>Pete, I'd hesitate about recommending 1% sulphuric acid on
a) health and safety grounds
b) it performed only marginally better
c) acids are bad news for paper<
I am not recommending anything Philip just reporting what happened when I
carried out a series of practical tests. However I will respond to your
comments
> health and safety grounds<
Firstly the 1%sulphuric acid is very dilute, I have personally used it for
the past ten years without ill effect to myself or my work,which includes
some paper based direct carbon printmaking. The health and safety issue is
important , but lets not, let the tail wag the dog.I reported to the list
as it includes a large number of educationlist's who are more than aware
of the risks involved in handling strong acids, indeed we are not dealing
with children in this respect , in fact as a schoolboy I remember being
taught at school the dangers of sulphuric acid and how to mix it safely
with water.I think your comments in this respect are what Terry King calls
being nannied.
> acids are bad news for paper<
Next this chemical has been used for a considerable length of time in other
photo processes such as chemical reversal the acid permangernate and
dichromate both use it as part of the technique.How I first came across it
was when I followed up a reference in Clerc's theory and practice, which
states in the Pigment Prints from Silver chapter 699.Bleaching the image
describing HJPVenn 1923 bromoil process :- the bleached print is washed in
several changes of water until the water is no longer coloured.It is then
placed in a very dilute bath of sulphuric acid {about1%} in which the
last traces of the image disappear.
Even recently our good klaus has described this method which he uses on
this list
Klaus Pollmeier Sat, 20 Jan 1996 Wrote
My way of using the KENTMERE Document Art paper (marketed in the US under
Luminos SW-Art) for bromoil:
10) Soak in diluted sulphuric acid (1-5%) for 1-5 min. This acid bath is
not used to let the last traces of the image disappear, but to bring up the
relief. The more concentrated the acid and the longer the soaking, the
higher the relief
will be. (Never pour water into the acid, but acid into the water!)
11) Wash 5 min. (This water must not be colder than the acid bath.)
So I ask the question if it is perfectly all right for bromoil ,why cant it
be used for direct carbon?
> it performed only marginally better<
This is quite true,but for an overexposed print it would perform a great
deal better providing a save it option.Giving the cleanest white it is most
useful when doing polychrome colour work allowing very clean rendition.
>Did you make any attempt to test sodium dithionite? Although since
suggesting it I was told it's been tried with EDTA for clearing stained
platinum prints and removed some of the paper fillers. This may not,
however, be a problem with a pure cotton paper.<
No Philip but I intend too, it is just a case of getting hold of it
> A series of doubling
>exposures were given to the three substrates in the classic
>photographic manner. with each colloid, in turn. ie
>1+1+2+4+8+16 mins Nine test's in all. The maximum exposure
>being 32 mins.
>In that case you'd be better off saying 16+8+4 etc. Because of the
"continuating" reaction in dichromated colloids, the area of the test strip
to be exposed least needs to be exposed last.<
Yes I will do it your way next time and see if there is any difference
>I think one of the most interesting thing to come out of these tests is the
>surprising long tonal scale possible with just dichromate alone,
>But as Judy has said you really need to go on to test whether the resulting
>image is archival. The oldest pure dichromate image I have has gone from
>brown to green and is now very faint.
Could you give me some more info on this, like how old is the image, has it
been exposed to the atmosphere, was it washed at all, and if it was for how
long.
Judy's point is well taken, I will need to test archival permanence somehow
there is of course another possibility if the stain image is impermanent
perhaps it could be stabilised by toning or some other after treatment if
we can do it for silver why not chromium oxide.
my best wishes
pete