Liam Lawless (lawless@vignette.freeserve.co.uk)
Fri, 21 May 1999 16:11:06 +0100
Hi,
Didn't have time earlier for a full reply. Clerc gives a good explanation
of how mercury works, but in case you haven't got it, bleaching with
mercuric chloride changes silver into a double mercurous-silver chloride
(200 parts of mercury are added to every 108 parts of silver that you
started with). With a darkener such as ammonia, the silver is dissolved
while the mercury turns black, whereas with sulphite the silver and mercury
are both reduced to the metallic state, but some of the silver is dissolved.
Developers (without a high sulphite content - hence my earlier
recommendation of PRINT developer) avoid this loss. Sod. sulphide gives
silver and mercury sulphides, and is probably the most permanent for
intensifying with mercury.
Mercuric iodide works directly to change the silver into silver iodide, at
the same time depositing metallic mercury on it. The process can be stopped
by washing at any desired point. Images become yellowish in the presence of
moisture, possibly due to the formation of a complex of mercuric oxide and
silver iodide, but they can be stabilised by a developer which reduces the
silver iodide to metallic silver, or with sod. sulphide which changes both
metals to sulphides.
Sounds to me like your intensifier is of the direct (mercuric iodide) type,
but made up with mercuric chloride rather than iodide. No harm in that, but
my guess is that it does not contain enough pot. iodide, or something else
which would stop the m. iodide precipitating out has been omitted. A recipe
from Clerc is:
Pot. iodide 20g
Mercuric iodide 20g
Hypo 20g
Water to make 1000ml
In Clerc's time, I'm sure he'd have used crystalline hypo; the equivalent
amount for anhydrous is (I think!) 12.5g. I've used it with 20g cryst., and
found it fine.
Don't exactly remember now, but I think you'll find a thiocarbamide sepia
toning solution can be used instead of sod. sulphide.
Liam
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:39:34