Monday, September 8, 2008

A Bichromate-Mercury Iintersifier

It was while working under active service conditions that the experiments leading to the discovery of a new method of intensifying negatives .was made. Some very brilliant results were required in the way of transparencies, and the only plates in tock of the size wanted were very stale, and though labelled "Process," would not give even ordinary printing density. So the only thing to do was to make the best possible, and then clear and intensify as much as possible. Lead was tried, but owing to the lack of proper washing accommodation, bad water, and also to the strong colour it gives to the very slightest trace of veil in the whites, it did not answer in this case. After trying every method that I could to persuade the "quarter-bloke" in charge of the stores to let me have the material without the usual circumlocution, and still not getting enough density, I began to experiment, and eventually found a method of greatly increasing the density without risk of stain provided that the negative was thoroughly fixed.
The procedure finally adopted was to bleach the negative in an acidified solution of potass bichromate (as for chromium intensification), and then, after washing for a short while, immersing in a mercury-iodide solution, and after a further wash to darken the bleached image in a sulphide bath; or else in a hydroquinone developer if there were any likelihood of subsequent reducing being called for.
I found that a lot of washing, after fixing the plate, between the various baths, was not essential to clean working, and the increase in density was far greater than I have been able to get with any other intensifier except lead. If the bichromate is not all out before the negative goes into the mercury bath, it comes out into that solution, but does not seem to affect its working.
Like the mercury-ammonia intensifier (which it easily beats for density-giving power) this new method can be worked with out accurately weighed and measured solutions, but in that case it requires rather a lot of bottles. Being minus reference books or any accurate measures at the time referred to, I got on quite well without, and did not find any appreciable difference resulting from varying strengths and proportions of ingredients of solutions. The way I arranged matters was, first of all, to keep a saturated solution of potass bichromate, of which a little was diluted for use as required, and a few drops of hydrochloric acid added. If this did not bleach it was poured into a jar, and a few more drops of acid poured in. This bleaching bath does not keep, so it was thrown away after use. The mercury-iodide bath, on the other hand, keeps well in the dark-room, and can be used over and over again. As I had no formula by me, I made a fairly strong solution of each of mercury bichloride and potass iodide. Then a little of the latter was put aside, and into the remainder I poured the mercury solution a little at a time, well stirring and shaking to dissolve the red precipitate that forms when these two chemicals are mixed. A point is reached when a little of the red powder fails to re-dissolve, and it was to get this into solution that the email quantity of the potass iodide liquor was kept aside. On adding this to the bulk the precipitate disappeared. This strong solution was kept for stock, and was used diluted, but both the stock and working solutions appeared to keep well. The sulphide solution was made as required from the crystal, but there is no reason why a stock solution should not also be employed for this. The used liquor should not be kept after the same day, as in the case of bromide toning. It seemed difficult to get the image thoroughly sulphide right through, so that if the density was too great some reduction was obtained by simply immersing the negative in a hypo bath. Another use al point with this intensifier, as in some others, is that if the plate before sulphiding is seen to be too dense or the lines are veiled, a dip in hypo solution will clear it. Of course, this means another good wash before sulphiding, and it should be pointed out that these extreme methods of working are seldom suitable for anything but line work, as the uneven nesses of the emulsion are usually very much accentuated by employing strong measures.


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