>The others guys from the list should know it better than me. I would say at
>least double.
How long is a length of string :-) it depends how you use it ,most
dichromate workers use a saturated solution this is 20% to 25% for
Ammonium and 8% to 10%
for the potassium so the Ammonium is twice as fast as it has twice the
amount of salt in solution.However my own imperical tests have shown an
increase of four to five times in sensitivity using whole egg as a colloid
an increase which is quite significant
>I have also read that the potassium salt cannot be used with methyl
>alcohol, because the salt precipitates out. Acetone was recommended as
>a substitute, resulting in "an unusually quick-drying sensitiser." Can
>anyone comment on this? Is it possible that an alcohol other than
>methyl would work; in other words, could I have a chance of appearing in
>an Absolute Vodka ad? :-)
>Try it but remember tht alcool test for cops are (were) made with dichromate
>as an >indicator. The potassium dichromate reacts with alcohol
>and turn into green potassium chromates (as far as I remember). The coloe
>passes from orange to green. For that reaction to take place, you need to
>be in an acidic solution (again if I remember well).
Correct, so much for health and safety
>All of the processes which use dichromates end with instructions on
>"clearing" the image to remove a yellowish dichromate stain. The
>clearing solution given for the "dusting on" process is a mixture of
>sulphuric acid, water, and methyl alcohol. But for other processes using
>Ammonium dichromate, the recommended clearing solution is a 5% aqueous
>solution of either potassium metabisulfite or sodium bisulfite. So:
>Can anyone see any earthly reason to use sulphuric acid?
>Potassium metabisulfite is a good choice (from The gum Bichromate book of
>David Scopick) because it does not alter the colours.
There are a number of agents that will clear or more accurately, change
the colour of the dichromate stain. I did a number of tests several years
back and found that a 1% solution of sulphuric acid changes the stain to a
very light neutral grey which for most purpose is undetectable in the final
image.I am sure my old friend and associate David Scopick will not object
when I point out that Potassium metabisulfite does in fact leave it as a
slight green blue grey ,which is a very beautiful colour but can give
problems in trichromatic work
>Most important, does the clearing solution actually remove chromium from
>the image, or does it simply change it to a non-yellow form? If the
>chromium is not removed, then the clearing step would be irrelevant in
>ceramics applications.
>Yes it is removed because that salt acts as a solubilizer of potassium
>dichromate (and Ammonium dich. too)
This clearing stage would seem a bit strange in respect to the dusting on
process as you in fact dust on the unexposed emulsion which you certainly
do not wont to remove, so the clearing must just be to remove the unexposed
salt only and not touch the colloid at all, a tricky business! .What must
be remembered is that the dusting on process is a unique process and works
very differently to normal dichromated colloid methodology.
>To those of you who are still reading: Thanks. Any answers will be
>appreciated--but even more appreciated will be any questions I haven't
>thought to ask.
>
>--
>Dennis M. Southwood
>dms1@home.com
>
>
You're welcome
Pete