reaction between an acid and a base (alkali). The silver salt crystals are typically 50 to 250 millimicrons in diameter (1 millimicron = 1 billionth of a meter).
The Gelatin is manufactured from cow parts (hides and bones). Photo-grade gelatin requires careful control of raw materials and of the manufacturing process, more so than the gelatin that is used in the food industry. It may seem odd that a key ingredient in the manufacture of photographic film is dependent on cows, yet gelatin has many special properties that make it ideally suited as a binder for photographic emulsions. Some of these properties include:
1. Light-sensitive silver salt crystals remain evenly dispersed in a gelatin emulsion, a critical factor when the need for consistent photographic sensitivity and image characteristics is considered.
2. Once it is coated on a support, gelatin has no chemical effect on the product, a key factor in the shelf life of the product.
3. Gelatin swelling properties increase its permeability to processing solutions.
During the manufacturing process, all the raw materials, including silver salts, gelatin and sensitizers (sensitizers determine the type of energy to which the film is sensitive) undergo a complicated manufacturing process called "ripening". This "ripening stage" is analogous to letting the bread rise in baking. Once the emulsion is ready, it is coated on a base support. Most products for micrographic applications have emulsion layers which are between 3 to 5 microns in thickness (1 micron = 1 millionth of a meter).
Regardless of the camera type used the same principles apply to photographic exposure; energy in the form of light is focused on the surface of the film. The amount of energy that reaches the film is dependent on the intensity of the source and on the duration of the exposure. The simple formula E = I x T defines exposure (Exposure = Intensity x Time). When exposing from a source document, the energy reaching the film is light reflected by the document, thus the characteristics of the document play a critical role, e.g., dark documents reflect less energy than light documents. Even with a constant amount of exposure (same lamp intensity and same exposure time), the amount of light that reaches the film will vary depending on the light reflecting characteristics of the document. To add to the complexity, the color of the document and the quality of the exposing lamps also play a role. This particular issue is meaty enough for further discussion but for the present, it is sufficient to understand that the energy necessary for exposure is reflected as in the case of document filming.
When the crystals are exposed to sufficient energy (the exact amount of energy necessary for the reaction is dependent on the characteristics of the silver salt), a small speck of metallic silver (latent image) is formed on the surface or inside the crystal. The exact mechanism behind this change is too complex to describe here and is it unnecessary for the average user to understand the physics behind this change. It should be sufficient to understand that the relationship between exposure and silver specks is "generally" proportional: more exposure creates more silver specks, while less exposure has the opposite effect. We say "generally" because there are two exceptions: reciprocity failure and latent image fade.
As you recollect, the total amount of energy which strikes the film is the product of intensity and time (E = I x T). Ideally, this means that a very short exposure with a powerful lamp should have the same effect of a long exposure with a low intensity lamp. This holds true over a fairly wide range (from 1 second to 1/10,000 of a second) but breaks down with very short or very long exposures. This breakdown is referred to as reciprocity law failure. The cause of the failure is due to the way the silver salt crystals respond to the extreme exposures. The reciprocity characteristics vary from film to film. Current formulations have a consistent response between 1/30 and 1/1,000,000 of a second.
Regarding latent image fade: silver specks are not permanent. With time, a certain percentage of crystals will lose their silver speck and revert to their former state. Most products go through an initial loss of latent image and then stabilize. Most microfilms experience a loss in density over a period of time after exposure. The developed image can easily lose .05 density points during the first 12 hours due to latent image fade, then stabilizes. For the amateur photographer this change goes unnoticed, while in micrographics it can be cause for concern. Some users, for example, may encounter difficulties when filming takes a long time from the first image to the last image on the roll. For example, if it takes as long as nine hours to expose an entire roll of film it could mean a .05 difference in density between the first frame shot at 8:00 AM and the last frame shot at 5:00 PM. Time permitting, the simplest solution to this problem is to hold the film for 12 hours before processing so that both the first and last frame will have gone through the 12 hour latent image fade cycle.
Development of the Image
Immediately after exposure a section of the crystals is exposed and contains silver specks while the non-exposed section of the film contains unaffected silver salts.
The role of the developer is to selectively convert the exposed silver salt to metallic silver while ignoring the crystals that were not exposed. The key to this reaction is the developing agent. The developing agent converts (reduces) the silver salt to metallic silver and this reduction will not as easily take place unless a speck of silver is present to trigger it. A wide variety of developing agents are available, each with a particular characteristic. Photo sensitivity (speed), contrast and acutance are all influenced by the choice of the developing agent. In addition to the developing agent, most developers contain the following chemicals:
Developing agents will react with the oxygen present in the air (oxidation) and become less active. A preservative is included in most developers to control oxidation.
It controls the activity of the developer by controlling the pH of the solution. For those who are not familiar with the concept of pH, a brief explanation may be in order. A formal definition of pH would be the negative logarithm of the hydrogen ion concentration. This definition, though accurate, leaves many of us in the dark, so we shall try a less technical description. Most of us have came across pH in relation to the concept of acidity or alkalinity of a substance. Frequently, a numerical scale is supplied to provide a rating of alkalinity, neutrality or acidity for that substance. It is important to remember that since these numbers are derived from a logarithmic system to the base 10, an increase in one unit is equivalent to a tenfold increase, thus 8 is ten times higher than 7.
Properties of acids include: turn litmus paper red, neutralize bases, conduct electricity in a water solution and have a sour taste.
Bases (alkalis), on the other hand: turn litmus paper blue, neutralize acids, conduct electricity in a water solution and have a bitter taste
To continue our discussion on the role of the activator; the activity of all developing agents is sensitive to pH. Typically, developers, which are low in activity (soft working), have a pH between 8.0 and 9.0 while developers with a pH above 10.0 are very active. Thus control of the pH of the developer by the activator also controls development activity.
Proper development will occur over a range of conditions, the aim being to achieve desired image density while keeping the D-min as low as possible. Ideally, only the exposed salt should be reduced to metallic silver.
It is possible, however, through improper processing conditions or through a chemical imbalance, to develop the unexposed crystals. When this happens, the clear, unexposed areas begin to darken. This is referred to as chemical fog. Some developers contain chemicals that reduce fogging without affecting the development of exposed silver salt.
The conditions of development also play a critical role and they include time (dwell), temperature, agitation, concentration and developer exhaustion.
After the exposed film is immersed in the developer, a minimum dwell time is required before any reaction occurs. This is referred to as the induction period - after which the exposed silver salt begins its conversion to metallic silver. Immersion time after the induction period determines both the density and contrast characteristics for a given exposure. Extended immersion will eventually cause chemical fogging. The time window between induction period and chemical fogging is the functional dwell time for most developer/film combinations. Longer than optimum development times increase D-max, D-rain, and contrast, while shorter development times do the opposite. For source documents, D-max, D-min and speed are of concern but contrast must also be kept within certain limits
Like dwell time, temperature also affects development. Increased temperatures speed up many chemical reactions; this is certainly the case for the development of silver salts. Higher temperatures result in higher D-max, D-min, speed and contrast, while lower temperatures have the opposite effect. It should be pointed out that every developer/film combination has an optimum temperature range that must be adhered to. Such range is usually between 60° and 100° F. This is especially true for development at high temperatures where excessive temperature can cause chemical fog. Since changes in either time or temperature affect sensitometric characteristics of a film, a single curve is often used to describe the relationship between the two. The advantage of a time-temperature chart is that it allows a quick determination of the necessary adjustments, when one of the parameters is changed. The combination 68 F/four minutes development time gives the same result as 74° F/three minutes. This type of information can be most helpful when setting up a processor.
Once development has started, the developer in contact with the emulsion becomes exhausted as a result of the reduction reaction. For complete development of the exposed silver salts fresh developer must be brought into contact with the crystals. This is the role of agitation: to remove the exhausted developer from the emulsion and replace it with fresh developer. In micrographics, automatic processors are almost universally used. The transportation of the film through the processing solutions provides the necessary agitation. Since different processors have different transport mechanisms, the processor itself, temperature and development time aside, can influence the development process. This is an important consideration when attempting to achieve similar results with different types of processors.
The concentration of the developer solution affects development. Developers are available as a pre-mixed solution or as a concentrate. Pre-mixed developers avoid the concentration variable because they are used without further dilution; developers in concentrated form require dilution that may introduce another variable in the process but cost considerably less. Since concentrated developers are more active than dilute developers, a diluted developer generally requires either more dwell time or higher temperatures to achieve the same results as a more concentrated developer. Shallow tank processors utilize more concentrated developers than do their deep tank counterparts because they cannot provide sufficient dwell time to use dilute developers.
As development progresses, both oxidation of the developing agent and the by-products of development begin to affect the activity of the developer. Depending on the processor, each developer/film combination is rated for a number of square (or linear) feet of film that can be processed before a loss in developer activity results. A common solution to loss of developer activity is replenishment. Depending on the type of developer, the replenisher solution can be either standard developer or developer replenisher adjusted for the specific needs of the job. On automatic processors, as the film moves from tank to tank, there is a normal loss in developer solution (carry-over). To keep tank levels constant (for constant dwell times and to avoid formation of foam), the processor is automatically replenished with fresh chemistry. Replenishment is thus used not only to extend the useful life, but also to maintain a constant level of the developer solution.
FIXING THE IMAGE
After development, the exposed part of the image consists of stable metallic silver. The unexposed portion contains silver salts that are light sensitive. To complete the imaging process the salts must be removed to avoid their exposure that would result in the formation of density in the clear areas of the image.
Since a portion of the image area remains light sensitive until after fixing, fixing must be done in the dark (like development). On most processors the film exits the developer tank, passes through a water rinse that stops the action of the developer and removes any excess developer from the film, then enters the fixer tank.
The most common fixing agents used are either sodium thiosulfate or ammonium thiosulfate. Ammonium thiosulfate is the preferred fixer for micrographics because it is more effective with automatic processors where the dwell time is limited. Both ammonium and sodium thiosulfate dissolve silver salts and hold them in solution. Within reasonable limits, neither of these thiosulfates will dissolve metallic silver. Fixer is frequently referred to as "hypo": the word comes from hyposulphite of soda. Surprisingly, hyposulphite of soda has never been used as a fixer. The term hypo was introduced in the early days of chemistry before the photographic process was fully understood, thus "hypo" is a technical misnomer.
In addition to a silver salt solvent, fixers may also contain hardeners, acids and buffers. Hardeners are designed to provide protection for the gelatin that softens during processing. The most common hardener is potassium alum. An acid may be included in the fixer to neutralize any developer that is carried into the fixer. Acetic acid is the most commonly used acid. Remember that developer solutions are alkaline and require an alkaline environment for development. Buffers are chemicals that stabilize the pH. In the case of the fixer the developer carry-over may raise the pH and cause loss of fixer activity.
Fixing, like development, is dependent on dwell time, temperature, agitation and concentration.
Most automatic processors are designed to provide sufficient fixing within their normal dwell time range. Since, up to a point, film cannot be over fixed (very long dwell times or highly concentrated solutions can begin to remove the silver metal) a wide range of dwell times are possible. The dwell time accuracy required for development is not necessary for fixing.
The activity of the fixer is affected by temperature. Higher temperatures promote activity while lower temperatures restrain fixing. All fixers provide satisfactory results within the range of normal processing temperatures, i.e., 70 "-100 ° F.
As the silver salts are dissolved, they have a restraining effect on the fixer. Agitation is necessary to remove the salt laden fixer from the surface of the film and bring fresh fixer in contact with the emulsion. Automatic transport provides the necessary agitation.
Fixer is supplied either as a concentrate requiring dilution or in pre-mixed form. If you dilute your own fixer, the proper concentration depends on the processor (deep tank or shallow tank) and on available dwell time. Generally the fixer manufacturer will provide specific recommendations for each type of processor and film type. Care should be exercised when diluting a concentrated fixer; incorrect fixer, dilution can cause incomplete fixing, easily noticed when the film is not fully cleared.
Like the developer, the fixer has a specified tank life. Once the dissolved silver salts reach a certain concentration, additional fixing cannot be achieved. The volume of square feet of film processed plus the reduction in activity due to developer contamination will determine the fixer life. Replenishment is used to extend the fixer life. A second method of extending the life is the use of a continuous silver recovery system, e.g., an electrolytic system. With such a system the fixer solution is passed over an electrode which causes the silver salt to plate out on the electrode as metallic silver. Not only does this system help rejuvenate the fixer, it also provides a slight financial benefit resulting from the recovery of the silver metal which can help pay the cost of film recycling.
The following general points should be kept in mind when working with fixers:
1. Thicker emulsions require longer fixing time than thin emulsions.
2. Exhausted fixer requires longer washing times than fresh fixer. Thus fixer concentration can directly affect the ability of the final rinse to remove the fixer from the film.
3. The pH of the fixer also affects washing time. Fixers with a pH equal to 4.9 are easier to remove during washing. This point is more important with shallow tank processors with limited washing times.
4. Most chemistry manufacturers supply a variety of fixers for the micrographics industry. Fixers for shallow tank units are usually acidic to neutralize developer carry-over and to facilitate washing. Fixers formulated for deep tank units often lack the acid because of the rinse available between developer and fixer. When selecting a fixer, make sure it is designed for your film and processor combination.
The purpose of the final washing step is to remove the residual fixer from the emulsion.
If an excessive amount of fixer remains in the emulsion, image permanence will be severely affected. Before washing, the emulsion contains dissolved silver salts in combination with the fixer. These chemicals, if not removed, in time will cause decomposition of the gelatin, stain and loss of density in the image areas. In micrographics where great emphasis is placed on image permanence (archival characteristics), adequate washing is of greater importance.
A variety of factors control washing efficiency. They include water temperature, water purity, flow rate, the wash mechanism, dwell time and pH.
1. Water Temperature - All things being equal, higher water temperatures are more effective in washing than lower temperatures. Thus a rinse temperature of 80° F will require a longer dwell than 100° F. Upper temperature limit is a function of excessive gelatin swelling which will result in emulsion softness. For this reason, most processors limit wash temperatures to 110° F maximum. Another point to remember: all processing solutions should be temperature equalized; in other words, the film should not go from a hot solution to a cold solution or vice versa. Temperature differences greater than 5 °-10 ° F can shock the gelatin causing small wrinkles to form. This is referred to as reticulation.
2. Water Purity - When installing a processor it is wise to have the water supply checked for purity. There are a variety of contaminants that should be investigated. If your water supply exceeds one of the limits, corrective action can be taken through filtration and/or water softening systems. For advice on the purification system needed for your system, checking with a local firm is recommended. Firms specializing in water purification are listed in the yellow pages.
3. Flow Rate - During the final wash cycle the water becomes contaminated with thiosulfate and by-products of the fixing process. As the level of these chemicals rise, the water loses its ability to reduce thiosulfate to the desired level. To combat washing water exhaustion, fresh water must be constantly introduced. The water exchange rate, flow rate, is dependent on the dwell time and square feet of film being processed. The processor manufacturer usually prescribes the level required for each system. In addition to the problems mentioned above, insufficient water flow rate can also lead to cosmetic defects such as spotting and stains.
4. The Wash Mechanism - Flow rate by itself is not the only factor that determines washing efficiency. Water movement also plays a key role. For ideal water movement, the water should enter the bottom of the tank and flow out from the top carrying the wash by-products with it. A second effective technique is to have water jets impinge directly on the film. In either case, water movement is a function of processor design that should be considered when purchasing a film processor.
5. Dwell Time - As is the case with all processing steps, dwell time plays an important role. The film must remain in the water bath long enough for the water to remove all the unwanted chemicals. Lower water temperatures or reduced water flow require an increase in dwell time. Most automatic processors are designed to provide adequate dwell times within the normal operating range of water temperatures and flow rates.
6. Water pH - A pH between 7.0 and 8.5 is the desirable pH range. Higher pH values (9 to 11) provide more effective washing but tend to swell the gelatin layer excessively and promote softness. Lower pH values (below 7.0) result in shrinking of the gelatin and require longer washing times.
Footnote: After installation it is wise to conduct a methylene blue test to determine the exact level of residual fixer in the emulsion. For archival requirements the residual thiosulfate must be below prescribed values.
This is the goal of adequate washing.
The final step in the conventional process is drying. The following simple guidelines should be adhered to:
Most processor dryers are set at a temperature between 100 ° and 125 o F. Specific operating temperatures are a function of airflow and dwell time. Both longer dwell times and higher airflow allow operation at reduced dryer temperatures. Provided complete drying is achieved, lower drying temperatures are preferred. Adequately dried film should not be tacky to the touch.
Incompletely dried film tends to stick to itself (the emulsion sticks to the layer wound on top), which can result in de-lamination when the film is later unwound. Emulsion pick-off of this sort is obviously a serious problem, thus it is important to dry film properly.
Over drying can result in excessive curl which cause jamming in some processors. Some users run their processors at maximum speed and compensate for the reduced dwell in the dryer by increasing the drying temperature. This regimen will dry the film but the high temperature may permanently "shock" the emulsion and result in high curl. The trick to correct drying is to find the best balance between dwell time, temperature and airflow.
The mystery to processing silver film, once the mechanisms are understood, is readily resolved.
1. Determine the optimum processing conditions for your system through direct experimentation and through manufacturer’s recommendations
2. Next, implement process control guidelines based on your results.
3. Finally, establish operating procedures for the processor and adhere to these procedures religiously.