Carving Raised Letting in Georgia Marble

["Simon Brown" <moonsong@zzzzzzzzzzzzzzz>]

Norman,

I'm no expert, but I'll have a go at answering. This is the way I see it:

The abrasive blasting medium is resisted by thick rubberised stencil tape,
which is cut to a customised design. The tape is treated as a single sheet,
usually 600mm wide and almost limitless length. The design is cut through
the adhesive-backed body of the tape, leaving a backing sheet intact, which
holds the entire design together during cutting.

A sheet of low-adhesion tape (application tape) is applied to the face of
the design to hold it together so the backing sheet can be removed, exposing
the sticky side. The stencil is then applied to the stone surface. The
surface of the stone can be pre-sprayed with a fine mist of soapy water to
delay adhesion until the stencil is accurately located.

The stencil can then be weeded, after the application tape is removed. That
is, the parts that are to be etched are peeled off. Of course, the design
would be planned beforehand so that it would be a positive or negative
etching. To create raised lettering, the stencil background would be
removed, and for incised lettering, the stencil letters are removed.

The stone is now ready for blasting. If the corners were meant to be
rounded, the letters would have been cut this way in the stencil. If the
abrasive-blasting operator is not careful, sharp detail can be lost through
the corners of the stencil tape curling up or being undercut.

The letters are not stuck on individually, because it would be too
time-consuming to line them up. Yes, you can buy the letters pre-cut, but
they come as a single-sheet stencil that you have to apply and weed
yourself.

I believe that gypsum (Calcium Sulphate) and some limestones may chemically
attack marble (calcium carbonate) and some sandstones. The calcium sulphate
in solution dissolves the calcium carbonate which makes up or holds together
the stone. This is why alternate bands of limestone (containing some CaSO4)
and sandstone (containing some CaCO3) in wall structures leads to decay due
to an acidic solution washing over the wall face every time it rains. There
is some illustration of this here: http://www.conservatrix.co.uk/ (scroll
down to 'Thursday, 25th March 2002' and click to see the picture). The
necessary factor is migrating fluids, so this wouldn't be an issue for
plaster-fixing marble panels indoors out of the rain. There are other
considerations peculiar to the situation such as the concentration of salts
and the resistance of the calcareous material, as well as the quantity of
water, drying rates etc.

Simon