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All dyes used in photography fade with time, and this effect has been dealt with in a previous paper.
Chemical methods of "restoring" faded tripack dyes
These processes were regularly recommended in the past and there is some literature relating to the theory but the practice is exceptionally difficult. The Editor has experimented with the techniques but, not surprisingly, has never been permitted to carry out the processes on real archival film of any value by an archive! The processes generally work but a fundamental reason why they cannot be recommended is that they are impossible to control, and it is not possible to know the long term effect of the procedures on the film treated, and for these reasons the following are summarised for interest but are not advised.
One chemical process is to reconstruct the original dyes from the breakdown products that still remain in the emulsion by "reprocessing" film in various solutions. Cyan dyes seem to respond to this chemical process but yellows are not so responsive. All the formulae for the tests carried out by the Editor were from verbal sources! Some of the past literature mentions that manufacturers publish chemical formulae for this procedure but none are known to the authors and certainly Eastman Kodak do not recommend this. In any case specialised solutions are difficult to handle, as they need to be located in a processing machine to treat long lengths of cine film.
The only procedure that can be in any way recommended is to reprocess faded film in a modern process similar to that originally used - a negative process using CD3 [e.g. ECN2] for a faded negative film - a print process using CD2 [e.g. ECP2] for a faded print. The temperatures should be reduced to about 25C to avoid excessive emulsion softening. There is some unpredictable beneficial effect on faded cyan dyes of old Eastman and Gevaert colour print films but little effect on faded negative film.
A more effective, but drastic, chemical treatment of faded print film is to intentionally destroy a proportion of the magenta dye until a low contrast but more neutral image is achieved. Azomethine magenta dyes are very stable but are susceptible to acids. The film can be held in a 3% [about 1N] sulphuric acid solution for up to 3 hours during which time the magenta dye diminishes. Once treated until the balance is about right the film contrast is now too low and must be copied onto a high contrast internegative film to restore a print of the original contrast. This method does work - however is messy, very difficult to carry out on a long film length, can damage processing machinery and can be uneven in effect. It also seriously softens the film emulsion and it is a race against time to achieve the right degree of magenta fading before the emulsion falls off. This is also not recommended.
A mask is a separate image that can be combined; in register; with the original image in order to create produce a modified image. The technique has been extensively used in restoring still photographs, and was also used for the correction of unwanted dye absorptions in preparing prints from colour photographs in the graphic arts industries. The old Dye Transfer, Carbro and Vivex paper colour printing systems all used masking wherever the original was a colour transparency.
The technique will provide excellent correction for faded dyes but is extremely difficult to apply to small images repeatedly. The process entails printing the original to make a mask image and combining the original and the mask or masks, in register, to then print.
In order to reduce the problems of "fringing" caused by a mask not being perfectly in register with the original image, masks are usually made slightly unsharp, and since contact printing is almost universal in order to make the image exactly the same size, the mask is usually exposed through the base of the original. Masks may be made on black and white or colour stock, negative or positive, depending on the purpose of the mask.
This procedure is extremely difficult on any printer, and is generally only done on a step contact printer. Printing on an optical printer is possible only if the printer has been specially designed for this. Rotary contact printers are also difficult although this is more commonly done in order to produce subtitles and the degree of slip and imprecision makes registration very imprecise.
An extra set of supply and take-up spools and a drive are needed to supply and rewind the mask, roll which will usually be the same length as the original roll.
Masks are usually used for two procedures:
1 To combine with an original during the printing of a separation negative or positive. In this case the mask is used to produce a separation image that matches the other two separation images in contrast. The process is used for the restoration of faded negatives or positives where one dye is more faded that the others. The mask is made on black and white panchromatic film by printing the original with colour light of the colour to be restored. A faded cyan dye on a negative requires a black and white mask made through a red filter. The resulting positive mask is printed to make a negative mask and the mask and the original combined in register to make the corrected red separation positive.
This process is extremely rarely performed, it is costly, time consuming, and requires a technician capable of matching the resulting separation contrasts. Considerable technical understanding of the process is essential to calculate the process gammas required. This makes the procedure one that will only be carried out as a last resort in organisation that specialise in colour film restoration.
This process can correct faded colour film when one or more dyes are virtually undetectable to the eye. Varying the development contrast of the separation image can also produce similar but less correction. This is explained in Colour Duplication chapter [cv].
2 To combine with the original when printing a colour internegative or intermediate.
In this case the mask can be black and white in order to correct contrast [an unwieldy technique almost never used since flashing or development control is easier] or a coloured mask in order to correct for faded dyes. This last technique was a very effective and relatively simple procedure that has not been carried out for some years. C. Bradley Hunt in SMPTE 1981 describes two techniques. One is very complex, the other easy and sophisticated.
In the Flashed Dupe Mask Method a positive print of a faded original [usually a positive] is made on a colour reversal print film to a colour balance and contrast more or less complimentary to the faded image. For example a faded colour print is often magenta due to the fading of the yellow and cyan. The mask would be visually green in this case, with a low green contrast, produced by flashing the print film. When combined in register the three contrasts can be equalised, and the process is easy to control without sophisticated sensitometry since the combined mask and original will appear neutral, or "correct" in balance when the mask is correct. From the combination of the mask and the original a colour internegative or intermediate can be made.
This procedure is easy to operate although making the mask may need some trial and error printing. Dyes can be corrected when the contrast reduction is up to 30%.
The process is hardly used today because the majority of the need is to restore 35mm and 35mm reversal processes do not exist in commercial laboratories. The technique could be used for 16mm restoration [for example of faded news film] but has not been used so far.
The Dupe Mask Method is far more complex but can correct up to 50% dye fading.
Separation methods from coloured originals - three-colour subtractive films
In the Colour Duplication chapter the procedure for making separation positives from colour negatives is described, and the procedure is similar when making separation negatives from positives. The same sensitometric control calculations apply. The technique can be adapted to a wide range of restorations provided the original characters of the colour process are known.
The usual reason for making any separation during a restoration process is in order to correct for fading. If fading has not occurred or a copy of a film image is required in its present condition separations are an unnecessary process.
Separations are still however considered the best method of preserving a coloured image, because the black and white image has a longer life than any colour image. However most laboratories, when asked to reconstruct a positive film image from archive stored separation negatives or positives find that the quality of their production was not very high. Contrast often does not match, registration is not good and storage sometimes results in separations having different shrinkages.