[Alt-photo] Split exposure of cyanotype
mike at mikeware.co.uk
Mon Jan 29 16:17:13 UTC 2018
Cyanotype exposure can be surprisingly complex:
Cyanotype (especially New) is a print-out process and is therefore self-masking to some extent.
But it suffers tonal reversal (“solarization”) as the Prussian blue in the shadows is further reduced to Prussian white.
Prussian white does not absorb the incident UV as much as Prussian blue.
Consequently, with extended exposure it becomes less self-masking in the shadows.
When all the Prussian white is restored to Prussian blue (during/after wet processing) the final Dmax will be higher than that of a fully self-masking print-out process.
Now consider the effect of a “split exposure” on this:
The “interval” between exposures provides an opportunity for the Prussian white in the shadows to be re-oxidised to Prussian blue (will depend on access to the air, or dichromate in the sensitizer).
Consequently the sensitizer becomes more self-masking again, and the ultimate Dmax will be lower - as observed.
Critical test of this hypothesis: is the tonal reversal in the shadows observed to be (partially) restored during the “interval”?
“Split exposure” has to be described by three time parameters instead of simply one. I think I would find that too difficult to get my head round for practical purposes!
> On 29 Jan 2018, at 15:10, John Isner via Alt-photo-process-list <alt-photo-process-list at lists.altphotolist.org> wrote:
> My split exposure print came out too light, not too dark. I will re-try
> the split exposure with the same negative to make sure I didn't mis-read
> the exposure meter.
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