From: Sil Horwitz (silh@earthlink.net)
Date: 01/18/01-09:23:25 PM Z
At 2001/01/18 07:20 PM -0700, Christina Z. Anderson wrote:
> Question: what is the difference between sodium carbonate and sodium
>bicarbonate, common baking soda; are both interchangeable? Is sodium
>carbonate more caustic?
Baking soda IS sodium bicarbonate, which is only slightly alkaline. Sodium
carbonate hydrolyzes in water in a reversable reaction creating sodium
hydroxide (lye) and carbonic acid (a very weak acid), so it is very
alkaline. (The term "caustic" is used in many contexts, but actually means
"to burn the skin" so even silver nitrate, not an alkali, is very caustic,
and at one time was used to burn off skin growths.)
Modern films are hardened during manufacture, and very difficult to
reticulate. At one time, before films were so tough, reticulation was
accomplished by alternating hot and ice water soaks. In fact, it often
happened when the photographer didn't want it, which is why modern films
were created to be so tough.
Sil Horwitz, FPSA
Technical Editor, PSA Journal
teched@psa-photo.org
silh@earthlink.net
Visit http://www.psa-photo.org/
Personal page: http://home.earthlink.net/~silh/
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