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Herschel's Ageing Test

Herschel's Ageing Test

The survival of Sir John Herschel's photographic experiments and of some specimens among them with excellent images is improbable by any standards. It is an accidental experiment in durability. The very difficulty of arresting the chemical changes on which the formation of images depended was one of the challenges Herschel's research was addressing. The chemical 'susceptibility' (Herschel's word) that created the images also destroyed them; if well fixed a photograph fades in light, if badly fixed it darkens. Even at its most sophisticated, photographic chemistry had that inherent instability. Some of Herschel's most interesting inventions, such as phytotypes and amphitypes (see narratives to 32274 and 34240 respectively), refused to be fixed at all and disappeared even when kept in darkness. Yet specimens not in fact 'fixed', even of these recalcitrent processes, and specimens from his long series of fixing experiments, by definition badly fixed or fixed by methods that Herschel tried and rejected, have in fact in some cases retained their images. (Some of course have not.)

One experiment that Herschel commenced however was specifically related to the effect of time rather than light on photographic chemistry - or anyway to the chemistry's inherent behaviour over time when few other significant factors were in play. In effect, it was an experimental control, the treated papers revealing their natural darkening or discolouring characteristics without exposure or processing, for comparison to exposed results. This experiment was commenced in September 1839, when he put away in a brown paper wrapper (packet 15 in the Museum's numbering, 94276) a quantity of 'Numbered Specimens. Reserved for trying effect of time - in darkness'. These are prepared papers (treated with sensitising chemical solutions). What Herschel meant by 'numbered' is that each chemical mixture he made up had a number, and all the papers treated with it were marked with that number. One of each was put away in this packet, and the rest were used in his experiments.

He obviously revisited the packet, making a note on one of the specimens (32736) to the effect that it had 'So changed' in 12 hours. We cannot know what 'so' was, but the specimen now (in 2010) exhibits the startling characteristic of being a dull mauve colour on the treated side, and olive green on the back. While the mauve is a more normal and predictable colour for early photographic chemistry, the incidental and sometimes unexpected manifestation of other colours is partly what inspired the later phase of Herschel's experiments, in 1841-43, when he explored sensitising and dyeing agents beyond the conventional silver salts, inventing the blue cyanotype and purple chrysotype as well as the phytotype using juices extracted from flower petals. In all probability many of the ageing test specimens discoloured in quite a short time, and reached a point of natural arrestment or stabilisation, after which the chemistry remaining in the papers becomes relatively inert. This is also perhaps the reason why, elsewhere in the collection, unfixed or unfixable images that Herschel will have thought stood no chance of surviving can, in some cases, still be seen.

The effects Herschel wished to monitor (and commented upon in his published accounts) were not confined to the inherent behaviour of his numbered solutions. The paper itself was found to play a significant part in the chemical and photo-chemical processes, and different types or makes of paper affected the colouring of the end product (see narrative to 51764). Size or bleach used in the manufacture of the paper was a relevant variable; and the photographic experimenter's preparatory treatment (washing in sodium chloride before sensitising was usual) could be varied with dramatic results. It was important to make observations of these kinds of effects not only under the conditions of exposure and use intended, but under neutral control conditions.

What of course is most peculiar about Herschel's ageing test, perhaps accidentally, is that it has been left to continue, long after he ceased his photographic researches and long after he ceased his mortal existence. There are 69 papers taking part in this interesting experiment (Schultze reported 70, but counting them can be difficult because some are interleaved with pieces of scrap paper, merely as separators, and these have often become discoloured through the effect of adjacent chemicals; there are 33 such spacers). Schultze recorded on the basis of his study of the collection in 1963-64 that 'they now look mauve, dark or light brown, green, black, etc. - not one has remained white or off-white'. They remain essentially as he described them now (in 2010) after 170 years of the experiment. The fact that they do not tend to complete blackness or even brownness but to a variety of hues, not necessarily dark, is itself interesting. While dark mauve to murky brown colours predominate, some have stabilised at a much lighter colour, including a number of paler mauve or lilac specimens (such as 51409, 54188, and 78259), and individual examples showing rust red (81706), khaki green (62335), pale green to buff (67327), yellowy (99437), and a distinctive copper colour (95146).

Obviously on a number of occasions, especially from Schultze's examination of them onwards, the experiment (or at least its darkness criterion) has been interrupted. Nor are they stored in their crowded little brown-paper wrapper any longer. Their removal into the light for examination by Schultze and other scholars demonstrates in fact how inert they have become, since although it would be rash to say that none of them has darkened further as a result, no dramatic change is ever discerned in them (whereas when fresh, exposure to daylight would have perceptibly darkened them). So even in spite of the interruptions, perhaps Herschel's ageing test belongs with a more famous and nearly contemporary experiment, the Oxford dry pile - an extremely durable battery dating from 1840 - as one of the world's longest-running scientific experiments.

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