9. Preparation of chemical solutions

The process of chemical processing of holograms is one of the most critical steps in hologram making: first of all, it is almost impossible to buy solutions for chemical processing of holograms, you have to prepare them by yourself. Quality of holograms depends greatly upon accuracy of the chemical solution composition. Holography as well as photography has a lot of methods for chemical processing of holograms for different types of photoplates and schemes of hologram recording [1].
We are not going to consider extraordinary formulas and methods. We'll examine a standard method of PFG-03m photoplates chemical processing. You have already studied that these photoplates have the highest resolving power to register the finest interference pattern of a reflect hologram recorded according to Denisyuk's scheme. The main aim of the developing process of Denisyuk's hologram is saving of this interference pattern in the developed hologram.

Developer preparation

Usual photographic developers referring to the type of "chemical" developers are not suitable for this purpose. During developing silver bromide microcrystals in the emulsion layer of a photoplate turn into tangles of metallic silver. Their size is equal or exceeds the size of the interference pattern period (see fig.). For the same reason developed negative films and photographic papers are black - silver balls absorb light (!). It has nothing to do with an ordinary silver plate.
So called "physical" developers are used for developing super high resolution photoplates PFG-03m. There is a special component in these developers. It is ammonium thiocyanate. Ammonium thiocyanate dissolves crystals of silver bromide. Practically the same process happens in the fixer, but in the developer atoms of silver bromide being in the solution are reduced to metallic silver and then they fall on silver bromide microcrystals (this process is called diffusive transfer). Kinetics of this complicated process is such that after development compact grains of metallic silver appear and their size is close to the size of silver bromide (see fig.). As a result the interference pattern of the developed hologram is completely preserved.
Let us consider composition of GP-2 developer which was worked out by Russian scientist Usanov Y.E. Formulation of GP-2 developer and its modification GP-3 are given below.

You can see the developers have the same composition, the only difference is quantity of some chemicals. Let us consider the purpose of each chemical in a brief way.
1. Methylphenidone and hydroquinone are developing substances. If they work together they are more active than each taken separately.
2. Sodium sulfite prevents rapid oxidation of the developing substances and stabilizes work of the fixer. It provides fine-grain development since microcrystals of silver bromide are partly dissolved.
3. Potassium hydroxide creates alkaline medium in the developer. The developing substances can actively work only in alkaline medium.
4. Ammonium thiocyanate is the most "holographic" component of the developer. (It is not used in the photographic developers). It actively dissolves crystals of silver bromide and provides super fine-grain development.

To prepare the developer it is necessary to have all the mentioned chemicals, a balance of 0.01 g accuracy, a thermometer of 0.1^oC accuracy, 2 retorts of 1l, a graduated cylinder of 1 ml graduation, distilled water (see photo). Cut several sheets of A4 paper in two and weight out each chemical. You should put rubber gloves on. Be very careful with potassium hydroxide. Avoid its contact with your skin or clothes.
It is known phenidone and methylphenidone are dissolved in water badly but their dissolubility in the alkaline solution is good. Pour 300 ml of water into a retort and dissolve there potassium hydroxide. Add the alkali by small portion and mix the solution by a plastic stick thoroughly. Alkali dissolution in water is an exothermic reaction (a reaction with heat release). So that if you add all the alkali weighed amount, the retort may be blown up because of overheating. After the alkali has been dissolved completely, dissolve phenidone. The solution will become pinkish.
Pour 500 ml of water into the second retort and then dissolve sodium sulfite and ammonium thiocyanate. Ammonium thiocyanate has an unpleasant smell of ammonia therefore you should work in a room with good ventilation or under a hood. After all the chemicals have been completely dissolved pour both solutions together and mix them thoroughly and add water to 1l. The prepared developer should be absolutely transparent and have a significant smell of ammonia. Filtrate the developer using a cotton wool tampon and a funnel. Pour the developer into a bottle with a hermetic stopper. Mark the bottle (name of the developer and date of its production) and put it in the refrigerator. The developer can be stored in a refrigerator not less than a month.
Wash the retorts, the funnel and the plastic stick carefully.
If the developer is turbid or it is brown or it has another color or if there are flakes or sediment, or there is no smell of ammonia, it means that one of the chemicals is not suitable for usage. Check if the chemical names and date of their storage life are correct and if their weighed amounts are accurate. Then try again to prepare the developer.

Fixer preparation

To protect the developed hologram from light influence (light can cause darkening), unexposed microcrystals of silver bromide should be removed from the emulsion layer. For this purpose a photoplate is processed by a fixing solution where the rest microcrystals of silver bromide are dissolved and removed from the emulsion layer. The simplest neutral fixer is used to fix PFG-03 photoplates. Microcrystals of silver bromide have very small dimensions and their dissolving in the fixer takes several seconds. Some specialists even convince that it is not necessary to fix the photoplates in GP-2 or GP-3 after their developing since the most part of silver bromide has been already dissolved in the developer. It is right, but we should use the fixer to make the process reliable.
Here is the fixer formula:

Dissolve sodium sulfite in 700 ml of water. Then dissolve sodium thiosulfate and increase the solution volume to 1l, using water. Thiosulfate dissolving is an endothermic reaction (a reaction with heat absorption), so it is desirable to warm up water for this solution to 40-50^oC. The ready fixer is absolutely transparent and it does not have any smell. Filter the solution, pour it into a bottle with a hermetic stopper, mark and put it in the refrigerator.
Wash the retorts, the funnel and the plastic stick carefully.

Preparation of alcohol solutions for hologram drying

Ethyl alcohol solutions are used for qualitative drying of holograms. It is possible to dry a hologram in 100% alcohol at once. But in this case alcohol will absorb water rapidly and quality of drying will worsen rapidly too. Therefore to save alcohols, drying is executed by three steps - successively in 50%, 80% and 100% alcohol. (To be precise, the usual alcohol has 96% concentration). Prepare the alcohol solutions in the following sequence:
50% alcohol - pour 500 ml of water and 500 ml of alcohol and mix,
80% alcohol - pour 200 ml of water and 800 ml of alcohol and mix.
Pour the alcohol solutions into hermetic bottles, mark and put them in the refrigerator.
Wash the retorts, the funnel and the plastic stick with running water carefully.

References
1. H.I. Bjelkhagen "Silver-Halide Recording Materials (for Holography and Their Processing)", Springer Series, USA, 1993.
2. T.H. James "The Theory of the Photographic Process", Macmillan Publishing Co., New York, 1977.
3. Usanov Y.E. "Influence of the Developer Composition upon the Holographic Image Properties", Recording Mediums for Holography (Collected Articles), Science, Leningrad, 1975, p. 98.
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