Limestone

Jim Mann <jmann [at] amdel__com> writes

>Fran Gale <fgale [at] prosoco__com> writes
>
>>This question is posted on behalf a Mary Weisert, a sculptor in the
>>Midwest who is currently having difficulties working with a local
>>limestone that contains clay minerals and fractures easily.
>>
>>    How do I repair or keep it from fracturing more, a hairline
>>    fracture in Cottonwood limestone. I am using hand tools and have
>>    encountered several losses of integrity. any assistance would be
>>    appreciated.
>
>The application of an ethyl silicate consolidant such as Wacker
>OH100 may prove useful in strengthening the stone especially in
>reducing the tendency to crack along bedding planes.  The
>consolidant deposits silica that binds the particles of the stone.
>The method of application will depend very much on the properties
>and composition of the limestone especially the stone's absorption
>characteristics.   The use of the consolidant may produce a slight
>darkening such as a "damp appearance" but this may be minimised
>after the surface has been worked with a chisel and allowed to age.
>The suitability of a consolidant also depends on what the intended
>use for the sculpture and its environment is as localised changes in
>properties of the stone may lead to variations in weathering
>characteristics down the track.

When I taught stone carving in the south of France in my previous
life as a sculptor I learned the technique that was traditionally
applied to harden the very soft limestone there to both allow for
finer finishing and provide a waterproofing characteristic to reduce
the growth of biota on the surfaces over time.  They would apply
silicate de soude (potassium silicate I believe) in an aqueous
solution that you could purchase from the drugstore.  It was
remarkably effective at hardening the surface through repeated
applications until the fresh cut stone that could be scored with
your fingernail would cause a rasp to run off the surface like it
was riding on glass.  Potassium silicate is the binder used in
mineral paints like those marketed by Keim.  I wonder if Fran would
help us understand the difference between ethyl silicates (as used
in Wacker and Conservare products) and potassium silicates in terms
of optimum uses as consolidants, binders, surface treatments.

Mark Rabinowitz


                                  ***
                  Conservation DistList Instance 16:10
                 Distributed: Wednesday, July 31, 2002
                       Message Id: cdl-16-10-004
                                  ***