Limestone

Mark Rabinowitz writes:

> 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.

The main problem here appears to be the presence of clay-rich
bedding planes within the limestone. It is likely that the clay may
be of the swelling type, responding dimensionally to moisture
infiltration.  The alumino-silicate structure of the clay-rich
planes and the calcareous structure of the limestone itself have
very different chemical and physical properties, de-stabilizing the
stone.  Deterioration sometimes manifests as delamination at the
bedding planes, sometimes like a deck of playing cards.

In a three-dimensional sculpture, there will always be areas where
the stratigraphy of the stone is exposed, essentially in cross
section, analagous to the end grain in wood.  Moisture will always
be able to access the clay component preferentially in these
locations.

The idea of using a silicate-based consolidant to bind a calcareous
porous stone is problematic, and the conservation literature
reflects a long history of frustration with this approach.  Whether
the silica is deposited through the use of ethyl silicate ( or an
alkoxysilane), or through the use of an alkaline silicate solution,
there is still a fundamental problem of compatibility.  The
conservation literature also addresses the differences in the end
product, including the nature of the silica matrix, the depth of
penetration (alkaline silicates tend to form impermeable films or
crusts, leading to moisture migration-related problems within the
stone, and are liable to form potentially harmful soluble salts to a
greater or lesser degree depending on the type used).

I believe Prosoco is still marketing an hydroxylating agent as a
pre-treatment for calcareous stone to make it more receptive to a
silicate-based consolidant system such as Conservare or Wacker Stone
Strengtheners OH and H.  I have tested this product and used it in
one case where highly deteriorated salt-laden marble was
pre-consolidated well enough to allow poulticing of salts.  After
two years, results are promising.

The idea of using a calcareous-based consolidant for limestone such
as limewater or perhaps the Dispersed Hydrated Lime (mentioned by
Bryan Blundell in another previous response to this query) might be
effective in consolidation of deteriorated calcareous stone. Again,
the conservation literature reflects very inconsistent results, with
better results reported from certain global regions for whatever
reasons, but the clay remains a serious problem here.

I might suggest trying the combined approach of the hydroxylating
agent pre-treatment followed by an alkoxysilane.  Unfortunately,
this was probably not the best stone to select in the first place.
Careful environmental control might end up being the only option for
long term preservation of this type of stone.

John Griswold
Principal, Conservator
Griswold Conservation Associates, LLC
310-271-5255
Fax: 310-271-5277


                                  ***
                  Conservation DistList Instance 16:11
                  Distributed: Friday, August 2, 2002
                       Message Id: cdl-16-11-003
                                  ***