Casting techniques for wooden carving

Frank Hassard <f.hassard [at] tiscali__co__uk> writes

>I am working on a reproduction Chippendale cabriole leg chair
>(c.1860) carved in mahogany. One of the ear-pieces to the top of the
>proper right side is missing. ...
>...
>... I think it may be possible to cast a new one from one of the
>existing ear-pieces (and then tailor it to suit). However, I do not
>have much experience with such techniques. I understand that there
>are concerns with some of the epoxy systems regarding
>discolouration, texture etc. Could someone give me some advice on
>how to get the best results and which materials/techniques are
>presently favoured (and ethically acceptable)? ...

Depending on the size of the feature you want to cast, you may have
several options at your disposal.  One important factor is whether
you can remove the feature you want to use to make your mold or
whether you have to make it in situ.  That will determine what kind
of materials and form you will want to use.

I am assuming that the feature of interest is too three-dimensional
to simply use dental wax.

But depending on the complexity of the carving and the extent of
undercutting you may be able to use dental alginate.  This is a very
convenient material for simple mold making, made from seaweed (thus
alginate) and mixed with water for use so that it can be made in a
variety of proportions depending on the working properties you want.
To find out more about what is available and the working properties
of different brands/types check out dental supply sites on the web
or call a local dental supply company.  Actually I have found
dentists and dental technicians--the guys/gals who make crowns,
caps, etc.--to be very helpful resources in making molds to replace
decorative elements on frames as well as paint itself.

Another dental product that is worth keeping in the studio for such
use is dental silicone.  We use 3M ESPE Express.  It comes in a
variety of grades for different rheology, curing times, etc.  Cure
time is generally short, a few minutes, as it is used directly in
the patients mouth.  It is very convenient to use, as it comes in a
two-part syringe, so the proportions are always right and it comes
out of the tip ready for application to the surface from which you
want to take your impression.  It is often limited in its utility
because of its viscosity (the grade we use does not flow and cannot
be cast) and short open time.  And its viscosity can lead to air
getting trapped on the surface and it takes some experimentation to
develop a method that will work for you.  But it is the quickest and
easiest way to take impressions from surfaces, especially of
paintings, that I have found that does not require casting. And it
captures remarkable detail when used properly.  Again, for more info
check out the 3M ESPE web site:

    <URL:http://products3.3m.com/
        catalog/ca/en001/healthcare/-/node_CF9PG1KFFWbe/
        root_VJ3G0N3T7Ngv/vroot_3LPG2VJGS7ge/
        gvel_K89M1BN438gl/theme_ca_en_healthcare_3_0/
        command_AbcPageHandler/output_html>

    **** Moderator's comments: The above URL has been wrapped for
    email. There should be no newline.

Or, contact you local dental supplier.  The latter will also be able
to inform you of alternatives by other manufacturers.

For more complex forms I have had great success making molds with
Dow High Strength Moldmaking room-temperature vulcanizing silicone
(HS II RTV).  It is a two part silicone rubber.  Not too difficult
to prepare, although it often needs to be deairated to prevent
bubbles in the mold.  This is accomplished by drawing a vacuum on
the newly prepared mixture (an aspirator on your sink is sufficient)
and by pouring it very slowly, in a thin ribbon, into your
frame/mold.  It can be used with a variety of materials for making
dams/frames around the feature you want to capture, although a
sulfur-free moldmaking plasticene clay is preferred.  Once prepared,
the rubber is a viscous fluid that flows well into your form to
conform to surfaces and capture incredible detail. For more info on
this and other Dow-Corning products check out
<URL:http://www.dowcorning.com/content/moldmaking/>

Alternatively, if you do not want to make a form you may want to
consider one of their other grades.  Some of these are low viscosity
for use by application of a thin layer to the surface, i.e. paint it
on.  The body of the mold is then built up around this surface
coating.  More viscous grades are available for application as a
putty that will hold to vertical surfaces. Check out D-C's web site.
It is a very informative site regarding mold making.

Whatever you decide to use, you should give some thought to a
readily reversible, protective coating for your original.  The
silicone rubbers tend to be greasy and can stain porous surfaces.
They can also act as solvent for some coatings and may effect old
finishes.  Likewise, the alginate might cause bloom on old shellac
and varnish.

I would have suggested that you make your casting from Ren 306, a
castable epoxy filled with glass microballoons and tinted to
approximate the color of mahogany (in the imagination of some
plastic engineer) that takes great detail from molds and carves and
sands like an amorphous, hard wood.  Unfortunately, Ciba-Geigy sold
off their Ren division some time ago and the new owners have not
continued manufacture of this particular epoxy.  I am still hoping
that a recent query regarding wood fillers on this List will turn up
a viable replacement for this wonderful old product.

Hope that is enough to get you started.  Have fun.  A good casting
is a very rewarding endeavor.

Steven Prins
Santa Fe, NM


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