Francesco Curcio (msepa.fcurcio@ntt.it)
Thu, 18 Feb 1999 22:31:19 +0100
Jewelia,
the latest issue of the British magazine Ag+ Photographic (#14) features an interesting article
(to be continued in the next issue) on ascorbic acid as a developer by Michael Maunder :
"Vital developers".
He is a photo chemist as well as having been an amateur astronomer for many years,
and the article gives some hints on the hypersensitization (pre-exposure treatment) of plates
to limit the great increase of exposure times accompanied by unpredictable reciprocity failure
in astronomic photography. Maunder proposes a different method to achieve the same result
in a simpler way, by latensificaton (post-exposure treatment) with a prebath as follows (I'm
quoting):
"The results were based on Technical Pan developed in D-19, but apply to any film
suffering from underexposure. Here they are:
1) A simple pre-bath of 1% sodium sulphite recovered (equalled) a full stpo film speed.
This is such a simple option that it ought to be a routine regardless as it does
no harm and has that benefit.
2) The optimum prebath was
Sodium metabisulphite 5 g
Sodium sulphite 10 g
Sodium isoascorbate 5 g dissolved in 1 litre
3) Substitution of the same concentration (0,5%) domestic hydrogen peroxide
solution for the sodium isoascorbate also worked well.
"This optimum prebath of latensification formula 2 recovered virtually all the reciprocity
failure on the Technical Pan trials being done at that time and were about 2 stops in total,
a most impressive feat. Much simpler than baking in hydrogen gas.
"The sodium salt of ascorbic acid [L-ascorbate, the more common form: different
from iso- or D- ascorbate as requested by Maunder's formula] was not quite as effective.
"It should be noted that formula 2 is a buffered mixture and requires the sodium salt
of the substance to achieve this. Commercial ascorbic acid as vitamin C could be used
but this becomes a messy and dangerous procedure with sulphur dioxide gas
given off during the mixing and is too acid without a lot of rejigging thr sulphite levels."
End of quotation.
The speed gain Maunder speaks of is up to two stops, a bit less than the
"factor of about ten" Lumicon gives (to be applied, if I understand well,
to the film speed: from, say, ISO 100 to ISO 1000? more than three stops).
He also stresses this is not a real speed gain, but a recovery of speed
loss in long exposures.
hope this helps
Francesco Curcio
Milano Italy
jewelia wrote:
>
> one of the first things i learned about this list was ebay.com -- i haven't
> bought anything yet cause i suppose nobody has offered to sell my color of
> lipstick: however, i did notice this thing called a hypersentizing system on
> the auction block--i had no idea what it is and thinking it might be made of
> latex i clicked on it out of curiosity. turns out the seller was good
> enough to list the web site of the maker lumicon.com. the gist of it is
> interesting--the idea is to treat chemically to raise its speed--whether
> color or B&W. apparently, the target market is astronomy--or was it
> astrology--i always get these mixed up! it is supposed to raise film speed
> by a factor of about 10. i looked at the systems--you can buy systems to do
> it yourself or you can buy film--the company sells mostly optics--scopes and
> telescopes and apparently a major customer for this stuff is astronomy.
>
> they don't have a system big enough for 8x10 so i sent off an e-mail asking
> if i could build my own and buy their chemicals if i wanted to do it--from a
> technical standpoint--they wrote back today and said something to this
> effect: you need a vessel that can maintain a vaccuum of 27-30" or 15 psi
> (i know it seems confusing to me--but i am guessing both not at the same
> time) and can be heated to 50 degrees (i assume this is centigrade) .for
> several days best to use a rack to hold the sheets seperate. i asked them
> what the effect would be on film grain and resolution and their response was
> none that it pretty much only effects reciprocity--sounds to good to be
> true? actually reciprocity and i are good friends and get along very well
> but i was wondering if there are any astrophysicists or other lunars out
> there who have any experience with this? have i been in the dark room by
> myself too long?
>
> regards--jewelia
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