==Bleaching with Rehalogenating Bleach== by Jeff Blyth
A successful bleach for reflection Holograms comes from using:
60KB Rehalogenating Bleach
- 40g Ethylenediaminetetraacetic acid iron (111) sodium salt [Aldrich cat. No. 35,961-0]
- 60g. potassium bromide &
- 70 ml acetic acid
Dissolved up in 1 litre water (tap water is OK here).
This bleach is particularly good after a developer such as TJ1_Developer is used.
So what happens with in this type of bleach used WITHOUT FIXING (i.e.without removing unexposed AgBr) in thiosulfate, is that the developed up silver gets re-oxidized to AgBr. But instead of returning you to square one and leaving you with a uniform coating and distribution of AgBr again just as it was before you exposed it, it is energetically more favorable for the re-made AgBr to move over to the adjacent dark fringe made up of virgin AgBr and grow onto that dark fringe using the virgin AgBr grains as seeding centers. That actually requires the bleach solution to have some AgBr solvation ability to enable this carry-over effect to occur. This effect occurs with the help of the relatively high concentration of potassium bromide present because it does raise the solubility of AgBr in the solution through the formation of complexes.
Now the great thing about this carry-over effect is that it causes almost all the original Ag in the emulsion to build up the fringes whereas if you had had to use fix you would have removed about half of your original silver content in the thiosulfate solution.
A revealing experiment is to take a newly developed plate that has been in a stop bath of ~5% acetic acid, rinse it and then place it upright in a beaker so that it is half covered in a fix solution such as 20% sodium thiosulfate. After giving it gentle agitation over about 4 -5 minutes avoiding splashing the unimmersed half it is then all given a vigorous rinse under tap water.
Then the whole plate is immersed in the above bleach formula and given constant agitation.
The first interesting thing that will be seen is that the fixed half will take longer to bleach the dark silver than the unfixed half .
This is at first counter-intuitive since one would expect that initially removing the undeveloped AgBr in the fix would later have left the bleach plenty of spare room in the gelatin to react and oxidize the silver metal without being encumbered by lots of AgBr still present.
The second point that will be noticed is that when the whole plate is bleached there will be considerable scatter on the fixed half compared to the unfixed half.
The increased scatter in the fixed half also testifies to the truth of that carry-over mechanism. The scatter is a consequence of the carry-over effect being unable to operate because of the missing virgin AgBr. Therefore the newly formed AgBr builds up around the dissolving silver grains in solution before reaching a level where it becomes energetically favorable to precipitate out. The precipitate will be in larger grains and to some extent will occur in the dark fringe areas where the gelatin is supposed to be free of AgBr in order to give good fringe contrast with the new AgBr in the light fringes.
The finished hologram if it had been recorded in red will now be shifted to the green, scattery, and less bright than the unfixed half.
Bleaching Transmission Holograms
The formulation above has been found to work pretty well also with transmission Holograms (Hs). The not-so-good thing about it though is that the original sensitizing dyes become chemically locked into AgBr grains making the emulsion very vulnerable to print out, i.e. darkening slowly in ambient lighting, particularly sunshine.
The dyes can be chemically inactivated with a 2% potassium or ammonium dichromate bath-- it takes about a minute after you have used the Ferric EDTA bleach.
A Good Bleach for Transmission Holograms, (can also be used to make reflection ones with a shorter replay wavelength)
A better bleach for transmission holograms is to dissolve up 0.5 to1 gram of iodine crystals in about 200 ml alcohol (methanol or ethanol) and then about 200ml of water is added. However before putting the plate in, it is essential this time to use fix. This is because the bleach has no carry-over power. The fix bath can be 20% sodium thiosulfate and the plate given about 4 minutes in it with mild agitation. It is then given a thorough rinse under tap water to remove all traces of fix. After the bleach step the iodine stain can be removed in a 70% alcohol bath.
A very good point about this bleach is that the dyes are released by the fix and easily removed in the alcoholic iodine solution.
A comparison was made by cutting a developed and stopped (5% acetic acid) transmission H in half and then bleaching one half in the ferric-EDTA bleach and the other half after fixing was put in the iodine bleach.
The iodine bleached half finished up producing a slightly higher diffraction efficiency.
(This could be due to the carry-over effect being less efficient in the larger fringe spacing of transmission Hs compared to reflection Hs.) If you choose to use this bleach on say a reflection hologram made with a red laser then you can get a quite nice final yellow-green replay color because contraction occurs due to loss of the original virgin AgBr in the fix solution. (You also get a little bit of expansion due to AgI replacing AgBr.)
It may then look brighter than a red one would have looked because of the eye's extra sensitivity to light-green even though some valuable AgBr diffracting material has been lost
P. Hariharan, C.M. Chidley; Rehalogenating Bleaches for photographic phase holograms 2: spatial frequency effects. Appl. Optics. 27 No.18, 3852 (1988) )