From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 01/08/02-10:57:03 AM Z
At 08:21 PM 01/07/2002 -0800, you wrote:
>Not to quibble here but it would be better to say that 'formalin' is
>'formaldehyde' in the sense that 'formalin' 37% 'formaldehyde.'
>
>Chrome alum can certainly be used as a hardener and it is used in some stop
>bath formulae. Since Chromium is a known hazard . . here you might think of
>the intensifier Mercuric Chloride to get an idea of the hazard of metals . .
>. you might consider using Potassium Alum instead.
>
>I don't know of any specific advantages of one over the other but since the
>1960's (when I worked first w/color), formalin is used in many of the color
>processes. Today, for instance, it is still used in the E6 process. However,
>due to advanced chemistry, it is cleverly 'locked' in the Pre-Bleach rather
>than in the Final Rinse so it won't "escape" into the atmosphere.
>
>Formaldehyde is smart to use thought to harden emulsions for higher
>temperature processing.
>
>Jack Fulton
>
>
>
>> << My question is, what are the relative advantages of formaldehyde versus
>> formalin versus chrome alum? >>
>>
>> Clay,
>>
>> Formaldehyde is formalin. Its hardening is stronger than chrome alum and is
>> instanteneous, but both can be used as I have used even food alum (the
>> weakest).
>>
>>
>> Dave S
>>
>
Formaldehyde produces the greatest hardening of the three, next comes
chrome alum and then plain alum.
Chrome alum (potassium chromium sulfate) must operate a fairly low pH to
harden. There are fixer formulas using it. Mostly chrome alum stop baths
and fixers were used for tropical processing and in some motion picture
work. Its quite sensitive to pH so fixing baths must be checked for pH
frequently. Partially exhausted chrome alum fixing baths, and maybe the
stop bath also, leave a green stain in the emulsion which can't be removed
easily. For this reason chrome alum is not often used for prints.
Chrome alum was, and maybe still is, used in emulsion making.
An interesting thing about chrome alum is that is does not seem to mordant
hypo to the emulsion the way that alum (potassium aluminum sulfate)
hardener does. The mordanting effect of alum happens over a fairly narrow
range. It can be eliminated without loosing the hardening action by the use
of a neutral wash aid like Kodak Hypo Clearing Agent. The washing rate of
chrome alum fixed film is nearly the same as film fixed in a non-hardening
bath.
Formalin hardeners are used in emulsion making, for very high temperature
processing, and for use with auxilliary treatments, such as some
intensifiers, which cause extreme emulsion softening and swelling.
Chrome alum is a valance 6 chrome compound all of which are carcinogens.
I think formaldehyde is also but check the MSDS for details.
BTW, a very good source of MSDS is http://hazard.com which also answers
to http://www.siri.org.
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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