Consolidation of terracotta tiles

Tanja Roskar <tanja.roskar [at] vm__unit__no> writes

>... a tile floor from a medieval armory was lifted
>from a wet site and brought to the lab for cleaning and
>consolidation. Of the almost 400 tiles we are lucky to have problems
>with just a few. These are glazed tiles that after cleaning were
>showing signs of soluble salts crystallizing on the surface during
>drying.
>
>The tiles in question have gone through a desalination process and
>we now want to consolidate them. ...
>
>The tiles are still immersed in water and we are interested to find
>a consolidant that can be absorbed into the clay body while the
>tiles are still wet, then air dried.  Perhaps a vacuum impregnation.

Because the tiles are saturated, they must be dealt with as
waterlogged material and vacuum impregnation is not an option.  I
suggest two possible treatments: 1) consolidation by transferring a
tile directly into 4% Primal WS-24 (known in the U.S. as "Acrysol",
this is an acrylic colloidal dispersion).  There is no surface
tension or capillary draw problems here as the tiles are
waterlogged.  Consolidation will take place by equilibrium
exchange--the water will exchange with the WS-24.  Depending on the
thickness of the tiles, I would recommend soaking for 3-5 days. It
will probably help to suspend the tile on a plastic shelf in the
solution, to minimize settling out of the WS-24.  Some settling out
may occur and the surface may have a white build-up.  This can be
brushed off or left until the tile is removed and dry (it will then
turn clear and excess can be removed using acetone).  2) De-water a
tile using changes of acetone and consolidate by transferring the
tile directly into 7.5% Paraloid (Acryloid) B-72 in acetone.  Again,
this is an equilibrium exchange.

Try one tile by each method and determine which is more appropriate,
Re-consolidation can be done after either treatment using more B-72 applied
by brush or pipette once the tiles are dry.  Use B-72 adhesive for assembly.

For more information see:

    Koob, S.P., "Consolidation with acrylic colloidal dispersions",
        AIC Preprints, 1981, pp. 86-89.

    Koob, S. P., "The use of Acryloid B-72 in the treatment of
        archaeological ceramics: minimal intervention", Art and
        Archaeology II, Materials Research Society Symposium
        Proceedings, Volume 185, 1991, pp. 591-596.

    Roberts, J. D., "Acrylic colloidal dispersions as
        pre-consolidants for waterlogged glass", in ICOM, 1984,
        84.20.24-26.

Stephen Koob
Department of Conservation and Scientific Research
Freer Gallery of Art
Smithsonian Institution
Washington DC 20560
202-357-4880 x276

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                  Conservation DistList Instance 9:45
                 Distributed: Friday, December 1, 1995
                        Message Id: cdl-9-45-003
                                  ***