The Reversal Bleach system
by Jeff Blyth
The most popular form of this type originally used dichromate salt and sulfuric acid. It has been of particular value for use on the AGFA 8E75 HD plates and film because when used correctly it can produce the lowest levels of light scatter. Its popularity for Denisyuk reflection holograms made with 633nm from HeNe lasers also comes from the inclination to hit the fairly elusive yellow replay coloration due to the right amount of emulsion shrinkage. I consider that its chief asset today with finer grain materials available is as a bleach for transmission holograms. It has good printout resistance (i.e. low tendency to darken in ambient lighting over time) and can produce high diffraction efficiency. However for Denisyuk reflection holograms it does not achieve as much diffraction efficiency as can be obtained from a good rehalogenating bleach.
People using it have however failed to appreciate that the developer used beforehand should not contain silver halide solvents such as sodium sulfite and urea, and the need for the rigorous exclusion of halide contaminants as discussed in the Theory section.
A suitable developer for use in conjunction with this bleach is TJ1 Developer.
The proportions used in the formulation is hugely tolerant of variation. I have chosen to use a lower concentration of dichromate compared to previous publications because it leaves the bleached hologram with less yellow coloration from the dichromate salt and this means less rinsing is required to remove it. Increasing the dichromate proportion will reduce the time taken to bleach the dark silver. I have substituted sulfuric acid with the more manageable solid salt sodium hydrogen sulfate. (This is in fact a semi-neutralized form of sulfuric acid).
The basic formula is:
- 1 g. potassium dichromate (or ammonium dichromate)
- 10g sodium hydrogen sulfate.
- made up to 1 litre in distilled or de-ionized water.
The set up should include 2 baths of de-ionized water (DI) as follows:
- DI with
- 4% acetic acid (Acts as a “Stop” to stop developer action)
- Reversal Bleach
- After development, a brief 10 second rinse under running tap water then a good rinse in Bath 1. for ~1 minute .
- A good rinse in Bath 2. ~1 minute
- Immerse in Reversal bleach (Bath 3) and gently agitate until no dark silver remains
- Important- after the bleach bath the hologram should be put first back in DI (Bath 2) for ~ 20 sec. before being rinsed under tap.
The idea might seem simple enough, after development the developed up silver is dissolved up into the solution and removed from the gelatin film so that then leaves the undeveloped virgin AgBr in the dark fringes to make the hologram. So in effect it both “fixes” and bleaches. The good point about it is that it has a high resistance to printout or darkening in ambient light and can have low scatter levels with holographic plates that do not have the smallest AgBr grains such as the old Agfa material, the lowest scatter comes about provided you understand what you must do to stop any soluble halide ions getting into your hologram before you have finished processing.
Because of the way this bleach operates, particular consideration has also got to be given the developer system used first. It is not satisfactory to have any “physical development “ which encourages silver bromide to be dissolved in the developer. We need to have as much virgin AgBr as possible to create our final diffraction and it makes no sense to load up the developer with sulfite ion a weak silver halide solvent and similar remarks apply to urea as in the CW developer.
The amount of development is also more important than in the case of the rehalogenating bleach system. Since all the developed silver is going to be washed away, if you develop too much for too long then you start to eat into your virgin AgBr in the dark fringes because even unexposed AgBr is developable given enough time.
The consequence is that reflection holograms made with red lasers may look a dull green instead bright yellow/green due to increased contraction .
This effect has also been shown to cause a peak in the graph of diffraction efficiency vs. developer/exposure level and after the peak the efficiency drops away. Whereas when a rehalogenating bleach is used after the same developer conditions, the diffraction efficiency flattens off. [Joly]
Importance of De-ionized rinsing water.
After the developer the hologram needs a good rinse under tap water to remove the developer and soluble bromide and iodide ions in it . Even if the developer had no halide ions initially, the development process means that the AgBr and AgI in the emulsion had to be broken up and turned into dark silver and soluble Br- and I-. The tap water rinse then leaves the emulsion with just chloride ions from tap water which are less of a problem to deal with later than soluble bromide or iodide ions.
Before the dichromate bath is used you have to have two pre-baths of de-ionized water (DI) to remove all traces of dissolved halide ions. If you don’t do this then some of the developed up silver fails to be removed from the light-struck fringes and deposits itself back in the fringe as silver halide. This causes scatter in the finished hologram and reduces diffraction efficiency because the light struck fringes have failed to be properly cleared of AgBr . Where even experienced holographers commonly go wrong is that after removing the bleached hologram from this reversal bleach bath, they rinse it under the tap instead of first putting the hologram back in de-ionized water for a second time .
This is because after leaving the bleach bath the hologram is full of silver ions in solution which can instantly form silver chloride particles with the chloride ions in tap water. So this causes scattering from inside the emulsion which cannot be wiped away even if surface silver chloride can be.
After using the bleach bath you may notice a red-brown precipitate or scum in the bath. This is normal and it is actually good to have it in there. It is made up of silver chromate or dichromate which is not very soluble but is far more soluble than are the silver halides. So what this red sludge means is that your bleach bath is saturated with silver chromate in solution and any stray halide ions in solution are effectively precipitated out before they can get inside your emulsion. Even though some precipitated silver chromate may form in your gelatin layer it comes out easily in the DI bath. After this final DI bath you can then rinse the hologram in tap water to eliminate any dichromate ions if you wish, because there will be no soluble silver ions to cause trouble in a final tap water rinse. (Personally I like having a trace of dichromate in the hologram not washed out because it helps to prevent future printout.
However dichromate is quite poisonous and who knows what future use your hologram may be put to particularly with young children around).
Tap water rinsing
Prolonged tap water rinsing can remove some of your AgBr with significant differences depending on time of year and the temperature of your cold water supply. Any AgBr loss causes a shift to a shorter wavelength replay in the case of reflection holograms and of course some loss in diffraction efficiency but sometimes people prefer to simply shift the color from orange-yellow to yellow-green using a hot water rinse. The result can look brighter, also any scatter from AgCl contamination can be removed because AgCl is about ten times more soluble than AgBr.
Some idea of the temperature effect can be seen from this graph:
[Joly L., Jacobs P. Spectral Response of reflection gratings on Holotest 8E75 HD Proc. Int’l Symp. on Display Holography, ed. Jeong, T.J. Lake Forest College IL. Vol III p115-126 (1989).]
[Owen, B.B. and Brinkley, S.R. J.A.C.S. 60, 2237 (1938).]