Wow John ! Oxalic acid is an old standby here in the "States"
...It has been a standby in rock shops for the removal of ferrous
stains from rough rock...
G’day; Whilst Oxalic acid is indeed poisonous, there is no need to
be over worried about it’s use; just treat it with all reasonable
care. And it is no more powerful an acid than Sparex.
As I suggested in my post, the reason it is poisonous is for the same
reason it is good at removing iron stains; it has an affinity for
iron and doesn’t really care where the iron comes from. So it mops
up the iron from blood haemoglobin and puts it in a sort of molecular
cage from which it can’t escape very easily. This ‘chelation’ causes
it to lose it’s colour too.
Perhaps I might mention here that in the old days (when I was a boy,
in fact!) ink was made with iron, and a solution of oxalic acid was
the housewife’s cure for ink stains on boy’s clothing. And fingers!
It was often referred to as ‘iron mould’. After such a use the cloth
and/or fingers would be washed in dilute washing soda, then
thoroughly rinsed under the tap.
Oxalic acid also exists in rhubarb leaves, hence all the old cook
books bore an admonition not to cook or eat the leaves, though the
stems were perfectly safe. The plant/weed ‘oxalis’ also contains the
substance - which is why you never see bugs and caterpillars eating
I have been using oxalic acid for decades and have never used
a precise acid to water ratio. I simply mix a warm water / oxalic
combination and wait about twenty-four hours and examine the
results. If the results are efffective or promising, I proceed
That’s fair enough; in many such applications of chemicals the
required strength is %$& “enough!” –
John Burgess; @John_Burgess2 of Mapua Nelson NZ