Removing tool marks - Hydrochloric Acid

[Don Dougan <dondougan@xxxxxxxx>]

Hello all;
I have been using Muriatic (hydrochloric) acid as a tool to modify
surfaces and clean marble & limestone for years, and one thing I must add
to the discussion is my preferences:  I use it outdoors on days when
there is a slight breeze - I avoid using it when there is mist or rain
because the fumes just sit in the air making it hard to breathe
comfortably.  I always have a water-hose handy to rinse down the stone
(and off my skin) while working.  Unless the specific effect is desired,
I avoid applying the acid to stone surfaces that are dry -- instead
applying the acid to rinsed surfaces to prevent sharp 'edges' of the acid
etch on the surface.

I usually use the acid full-strength from the bottle (the
bottled-solution as sold for cleaning masonry) and apply several ways to
suit the job.  Note: On using methods 1 or 2 below, I am careful about
positioning the workpiece so run-off will not produce un-wanted etching
on surfaces I want to keep as smooth-sanded or natural cleavage areas.  I
also try to do the acid-etching prior to any final sanding or polishing
of the piece -- it is just too difficult or too much trouble to attempt
to control the acid!

1. Overall cleaning/etching -- by using a plastic squirt bottle lid (like
on ketchup/catsup containers in restaurants, or carpenters chalk refills
at the hardware)  that fits directly on the 1-gal. bottle of acid -
followed by scrubbing with a long-handled plastic scrub brush.  Rinsing
and repeating as necessary.

2. Spot cleaning/etching (removing specific tool marks) -- pour some in a
plastic cup and brush on with a natural bristle paint brush as many times
as is necessary (usually many times!).   The info someone shared about
making a poultice was good to know - my similar and not-too-satisfactory
solution was to wad-up a rag and place over the spot, and then saturate
the rag with acid, repeating as needed (I'll have to try the poultice
method to see how it compares).

3. Heavy overall etching for effect (depending on the size I sometimes
cut the strength of the acid by adding water to the solution -- total
immersion bath where the stone object is placed in the acid-bath within a
plastic tube of suitable size and left to soak.  The stone should be
raised on small supports to allow the acid to reach bottom as well as
top, and the bubbles are 'brushed' off the surface of the stone
periodically.
This method also works well for using  the acid to etch designs as
desired into the stone surface (masking off surrounding areas with
different materials depending upon the desired depth of etch and type of
stone being worked -- usually the mask is just paraffin wax that was
applied with a dedicated natural-bristle paintbrush while liquid, but
sometimes a layer of duct-tape, adhesive-backed vinyl Contac©-paper, or
sandblast-resist mask will be sufficient on smooth-sanded surfaces).  The
length of time I keep the stone in the bath will vary from minutes to
hours, depending on the depth-of-etch desired and the strength-of-acid
used.

On finishing the stone after using acid:  depending on the type of stone
worked (not all calcium carbonate marbles or limestones will react to the
acid the same way because of their varying crystalline structure and
impurities), the finish might be left straight from the etch, or I might
follow-up with 600-grit or finer abrasive to 'polish' the sharp gritty
areas into a less abrupt texture.  I am not trying to make the area
smooth -- retaining the visual 'roughness' but making the tactile quality
more pleasingly sensual to the hand.  This is sometimes followed by
buffing with wax or proprietary liquid marble polish like Akemi or Tenax.

Good Carving to you -

Don Dougan