British Kinematography (1952)

102 BRITISH KINEMATOGRAPHY Vol. 20, No. 5 means that any nearby film will also become involved, unless it is effectively insulated. Nature of Decomposition. Quite a small degree of heat will cause nitrate film to decompose rapidly, but further than this, the film at normal temperature decomposes slowly. In other words, it is an inherently unstable compound, and in this lies the most serious problem which faces any archive. Emphasis must be laid on the fact that film consists of grains of silver suspended in gelatin, and more commonly known as " the emulsion "; and the support or " base " on which it is carried. The emulsion is recognised to have very long lasting characteristics if it is properly processed.1 It is the nitro-cellulose base which decomposes. More specifically what happens is this:— From the time of its manufacture, the base begins to disintegrate, to break down into the chemically simpler substances of which it is composed, and gases, mostly oxides of nitrogen are given off.3 At first this occurs very slowly and in normal use the vapours are able to escape. Naturally, passing film through projectors, or printers, and rewinding are all a form of ventilation. Later the rate of production of gases increases, and probably the film is left untouched in its can for some time, so that in addition to an increasing rate of production, there is less opportunity for gases to escape. The gases eventually enter into combination with the gelatin of the emulsion, forming; a substance which possesses a great affinity for moisture. It will readily absorb moisture which is normally present in the atmosphere, and then it comes to a sticky state. This becomes more and more strongly acid, and attacks the silver image, which fades completely. The age which a film attains before reaching this state varies, and a number of factors seem to influence its life. Detail of manufacturing process is one. Stock produced by certain manufacturers at about certain dates seems to reach this condition while film of other manufacture at the same date is still good. I have mentioned that projection of film provides ventilation, and we have observed that films which have been packed in tins and never used are found to have come to the end of their lives before other copies of the same film, which have been much used. Conditions of temperature and humidity in which the film is stored are important factors. A serious practical fact is that at the time the film becomes sticky the base itself shows no change. It is still clear, colourless and reasonably supple. Not until the emulsion is sticky and the picture destroyed does the film look different or feel different. It might seem that if all the vapours produced were able to immediately escape, or if the film were kept in dry condition, that the problem would be evaded, but no satisfactory solution is possible on these lines; indeed we might be in a still worse position, for the sticky condition is not the last stage of decomposition. Subsequently, the film base, which at the onset of the " sticky " stage is physically and optically in good condition, proceeds to deteriorate to a brittle and friable state, becoming an even greater fire hazard, since in this condition it may ignite at a temperature as low as 106°F.4 It may be interesting to note that, while in the sticky condition, film is sometimes quite difficult to burn. It is well known that uncoated film burns more readily than that which carries an emulsion; the emulsion tending to inhibit the combustion. When the emulsion is heavily charged with moisture, than inhibition seems to be still greater It is possible to avoid the emulsion becoming sticky by keeping the film in very dry conditions. A sticky film placed in a dry atmosphere will lose its moisture and the emulsion become firm again, but a film kept in dry conditions becomes so brittle as to be almost impossible to handle. I will take this opportunity to make quite clear the distinction between the sticky condition of the reel and another cause of damage, because the results of the two look very much alike at first glance. This reel has been ruined because it has been in contact with water. I have just taken it from a