Keep your Cool - and go easy on the
Vinegar
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By John pearle (Area 13) March 2001
Sounds like a slogan from a healthy eating advertisement, it is, in a way. Not your health but the health of your films.Do you ever think about film? Of course you do, we all do don't we? But wait, what we are usually thinking about is really a mixture of fancy chemicals and beef extract, spread thinly on - you guessed it - the film. Much is written about keeping our pictures safe from the ravages of time. The light fastness and permanence of the dyes produced by different colour processes; the merits of toning black and white images; archival storage of negatives. Home processors are concerned about adequate fixing and effective washing. The humble film base gets scarcely a mention, we behave as if it were a permanent, dependable support for our priceless work. If we print our pictures we worry about the acidity of the paper, is RC or plastic better than fibre, will it fade in the light, should black and white prints be toned and so on. Those who live in the world of bits think about the compatibility of printer ink and paper whilst people like me who think that digital means counting on your fingers are happy if we can keep the mushrooms from growing on our slides. But don't forget the film...
Film itself is a thin sheet of plastic. This doesn't really tell us anything, there are thousands of materials we call "plastic", but nearly all those that are now used for film base are of two basic types. Polyester and acetate. The first is not the villain of this piece, so we will keep it until later. The villain is vinegar, or if you want to impress the others in the queue in the chip shop, acetic acid.
Acetate film base is cellulose acetate, or a similar compound. This is made by treating cellulose, probably from trees, with acetic acid, probably not from wine gone wrong but it could be. Hence cellulose acetate. This will react with water, which pulls off the acetate as acetic acid. The humidity in the air is enough to do this, albeit slowly. The fun starts when a small amount of acid is present, this acts as a catalyst for the reaction. The Ilfochrome (neé Cibachrome) printers among you will know what a catalyst is, but put simply, it is something which accelerates a chemical reaction without being consumed itself.We now have the interesting situation where a chemical reaction produces its own catalyst. The fancy name is an autocatalytic reaction. A tiny bit of acetate breaks down, forming a tiny bit of acetic acid, this helps the next acetate to break down, releasing another bit of acid. Now there are two so the next acetate goes in half the time... and so on. It literally self destructs, and it goes faster and faster. While this is going on there are other reactions taking place in which the acid chops the long molecules (strictly speaking, polymers) of plastic into smaller and smaller pieces. The acetic acid, as you might expect, gives a nice vinegary smell but its warning is late, print your pictures now.
The result of all this is that your films look perfect for a long time, decades maybe, then all of a sudden.... The removal of the acetate, as acetic acid, causes the film base to shrink. Eventually by 10% or 15%. The gelatin emulsion doesn't, so the bond between the emulsion and the base gives way leaving "tunnels" between the two. A film with gelatin on the back will be affected there, too. The base itself becomes brittle, because those nice long chains have been cut into smaller pieces. The plasticisers (compounds added to a plastic to give it special properties, such as flexibility) now start to come out of the base as oily liquids or, sometimes, feathery crystals. In the case of a chromogenic colour material (for us that means E6 and C41) the acid will also hasten the fading of the yellow dye.How fast does this happen? you ask, looking at your unblemished negatives and pristine slides. The home processors among you will realise that it depends on temperature and, in this case, humidity. According to the experts at the Image Permanence Institute in Rochester (the one in New York, not Kent, more about them at the end), film stored at 21°C and 50%RH will reach the autocatalytic point - where the reaction really takes off - in about 40 years, at 21°C and 80%RH it's about 17 years. The hotter and the wetter, the shorter.
A while back we mentioned polyester, and implied that this material does not cause a problem. In theory, polyester (polyethylene terephthalate - the stuff that fizzy drinks bottles are made from, hence the "PET" recycling code) does the same sort of thing, producing ethylene glycol (anti-freeze) and terephthalic acid. The difference is that the reaction runs five to ten times slower, so no real problem there. Polyester goes under the trade names "Estar" and "Mylar" and, no doubt, others, so why don't film makers all use this instead? Some do, we've all seen "Estar" on Kodak data sheets, but there are drawbacks. If you process your own 35mm films you are bound to have come across films that have been tightly rolled up in a cassette for a while. They are like steel springs and it can be quite a problem to load them into a spiral. The plastic "flows" and takes up the shape it's been coiled up into. Paradoxically, acetate does this more readily than polyester, but if you have noticed, when you take your incredibly curly film off the spiral and hang it up to dry... hey presto, nice and flat. Well nearly, certainly much better that it was when it was dry. This is because acetate film absorbs a few percent by weight of water, this sort of "lubricates" the long chains of molecules and allows the plastic to unwind itself. Polyester, on the other hand, only absorbs a tiny amount of water and comes offf the spiral as curly as it went on. Quite a job to handle and more difficult to print. There are also a couple of problems for the film manufacturer, polyester is more expensive and also takes the edge off cutting and punching machinery more quickly.So, what to do. Maybe nothing, it depends if you want your films to last. Black and white images have traditionally been stored as negatives so archivists have a serious problem. The motion picture people have all their work on film and have spent a fortune on cold storage to combat what they call "vinegar syndrome". Copying images onto polyester material or turning them into some digital format is not really practicable for the huge numbers of images existing. Careful storage seems the most practical way to preserve the film. Keep it as cold and as dry as you can. If you are responsible for a collection this means colder than 7°C and between 20% and 30%RH. Film stored in tightly closed containers - like motion picture film - would be at higher risk than slides stored in open racks in a box. Another point to note is that poor storage counts for more than good storage. By this I mean that as conditions vary throughout the year the temperature of the stored film changes. Even a short period of high temperature during the summer will have a significant effect on the life of the film.
As we (nearly) said at the beginning, keep 'em cool, dry and go easy on the vinegar.
It may be possible to use polyester material for new work, but as yet there are few 35mm or roll films available on this material, especially ordinary "still" films. Of course, there's always glass... There is a new material, polyethylene naphthalate, coming into use for motion picture films.
It is tempting for the computer experts to say they have the answer, but the technology changes fast. We have gone from floppy disks, magnetic tape, zip disks, to CDs. and DVDs with various flavours along the way. Magnetic tape has also had problems of chemical deterioration. None yet match the potential information storage density of a photographic film or plate. You have to keep copying and re-copying as things move on, I'm told that many early video recordings are no longer accessible because the equipment needed to play them is no longer available. Betamax video tapes are going the same way. There was an earlier Philips format, the name escapes me, which has probably already gone. The first magnetic recordings were done on steel tape - with brazed joints giving a whole new meaning to tape editing - I don't know if any survive but playing them now would present quite a challenge.
If you're still with me, you will remember that I promised more about the Image Permanence Institute. The Image Permanence Institute was founded in 1985 through the combined efforts and sponsorship of the Rochester Institute of Technology and the Society for Imaging Science and Technology. It is a university-based, non profit research laboratory devoted to scientific research in the preservation of visual and other forms of recorded information and is the world's largest independent laboratory with this specific scope.
This article sprang from a Usenet discussion led by Dr. Douglas Nishimura, of the Image Permanence Institute, Rochester Institute of Technology (http://www.rit.edu/ipi). Important contributions also came from Richard Knoppow and Francis A. Miniter. I am indebted to Dr. Nishimura for permission to use his material.