beginner in stone

[John Twilley <jtwilley@xxxxxxxxxxx>]

Sheila,

Terrazzo is a cement mixture containing coarse chips of stone
(occasionally glass) chosen to produce a contrasting color to that of
the cement in which they are mixed. The cement may be colored or white.
Usually the surface is ground down and polished to expose the chips.
Areas of different color or patterns may be divided by bronze strips set
into forms before the mix is poured.  This material used to be very
common in commercial buildings for floors but has also been used for
sculpture.

A couple of notes on the approach and materials for "restoring" the
markers that you are interested in:

"Restoration" means different things to different people.  Knowing that
all materials are affected by time and the elements, conservators
(people who treat the deterioration of historic artifacts) find it
helpful to distinguish between steps that are taken to arrest ongoing
deterioration and those that are of a more cosmetic nature.   In other
words, treatment that enhances the longevity of whatever survives up
till now might be followed by other steps intended to improve the
appearance.  They are not necessarily one and the same.

Unfortunately, the steps that might make an immediate or dramatic
improvement in appearance are often very detrimental to the stone over
the long term.  This can be the case with cleaners.  I'm not familiar
with the product that you cite.  However, you should know that may
commercial preparations are intended for fast dramatic results and may
achieve their effect by etching away the surface of the stone to expose
fresh material.  In some marbles this can be very destructive because
the individual grains of calcite lose their margins and begin to detach
 from their neighbors, leading to a roughened surface that is more prone
to soiling, biological growth (like algae and lichens - hence staining),
and erosion by  frost.

Since you started with questions regarding the recutting of letters, I
have to assume that the existing surface is heavily weathered.
Apparently a simple cleaning is not going to be sufficient at this
point.  However, since you have stated a concern about the qualities of
the old markers versus the idea of replacing them outright, I think
that  you might want to do a bit more reading up on the subject of
stone  conservation before deciding how to proceed.  There are a number
of  books on the  subject, many very technical.  However, with your
background in geology you might find them helpful.  An online library
search should turn up some things for you.  Other less technical
sources  are publications from offices like the National Trust for
Historic  Preservation (US), the Preservation Assistance division of the
Park Service (US), Historic Scotland (UK) and the referrals of the
American Institute for  Conservation (US) and  International Institute
for Conservation (UK).

Good luck.

John Twilley