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Subject: Exterior terra cotta ornaments

Exterior terra cotta ornaments

From: Mark Rabinowitz <markrabinowitz>
Date: Friday, February 27, 1998
M-100 Jahn is a proprietary cement based patching mortar marketed as
a patching material for stone restoration.  It is usually ordered
custom color mixed.  It is selected for its relative softness and
high water vapor permeability.  Some (like me) don't specify it
because of its high relative cost, inconsistent deliver schedules
and tendency of the cement component to separate if over-worked
leading to a blanched appearance if improperly handled.  It is a
mortar, not a consolidant.

It could be used as a fill material to make up losses in your
friezes.  It will resemble stone or cement when cured and will need
a top-coating to achieve a gloss similar to glazed terra-cotta.
Selecting that material can be difficult.  You will create a
sandwich of materials (original substrate, patching mortar,
top-coating) with each likely having different thermal expansion
co-efficient leading to early failure, particularly in your
temperature zone.  Edison Chemicals of Conn. (a competitor of Jahn)
markets an architectural ceramic repair system using their
restoration mortar (which unlike Jahn is latex modified, there are
several other suppliers of such modified patching mortars) and an
acrylic top coating.  I have no experience with it but the samples I
tested were not acceptable for the project I had. Mortars can also
be colored with potassium silicate based "paints" (Keim mineral
colors) which may be a better system as you can avoid one of the
layers.

If the cracks to be filled are due to movement within or between
units a mortar fill will not have the flexibility to take up
continued movement. "Gentle washing" is probably not a specification
that can be reliably bid out.

                                  ***
                  Conservation DistList Instance 11:74
                   Distributed: Friday, March 6, 1998
                       Message Id: cdl-11-74-002
                                  ***
Received on Friday, 27 February, 1998

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