Alum mordant for paper marbling

Anne Lane <alane [at] infoave__net> writes

>There is a discussion going on on the calligraphy list concerning
>the use of alum as a mordant for marbling paper.  Some of these
>people are using the traditional carragheenan size and watercolors;
>some are using methyl cellulose and acrylic paints; some are using
>combinations of the above. Their concern is for the effect that the
>alum may have on the longevity of the paper; questions have arisen
>as to whether deacidification with Wei T'o would be of any use after
>the paper has dried.  Someone has mentioned that the use of ox gall
>as a dispersant may be what has counteracted the acidity of the alum
>in historical papers.  Anyone who could provide a clear layman's
>explanation of the interactions between paper and alum and of the
>necessity or not for treating the paper after marbling would receive
>the undying gratitude of 600 calligraphers.

Before colours are applied to a paper, it must be treated with an
alum solution. Traditionally, it is applied with a sponge and left
to dry, but should be still damp in order for the paper to be
handled easily. During drying, the alum crystallizes on the fibers.
If dried too fast or if the paper surface is too hydrophobic, the
colours are washed away. The same happens if the paper is used while
still wet, as alum has not been allowed to crystallize on the
surface of fibers. Every marbler knows the problems and the hardest
thing in marbling is to find just the right paper.

The pH of an alum solution is approximately 3-4, depending on the
concentration used. The purpose of alum in traditional marbling is
to precipitate the pigment colloid solution on the paper surface.
The pigment is usually prepared with gall as a detergent/dispersant.
With inorganic pigments, a natural resin such as gum arabic is added
as a binder, while it is not needed for organic pigments, such as
indigo. Because of the alum used, the pH of a marbled paper (or at
least of the paper extract as measured by standard methods) is more
or less acidic, depending on the paper used.

The safest deacidification techniques of course include spraying,
but I don't think that saving a paper justifies destruction of the
Earth' s ozone layer by using freons or their surrogates. Equally,
if not more, efficient is spraying the paper from the back side with
a water solution of any usual deacidification agent or simply by
using a brush. If the paper is then glued on a suitable cardboard
and some calcium carbonate is added to the starch glue, the marbled
paper is probably quite safe, depending of course if the pigments
applied contain corrosive agents or not. But this is another
question. Hope this helps,

Matija Strlic
Faculty of Chemistry
University of Ljubljana
Slovenia

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                  Conservation DistList Instance 12:68
                 Distributed: Friday, February 19, 1999
                       Message Id: cdl-12-68-010
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