natural unfinished stone bases (acid etching)

[Don Dougan <dondougan@zzzzzzzz>]

Dulce;

I use muriatic acid (also known as hydrochloric acid) quite often on my
bases and sculpture -- the dilution varies depending on the surface I
desire.  It is the same type of acid that your stomach produces (though
in lesser strength).
Below are some general "Rules-of-Thumb" for working with acid:

Precautions of working with acid would include NOT working on a rainy day
(the acid fumes sit in the air and do not disperse).  Others would
include safety glasses (goggles) or at least a handy source of running
water to rinse with (believe me, splashes of acid in the eyes is NOT a
pleasant experience!).  If your skin is sensitive wear rubber gloves, if
not, rinse the skin after contact with the acid (if you have any minor
cuts you will know very quickly when the acid is on you -- it stings like
****!).

I usually use the acid straight from the store-bought container when
cleaning the stone.  The acid I buy is sold by the gallon (4-gallons per
case) at home-center/building-supply stores for cleaning-up mortar spills
on brickwork.  It is less than $4 per gallon in the state of Georgia,
USA.  Diluted to a 20% solution (recommended for cleaning concrete) the
acid will work more slowly.

To prevent 'runs' on the surface, wet the stone with water first.  I use
a squirt bottle (like for ketchup or mustard in a diner) to apply the
acid, and a natural bristle brush to wipe the surface while the acid
foams.  Heavy scrub brushes with coarse bristles for very dirty stones,
and several applications/rinses of acid/water.  I do it outside my studio
(where I carve when weather is good) and the acid is quickly neutralized
by the alkaline marble/limestone dust in the soil.  If your acid-working
area is not alkali-rich, you might want to keep some baking-soda handy to
neutralize the acid.

The acid is applied prior to any further finishing (sanding, specific
final tooling for texture, etc.) so as to prevent discoloration or
surface degradation of the 'finished' surface.  For particular problem
areas I might also apply the acid full-strength with a natural-bristle
paintbrush (reserved and marked for use with acid) to those specific
areas, brushing it on as many times as needed.  I re-apply the acid when
the vigorous foaming action from one brushful has subsided to little
dispersed bubbles.

Different calcium-carbonate stones (marbles and limestones) are affected
by the acid differently depending on their structure.  Most of these
stones will be cleaned (etched) by the action -- the veins with different
proportions of calcium-carbonate than the matrix will etch either faster
or less quickly than the surrounding stone giving interesting textures.
Sandstones, serpentines, granites, soapstones and alabasters will not be
affected by acid to much extent  - if at all (depending on the impurities
present).   Sometimes I re-work the surfaces with abrasives or tools
after acid-work.

For conservation or restoration/cleaning:  Acid should NOT be used on
calcium-carbonate stones where a finished surface is already present -
even if dirty - and unless you want to refinish the surface as was done
originally.  The acid will erode the finished surface whether it was
originally polished or textured, and that surface cannot be regained
without the full 9-yard polishing or tooling treatment.

Also, if you like shiny metal do not store the opened container of acid
indoors - the fumes WILL escape (even through the closed lid) at a minute
rate and cause any and all rustable steel or iron surfaces/tools within a
radius of several yards to develop a nice, even coating of RUST !

Good Carving to You,
Don
http://www.dondougan.com