Bleaching agents and masonry

Bleaching agents and masonry: calcium hypochlorite vs. sodium
hypochlorite

In much of the conservation literature I've run across that
describes the cleaning of masonry, one of the most frequently
mentioned bleaching agents is calcium hypochlorite. Sodium
hypochlorite is rarely, if ever mentioned.

The action of both compounds appears to be similar, but with
slightly different salt end products: CaCl2 vs. NaCl--neither of
which are desirable in masonry (I would argue that calcium chloride
is more insidious that sodium chloride). Also, apparently there is a
pH difference as sodium hypochlorite is in the range of 9.75-10-50
while calcium hypochlorite is rather more basic at 12.59-13-11 (at
least in concentrated solutions).

Why use one over the other? Is it an issue more of convenience
(powder vs. liquid)? Or is calcium hypochlorite a more gentle
bleaching agent? And if it is more gentle, couldn't you simply make
a more dilute solution of sodium hypochlorite?

Also, pool disinfectant seems to be one of the more frequently used
sources for calcium hypochlorite. Am I correct in understanding that
the powder bleach that you get in the grocery store is also calcium
hypochlorite? Is there a reason why pool disinfectant is superior?

Jeremy


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                  Conservation DistList Instance 17:46
                Distributed: Tuesday, December 16, 2003
                       Message Id: cdl-17-46-020
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