Posted from the Conservation distlist:

Date: 13 Feb 2002
From: Kate Helwig <[log in to unmask]>
Subject: Magnesium carbonate as a fill material for marble

I am analyzing a repair (fill material) from a marble sculpture
(believed to be sculpture grade Carrara marble). Analysis (by FTIR
and XRD) showed the fill material to be composed of a hydrated
magnesium carbonate with a smaller amount of quartz and probably
sepiolite (a magnesium silicate). No organic binder was detected.
I'm wondering if this could be a type of plaster fill based on
magnesium carbonate rather than calcium carbonate.

I have found reference to the presence of  magnesium carbonates in
plasters where the lime comes from a magnesium-rich limestone,
however I have not found any reference to plasters where hydrated
magnesium carbonate is the major component. Has anyone come across
this type of fill or a reference to the use of magnesium carbonate
based plaster?

Kate Helwig
Conservation Scientist
Canadian Conservation Institute
Ottawa, Canada

Am I right in assuming that magnesium is the reactive agent in hydraulic
limes formed from dolomitic limestones?  And that aluminum is the agent in
calcium aluminate cement?  I used to make sculptures with a method known as
"ciment fondu" (don't ask me why) using a calcium aluminate binder.  It was
available from a french firm LaFarge, and was an extremely fine dark olive
colored powder.  It produced  beautifully accurate fine castings that looked
like stone.  The technique was promolgated in the sixties as a cheap method
of making permanent outdoor castings that looked a lot better than cast
concrete,  The pure cement skin coat was backed by a series of layers of
type "S" mixes alternating with fiberglass mesh to about 1/2" thick.  It
took a 24 hour damp cure after the initial set but I was able to make quite
large castings that looked like stone and weighed the same as fiberglass.

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