Removing tool marks - Hydrochloric Acid

["Clive Murray-White" <cow_artclive@xxxxxxxxxxxxxxxx>]

I came across this use of Hydrochloric acid when I was making an enormous
marble sculpture for the City of Cairns in Far North Queensland, Australia.

The Cairns Marble Co mines stone from a place called Chillagoe, a huge
deposit of marble with a vast range of quality and colour, and they had made
any block in their yard available for my use.

The company pours acid on to raw stone outcrops on their lease to see what
colour the stone is, I am no chemist but it seems to work, hydrochloric acid
attacks the lime content in the stone causing it bubble and froth quite
vigorously, when this subsides it is obvious that the acid has become water,
the stone is clean looking and its true colour has been revealed.

Hydrochloric acid is most commonly used in this country by bricklayers to
clean cement off brickwork or to etch cement prior to painting, it is very
readily available in all builders supplies. It comes as a liquid in
containers ranging from 500mls to 4ltrs.

Yes, it is a little nasty to use, it will burn your skin and if you breathe
the vapours it doesn't make you feel as if it is doing you any good at all.
I wear industrial rubber gloves, eye protection and face mask. Having
discovered the hard way that the "fumes" also attack steel quite vigorously
I only use it in the open air. As soon as you open the container heavy
yellowish fumes seem to be produced!

I tend to use it undiluted most of the time but it can be diluted to suit
the amount of bite that you require.

Once I feel that the acid has done its stuff I wash the sculpture with clean
water, this is not absolutely necessary but the stone seems to stay wet for
ages if you don't.

Clearly it will etch any stone with a high lime content leaving it looking
clean and pretty much like it is damp eventhough it is actually quite dry.
It can leave the stone with a slightly polished look but feels roughish to
the touch I sense that over time the colour deteriorates and the stone
looses some of its colour.

Whilst find it very useful I just feel that there could be some
improvements, I suppose my dream product would come as a kind of  highly
concentrated acid jelly that I could trowel onto a specific area were it
could chew away at an annoying tool mark.

I would be nice to know what the geologists and chemists think of this.

Regards Clive