Hello Ross,

Since I have not noticed any yellowing problems after bleaching tens of
thousands of shells, I am led to suspect that the yellowing you observed
might be due more to the water you dilute the bleach with, than to the
actual bleach.  If your water contains a lot of iron, iron atoms can
actually replace calcium atoms in the shell.  Shells found on shipwrecks
where iron is present, or on pilings under large iron bridges, are often
stained yellowish to brown to dark red-brown, depending on the amount of
iron present.  Perhaps this could account for the yellowing you have
seen(?)  If so, this could be prevented by purchasing distilled water,
and using that to dilute the bleach.

Paul M.