Gas Hypersensitisation Of Film
In the days before CCD cameras I used the gas hypersensitisation process outlined below to 'hyper' Kodak Technical Pan 2415 film. It gave me excellent results especially when combined with deep red filters to enhance red emission nebulosity and cut light pollution. The process was incredibly complicated and potentially hazardous - it is hard to appreciate just how easy CCD imaging is by comparison! I have left this page here as an interesting and amusing reference to the technology of the very recent past.
Some examples of my work can be found on the astrophotography page.
The gas hypering process using pure Hydrogen gas is intrinsically hazardous and so great care must be taken at all stages to minimise the risks involved. Please read the safety section and ensure you are completely familiar with the chemistry and hazards involved prior to starting any experiments.
- What does hypering do?
- Equipment & Chemicals
- The Chemical Reaction
- Drying the gas
- Applying vacuum to the film
- The overall process
- SAFETY PRECAUTIONS
The process is hazardous and so it is best to consider whether you really need to hyper your films or not. Ask yourself these questions.
- Will you be using B&W film?
- Will your exposures be greater than about 10 minutes?
- Will you be using the film for deep sky photography via a telescope?
If the answer to all of these is YES then you should consider hypering. If the answer to any of them is NO then you need to think again.
The purpose of hypering film is to reduce the effects of Low Intensity Reciprocity Failure (LIRF) by removing water and oxygen from the emulsion and producing a low level of chemical fog within the film prior to exposure.
Removal of water and oxygen is performed by subjecting the film to as high a vacuum as possible, the chemical 'fogging' is carried out by 'baking' the film in hydrogen gas. The film is best treated out of it's cassette on a spiral reel or similar otherwise uneven results will be obtained.
Kits are available commercially from Lumicon in the USA that use forming gas which is a non explosive mixture of nitrogen and hydrogen and can be used for hypering colour film as well. Hypering with pure hydrogen is not recommended for colour film as the colour balance becomes strongly shifted after hypering. This occurs mainly because the film has three layers, the hydrogen does not reach all of them evenly. The following sections describe in general terms each of the process steps. Experiments will always have to be carried out for your particular set-up to determine the best process.
Laboratory apparatus is the best type of equipment to use for the reaction to produce hydrogen. Flasks should be Pyrex or similar, pre-holed rubber bungs can be used with glass tubing inserts to carry the gas out of the flask. The glass tubing is then connected to the main apparatus with clear PVC tubing. Rubber tubing decays over a period of time and is less suitable. Lab apparatus can be bought mail order or from a local laboratory supply house. Various gas tight valves will need to form part of your apparatus to control the flow of the gases and vacuum.
You may wish to avoid the chemical side of the process altogether and buy gas cylinders of hydrogen or forming gas. This is a more expensive option though as you will need a gas regulator to fit on the cylinder and these are expensive.
The hypering tank is best constructed out of metal, aluminium, brass etc. The connections to the vacuum pump, tank and valves are best constructed out of 6mm copper tubing with soldered or brazed connections to avoid leaks and make the whole apparatus more permanent. A photographic developing tank spiral is ideal to hold the film in the tank, these can be bought as spares for developing tanks from most photo shops.
The main chemical in use is sodium hydroxide. This can be bought in pellet or bead form from most good hardware stores. 500 grams costs about £1.30 sterling. Concentrated sulphuric acid, if used as the drying agent, can be obtained from laboratory supply houses. You will need to explain what you are using it for. Supply of such materials will be governed by local and national regulations.
Hydrogen can be produced in a pure form from the reaction between sodium hydroxide (caustic soda) and aluminium metal.
2Al + 2NaOH + 6H2O = 2Na(Al)(OH)4 + 3H2
If we calculate the amount of hydrogen liberated based on the reaction then we find that 1 gram of aluminium will produce 1.24 litres of hydrogen gas given an excess of sodium hydroxide and water. The reaction is extremely exothermic i.e. produces large amounts of heat. This must be controlled to avoid a potentially dangerous rapid evolution of hydrogen.
In practice, a 10-15%w/v solution in water of sodium hydroxide can be used to react with thin aluminium sheet (approx. 14 gauge) the thickness of the sheet helps to control the reaction rate and thus the temperature of the reaction. Use about 250cm3 of solution with a 5x5cm square of aluminium as a starting point, this will generate sufficient hydrogen to purge and fill a tank of 500cm3 capacity . The reaction temperature must be controlled to below about 30C. this can be accomplished by ensuring the aluminium sheet used is of sufficient thickness and that the reaction is cooled in a stirred water bath.
The hydrogen evolved will be saturated with water vapour and must be dried. This is best accomplished by bubbling through concentrated sulphuric acid. Concentrated sulphuric acid is extremely corrosive and very hazardous to handle and can be difficult to obtain. An alternative is to use the sodium hydroxide pellets that are used to make up the 10-15% solution for the reaction. Care must be taken when using pellets for the drying process to ensure that the path of the gas through the 'bed' of pellets is kept clear and not allowed to block otherwise a pressure build up could occur. Many other drying agents could be used including 'molecular sieves' which can be re-used. Silica gel may be effective if used in conjunction with a slow gas rate.
The water and oxygen in the emulsion must be removed prior to treatment with hydrogen. This is best done by applying as high a vacuum as possible to the film. A vacuum pump built for the purpose is of course best but these are very expensive when bought new. An alternative is to use an old fridge compressor in reverse. These pumps will usually pull down to about 25mm Hg pressure which will be sufficient. The vacuum will need to be applied for at least 20 minutes at room temperature. A vacuum gauge is not absolutely necessary though will indicate leaks etc. if present. Remember that Hydrogen gas in any concentration must never be allowed to pass through vacuum pumps or an explosion could result.
The chemical reaction is started and the hydrogen evolved allowed to purge the air out of the system. After the air is purged out the hydrogen can be collected in a toy balloon. Whilst collection is in progress, the film is put under vacuum in the tank. After the vacuum has been applied for sufficient time the vacuum in the tank is let down to the hydrogen in the balloon taking care that any valves are closed so as to prevent hydrogen entering the vacuum pump.
The flow of hydrogen is continued through the tank to vent whilst heating the film to 40-55C. The time this is continued will depend on the film type and vacuum applied, but a starting point of 4 hours can be used.
After the time is up the tank must be purged with dry inert gas such as nitrogen or perhaps more readily available, carbon dioxide from a sparkling drinks maker cylinder. This step is for safety reasons as the tank full of hydrogen would be opened to air to retrieve the film forming an explosive mixture.
Several experiments will have to be performed with small pieces of film which can be tested to determine the best conditions for your set-up.
8. Safety precautions
The chemicals in use are corrosive and must be handled according to the manufacturers instructions. If a safety data sheet is not supplied you should request one and read it carefully. As minimum protection, rubber gloves, goggles and a PVC protective apron should be worn when handling sodium hydroxide and sulphuric acid. Keep all chemicals and solutions away from children.
Hydrogen forms explosive mixtures in air from about 4% concentration, the explosive mixture is easily ignited by static electricity or very hot surfaces and so contact with any likely source of ignition must be avoided at all costs. This must be thought out carefully, small electric motors, even battery driven will provide a source of ignition.
All the apparatus must be earthed to avoid static build up. The apparatus is preferably purged with inert gas before starting the chemical reaction to produce hydrogen. Waste hydrogen during the process must be safely vented in the open air well away from any source of ignition. After hypering the tank must be purged with dry inert gas such as nitrogen or carbon dioxide otherwise a tank full of hydrogen would be opened to air forming an explosive mixture.
The set-up must take account of the possibility of the chemical reaction becoming out of control and frothing over into the drying agent. Two traps either side of the drying agent must therefore be used. The first to catch any caustic solution from entering the sulphuric acid and the second to prevent any sulphuric acid from entering the lines leading to the vacuum pump or film chamber Refer to this diagram
Caution must be exercised at all times to avoid the possibility of a serious incident occurring due to a hydrogen/air explosion.
No liability will be accepted as a result of personal injury or loss. It is the responsibility of the experimenter to fully assess the hazards associated with the hypering process.