Fixer
From Silvergrain Labs
Developed film, plate or paper is brought into fixer or fixing bath after removing much of the developer in the stop bath or mechanical squeegee. Fixer is a bath that dissolves and removes unexposed and undeveloped silver halide, thereby rendering the material insensitive to further light exposure.
Fixed materials are washed and then dried. Optionally, fixed materials are toned, or given other processing steps before they are washed and dried.
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Fixer composition
Fixing baths commonly use thiosulfate to dissolve silver halides in the emulsion. The concentration is typically about 1M (1 mole per liter). Fixer also contains:
- sulfite: usually sodium sulfite, to preserve thiosulfate
- pH buffer: usually acetic acid for acid fixer
- hardener: alum hardener may be used (optional)
- sludge inhibitor: boric acid is most common, but polycarboxylic acids are also used
Rapid v. non-rapid fixer
There are rapid and non-rapid fixers. Rapid fixers use ammonium thiosulfate as the fixing agent. This type of fixer is rapid in fixing the material as well as washing the fixing reaction products in the subsequent washing process. Rapid fixers also have larger processing capacity. Therefore, it is generally preferred for small scale fine art darkrooms.
Non-rapid fixers use sodium thiosulfate as the fixing agent. This type of the fixer requires longer time to fix and also to wash. Non-rapid fixer is still used in some large scale processing establishments to minimize the ammonium ions from the wash water effluence. (Ammonium ions from rapid fixer and washing water is biologically available nitrogen source, which works as a fertilizer.)
Hardening v. non-hardening fixer
Hardening fixer is used when films and prints are processed at an elevated temperature. Hardener is unnecessary for manual processing at 20-25˚C with modern materials made by major manufacturers. (Minor manufacturers may use old, less effective emulsion hardeners, which make their films more susceptible to damage during processing. In this case, hardening fixer may be useful.)
Hardening fixer has a major disadvantage: washing process is considerably slower when hardener is added to the fixer. That is, materials fixed in hardening fixers require longer washing time to meet a certain residual thiosulfate level. Therefore, non-hardening fixer is recommended for manual processing and whenever hardener is not required.
The reason for slower washing process is that, alum hardened gelatin provides sites to which thiosulfate is strongly adsorbed. Washing process requires thiosulfate to (1) desorb, and (2) diffuse out of the emulsion, and the first desorption stage is considerably retarded by alum hardener.
Emulsion hardener used in the processing stage reduces swelling of gelatin and therefore adds mechanical strengths in the wet condition only. The dry strength of the gelatin is the same regardless of whether hardener is used in the processing stage.
By far, the most common hardener in fixer is white alum (potassium aluminum sulfate) or aluminium chloride. This agent is effective in pH 4 to 6.5 range. Alum hardeniner is less effective at a lower pH. This is because alum hardener acts on ionized carboxyl groups in the gelatin, and lower pH makes carboxyl groups to deionize. While alum hardener is more effective at higher pH, there is an upper limit in the useful pH range, because insoluble aluminium hydroxide sludge will form at higher pH. Therefore, well designed alum hardening fixers usually contain an acid buffer and aluminium complexing agents (such as borate and citrate) for optimal performance.
Acid v. alkaline fixer
Acid fixers are most commonly used in b&w processing. However, although much less common in b&w processing, neutral or slightly alkaline fixer is most effective.
Fixer is traditionally made acidic because acidic pH (4.5 to 5.5 range is most common) is required for hardening fixer, which used to be much more common. Since hardener is no longer necessary for manual processing, there is no reason to use acidic fixers.
Neutral to alkaline fixer (pH range 6 to 9) can wash fixer from films and prints faster, thus expediting washing time and saving wash water. Neutral to alkaline pH is also important for tanning developers and color developers, since lower acidic pH may fade the dye formed during development process.
pH Buffering agent
According to research done by Green and Levenson, fixing baths containing acetic acid (or acetate) wash more slowly than those free of acetates. Acetates are very commonly used in acid fixing baths.
The formulator of TF-4 fixer (Bill Troop) mentioned that this fixer uses a large amount of borax to make the fixer alkaline. In my testing, borax has poor pH buffering property for this fixer, and there was no advantage in fixing or washing rate. On the other hand, borates such as borax are not very desirable compounds to use in large quantity, since they are harmful to plants.
Alkaline fixers
An effective neutral to slightly alkaline fixer can be mixed as the following formula.
| Neutral rapid fixer | |
|---|---|
| ammonium thiosulfate (60% solution) | 200ml |
| sodium sulfite | 15g |
| sodium metabisulfite | 5g |
| water to make | 1 liter |
| target pH | about 7 |
An excellent non-hardening rapid fixer can be formulated by using a suitable buffering agent to maintain the pH at neutral or slightly alkaline region to resist the change when acid stop bath is carried over. The buffering agent should be selected from the type that does not impact the chemistry of the fixing reaction as well as the washing process. Silvergrain Clearfix alkaline fixer uses a safe, nontoxic proprietary buffering agent to stabilize its pH, therefore this alkaline fixer may be used with acid stop bath or plain water rinse. On the other hand, some other alkaline fixers on the market are poorly buffered and they are not to be used with an acid stop bath.
Fixer exhaustion
Fixer solution should not be overworked from the maximum processing capacity. The fixer may appear to continue to fix as usual, only a bit slowly, but this leaves a risk of unwashable incomplete fixing reaction products, which is invisible until some time after processing, when the unwashable residue turns into visible stain.
In archival processing sequence, use of washing aid helps to reduce this risk greatly.
Also, if the material is toned in brown, sepia, selenium or other archival toners, the invisible stain can be made visible immediately after the toning step, and therefore leads to immediate detection of the problem. Toning process is highly recommended to improve the image color as well as permanence, and it is also an excellent way to detect this otherwise invisible problem.
Processing capacity
Fixers gradually deteriorate as it is used, due to accumulation of the silver compounds removed from the emulsion. Thus the exhaustion is determined by accumulation of the silver rather than depletion of the active ingredient, thiosulfate.
For b&w negative film processing, the maximum amount of silver allowed for archival processing is 5 g/L (sodium thiosulfate or non-rapid fixer) or 8 g/L (ammonium thiosulfate or rapid fixer). For b&w print processing, the maximum amount of silver allowed for archival processing is 3 g/L (non-rapid fixer) or 5 g/L (rapid fixer). The level of exhaustion (silver concentration) can be measured by analytical chemistry techniques, but for most darkroom quality control purposes, a simple potassium iodide test (Kodak FT-1 solution, consult Kodak publication O-3) suffices.
For manual processing, it is best to use fixer until it reaches the processing capacity limit, and then replace it with a fresh fixer. If there is fluid loss, top up with plain water or fresh fixer. However, there is no use in actively replenishing the bath with fresh fixer.
In automatic processing machines, such as minilab machines, fixers are rejuvenated by electric desilvering process. This removes silver compounds from the fixer, thus the fixer may be kept reused for a long time, with regular additon of replenisher to make up for the loss of active ingredients. Thus replenishment of fixer is effective only when desilvering or rejuvenating system is used in conjunction.
To illustrate this further, I'll use a very oversimplified model. Suppose 10 parts of fresh fixer can fix 100 prints. By replacing 1 portion of the exhausted fixer, you'll end up with 90% exhausted fixer. Does this fixer fix 10 prints before going to exhaustion just as well as the fresh fixer? The answer is no.
By the above scheme, the fixer is always operating between 90% exhaustion and 100% exhaustion, except for the initial period when the exhaustion level go from 0 to 90%. This is far worse than using the fixer until game over, and then replace it with fresh one. This way your fixer goes from 0 to 100% exhaustion, and then resets to 0% exhaustion.
This is why it is best to dump the entire bath of exhausted fixer and replace it with 10 parts of fresh fixer to fix another 100 prints.
In fixing bath, lower silver concentration is always better. There is no optimal silver concentration that is greater than zero.
This is different from replenished developers.
Archival processing
For archival processing, it is very important to make sure that the fixed and washed materials are free of residual thiosulfate and residual silver compounds.
Although fixer "dissolves" silver halides from emulsion, only fresh fixer can do so to completion, so that the fixing reaction products can be effectively washed out by plain water wash. In other words, as the fixer accumulates some silver in the bath, one last step of the fixing process becomes incomplete, and this incompletely fixed material cannot be adequately washed in plain water. This problem can be most economically solved by using a washing aid, which promotes removal of partial fixing reaction products.
Without a washing aid, the capacity of fixer for archival processing can be very low (a few 8x10 prints in a liter of fixer) and this is very uneconomical. It is less expensive and less wasteful to use washing aid; it is definitely worth an additional step in archival processing protocol.
Although any thiosulfate-based fixer can be used for archival process, the best fixer is an alkaline, non-hardening, rapid fixer. Other acid, non-hardening, rapid fixers are also recommended. Hardening fixers are not recommended unless there is a clear need for the additional hardener.
Shelf life
Shelf life of fixers vary greatly depending on the formulation and storage condition.
Acid fixers have limited shelf life (about 2-3 years) and the fixer can go bad much faster if it is in contact with air. This is because thiosulfate can decompose to elemental sulfur (insoluble in water) in acidic condition.
Alkaline and neutral fixers can last longer than acid fixers, and they do not decompose to elemental sulfur. This is another reason to choose a neutral or alkaline fixer.
Washing process
Washing process is described in wash section. Also refer to washing aid and residual thiosulfate tests sections.
Fixer stain
When fixer splashes onto cotton shirt, towel, or some other fabrics, the affected spots remain invisible for some time, but dark brown or black stain develops over time. This is because silver thiosulfate complex in the fixer decomposes slowly to make black silver compounds. This stain tends to be darker with more heavily used fixer solutions. The stain also tends to develop faster with rapid (ammonium thiosulfate) fixer.
To prevent this dark stain, the most effective is to wear a darkroom coat.
For the same reason, the fixer may leave stain or solid deposit if not washed from tray or other apparatuses after use with fixer.
