[Alt-photo] Split exposure of cyanotype

John Isner john.isner at gmail.com
Mon Jan 29 17:02:47 UTC 2018

Thanks!  You have given me a good start on understanding the process.

On Mon, Jan 29, 2018 at 9:17 AM, Mike Ware via Alt-photo-process-list <
alt-photo-process-list at lists.altphotolist.org> wrote:

> 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!
> Mike
> > On 29 Jan 2018, at 15:10, John Isner via Alt-photo-process-list <
> alt-photo-process-list at lists.altphotolist.org> wrote:
> >
> > Loris,
> > 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.
> > Thanks,
> > John
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