This is what the manual says about this:
"To the best of our knowledge, there is only one case where a powdered fingerprint will fade on a BVDA Gellifter. This is where BVDA gold powder is used (Patrick: this is a fingerprint powder containing brass powder (copper/zinc alloy)). Lifted prints will fade and ultimately disappear in time. Fading will be noticeable after days or weeks, depending on the storage temperature (the lower the better). Prints developed with silver powders (aluminium), on the other hand, are known to have been stored for several years without apparent fading. However, we always recommend photographing prints as soon as possible."
"Lifted shoeprints (dust marks) may slowly fade in time. For very weak prints this may be noticeable after storing them for a few days. This will depend on the temperature. The lower the temperature, the slower they will fade. This, however, should not be a concern, since lifted shoeprints can easily be photographed after removing the cover sheet. Oblique lighting will show details which are not visible before photography. So far, we know of no materials that fade on the Gellifter surface before adequate photographs have been taken."
We recommend photographing the prints as soon as possible, no 'immediately'. Photography is
indeed needed as you will always have to flip the image to get in the correct position, in case of a black lifter, to invert the image so that you have a black fingerprint on a white background, to get the print on the lifter into AFIS and, if needed, to enhance the print. As a reward for making these efforts you will end up with the best possible image of the print!
Over the last couple of year's I have visited a lot of labs for demonstrations with our imaging system for Gellifters, the GLScan. During the demonstrations we always scan actual case work as well that the audience has available. Most of the time they will not give you the easiest lifts and quite often they the lifts are a few years old. As far as I remember we have always been able to image the prints on these lifts, even after the lifters were stored for a few years.
One thing that will make you think that your print has faded is actually not fading but diluting. This diluting of the print is caused by repeatedly replacing and removing the transparent cover sheet. Part of the dust print will stick to cover sheet and will be removed from the actual lifting surface when the cover sheet is removed.
That's why the following is recommended in the manual as well:
"For best results you should not replace the cover sheet. Transportation of the lifted print is of course more difficult then. We suggest using double-sided adhesive tape and a clean, shallow cardboard box, like the ones that photographic paper comes in."
The Home Office Scientific Development Branch in the UK has done research about this and published the results in their Fingerprint and Footwear newsletter. This is the link to their website where the news letter can be read/downloaded:http://scienceandresearch.homeoffice.go ... iew=Binary
Kathy mentions in her reply that she heard of people washing the Gellifters after the case was done and then used them again. I don't think it is wise to reuse Gellifters, think about issues related to contamination. As importantly, a Gellifter used for a second time will never give an image a good as when it came from the package for the first time.
The quality of the image of a print on a black Gellifter heavily relies on the deep black color of the gelatin surface and on its mirror like surface. A part of the light used during photography is absorbed by the deep black color and a larger part is reflected by the glossy surface.
Where there is no trace material on the Gellifter the light is reflected in the same angle as it hits the surface (specular reflection). This light will not reach the camera and the camera will record this as black. Where there is trace material on the black surface, the light will be reflected in many directions (diffuse reflection) and will also reach the camera which will record this as grey to white color. This is even the case when you have black fingerprint powder on a black Gellifter, black powder contrasts perfectly on the black surface of the Gellifter.
If the surface of a Gellifter is cleaned, the surface will no longer be glossy. When you then photograph the lifter, the whole surface will diffusely reflect the light and the lifted print will not, or very poorly, contrast against the background.
I hope this helps. Please let me know in case further questions arise.