Philip Jackson said:
>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 would add:
a that the sulphuric acid is likely to soften the paper and the image as HCl
does when used as a the clearing agent in platinum printing.
b sulphuric acid,according to some authorities, will convert the gum to dextrine
c if the exposure is adequate, there is no dichromate stain, and no need for
anything other than water to clear the dichromate
>>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.
One calls to mind the interesting phenomenon of pure dichromate images produced
by Ponton in 1839 and the practical application of the phenomenon by Pouncy and
Poitevin in the 1850s. who added pigment to the mixture. Ponton's examples do
not survive but Pouncy's still have vigour to the extent that they look better
after one hundred and forty years than modern gum prints I have seen hanging on
gallery walls.
Terry King