British Kinematography (1952)

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160 BRITISH KINEMATOGRAPHY Vol. 20. No. 5 produced in these processes. Solution of tiie problem of perpetuating the hand and stencil coloured him is being sought along tne lines of obtaining an accurate account ol the original means ot production, in order to permit of eventual colouring of dupes by tne same means. The fugitive dyes of tripack present another unsolved problem of tnemselves, for long term preservation purposes. They are sufficiently permanent for the commercial run of films but certainly some of the dyes of films of tnis type, made a few years ago are now fading. Storage of Safety Film. All duplicates made for the Library are printed on safety base; and indeed have been since March, 1949. Since safety film is now being generally used, almost all of our future acquisitions will be on this material, and the best conditions in which to store acetate film becomes important for archive purposes. Safety film has some very valuable advantages for our purpose. Apart from the fact that it presents no extraordinary fire risk, its inherently superior chemical stability means that unlike nitrate it will not destroy itself within a short term of years. On the contrary, it is anticipated that it will have a useful life comparable with that of good rag paper, which may amount to hundreds of years. This is important because it will not be necessary to duplicate the film so frequently, and it is the slight degradations of quality, which are attendant upon duplicating, which, many times repeated, would eventually cause the image to deteriorate to the point where it would be useless. Its long life also raises the interesting speculation that with currently produced film, the factor limiting life will not be the base. It may be found that the limit of life will be imposed by the gelatin, which during the regime of nitrate base has been considered quite adequately permanent. Since, until recently, there has been only very little safety film, there is not the same body of actual experience in its preservation as is the case with nitrate; but some of the effects of unsuitable conditions are known. If stored in too high relative humidity the plasticizers are liable to separate from the base in crystalline form. If kept in too dry conditions it will become extremely brittle due to evaporation of plasticizers and moisture. There is agreement among investigators, that a relative humidity of 20 per cent, is a critical minimum. These investigators recommend storing in a relative humidity of the order of 50 — 55 per cent.,J It is impossible without air conditioning to maintain constantly the required relative humidity throughout the vaults. It is therefore proposed to condition the film in a small chamber in which the required temperature and humidity would be maintained. If the film has an excess of moisture it will lose it; if it is too dry it will absorb moisture. When it reaches the condition that it is neither losing nor absorbing, it is said to be in equilibrium. The film is then to be sealed in its can and placed in the main store. The most convenient way of sealing the cans is by means of a self adhesive tape. Such a tape requires three properties: (i) it must be resistant to the passage of vapour; (ii) the tape must be sufficiently mouldable to wrap round the ridges of the can to make a close seal; and (iii) the adhesive must retain its properties for a number of years. It is also desirable, for reasons of convenience and economy that the tape should be able to be stripped off the tin without leaving any adhesive behind, and capable of being re-applied. Tests for Water Vapour Permeability. At the request of the National Film Library a number of tapes of various materials were tested by the Chemical Inspectorate of the Ministry of Supply for water vapour permeability. Cellulose, paper and loose woven cloth tapes were all considered to be too permeable. The most satisfactory in this respect were found to be certain plastic