Although over time all photographic papers will be subject to degradation, it is normally desirable to do the utmost to postpone the effects of decay for as long as possible. Indeed silver-halide emulsions, if processed and stored properly, are considered to have a potential shelf life of around 200 years. Originally this was deemed to be the case solely for fibre-based (FB) papers but, with the advances in modern paper manufacturing, it is thought that a properly processed resin-coated (RC) print can now last just as long.
Whilst the term ‘archival processing’ seems to suggest that a great deal of extra effort has to be made, in reality only a little more care and attention is needed to help prolong the life of the image – mostly in the fixing and washing stages.
After development the highlights and lighter tones of an image still contain large amounts of undeveloped silver-halides. On exposure to light these halides will begin to ‘print out’ until the whole print darkens or ‘fogs’. The purpose of the fixer (sodium or ammonium thiosulphate) is to remove these excess silver-halides and give the image stability. It does this by dissolving any unexposed or undeveloped halide, and converting it to silver thiosulphate, which is soluble in water. When this reaction has taken place any residual fixer must be washed out of the paper or, over time, it will combine with the image silver to produce yellow silver-sulphide stains.
It is essential to adhere to the manufacturers’ recommended fixing times, as under-fixing will leave residual silver-halides in the emulsion, and over-fixing may well leave excess fixer embedded in the fibres of the paper that no amount of extended washing will counter.
A series of chemical reactions take place when fixing, with the first reaction producing a silver thiosulphate compound that is actually insoluble in water. If left trapped in the paper base this product will eventually migrate to the emulsion and stain the image. However, as long as there is enough excess fixer (thiosulphate) in the solution, two more chemical reactions will take place to convert the compound into water-soluble products, which can now be washed out.
If the fixer is partially exhausted there may not be enough excess thiosulphate to convert the insoluble compounds adequately, so it is essential to make sure that the fixing bath does not get over-worked. Most printers use fixing bath test-strips which, when immersed in the solution, show the residual silver level of the bath. If this level reaches more than 0.5 grams/litre, then the fixer should be discarded and a new bath made up.
Whilst applying these simple checks helps in maintaining the integrity of the print, it also means that normal single-bath fixing has a necessarily limited capacity, with the only apparent remedy being frequent changes of the solution. There is, however, a cleaner-working and more economical alternative to the single-bath process.
With this process the total fixing time is divided equally between two baths of the same dilution. The first bath does most of the work in removing the majority of the silver and producing the insoluble silver thiosulphate compound. The second bath contains the fresher excess thiosulphate that ensures all the silver salts are rendered soluble. By having the cleaner-working second bath, the silver content of the first bath can reach a much higher level before it is discarded: when the level reaches 2 grams/litre the first bath should be replaced with the second, and a fresh second bath mixed up.
When checking fixer longevity, it is also worth recognising that high-key images and prints with large white borders retain much larger amounts of unexposed silver-halide, and will therefore exhaust the fixing bath at a far greater rate.
The two-bath process is most professional printers’ preferred method of working with fibre-based papers, but what about resin-coated papers?
The emulsion for a resin-coated (RC) paper is coated onto a waterproof plastic base. Upon processing only the emulsion layer absorbs any chemistry and, as a consequence, because nothing permeates the base itself, there is a marked reduction in the time needed to both process and wash the image. This facility for shorter fixing times tends to negate the need for the second fixing bath – the excess fresh fixer in the first bath is reckoned sufficient enough to stop the build-up of insoluble by-products.
In the Photofusion darkroom we employ the traditional two-bath fixing set-up for fibre-based papers – two minutes in each tray followed by an hour wash. However, for resin-coated papers, two minutes in the first fixing bath is sufficient.
Gallery: Kim Shaw
Kim Shaw is having work printed by Nick for her upcoming exhibition at Willmott Whyte Studio. The American photographer will be presenting work that spans the past five years, including images from three separate projects: “Earthly Bodies”, “99% Humidity” and “We Built this City”.
10 Heathfield Terrace
Thursday March 21, 18.00 – 21.00