Horn

Gille Schofield <gille.schofield [at] virgin__net> writes

>I have a snuff-box made of horn in my care.  It reacts badly to any
>changes in the atmosphere and as a result has started to laminate
>slightly on its base.

Horn is particularly subject to dimensional changes (and
de-lamination) with changes in relative humidity.  Like wood, horn
is also subject to the phenomenon known as "permanent set."  This is
exemplified by what happens in a wood veneer tabletop surrounded by
a solid edge.  Let's say we're starting at 50% relative humidity. If
we bring the relative humidity up to 70%, the veneer attempts to
expand in the cross-grain direction.  It is restrained by the thick
wood edge.  The cells of the veneer compress a little.  When the
humidity comes back down to 50%, the veneer shrinks, leaving a
little crack along the side.  When the humidity goes up again, the
process will repeat, with some hysteresis.

I worked on an 18th-century horn snuff box, rectangular, with an
inlaid rectangular horn piece in the top.  The box had shrunken in
one direction and the inlay piece had shrunken in the other
direction.  On one end of the inlay there was a gap.  However, the
long side of the inlay had not shrunken, but the box under it had;
the only solution that I could see was to pare down the inlay about
1 mm so that it would fit.  It was glued back (with poly(vinyl
acetate, I believe).  It was easy to fill the gap at one end with
colored wax.  The reason to use wax is that when the box expands
again, the wax will extrude.  The wax will not contribute to
increased permanent set or to cracking as a more solid fill might.

This box had the additional problem that iron had been used as the
hinge pin. It was rusted through and had to be replaced.  The iron
had also expanded to crack some of the hinges and was firmly rusted
into the others.  I made a core drill out of hard brass tubing with
teeth cut in the end to drill out the iron pin.  After it was out,
the cracked hinges were reglued and a brass pin made to fit.

Basically, with this problem as described, I would suggest putting
the box in a somewhat elevated humidity (55-60%?), seeing if the
cracks close up, repairing them with wax, and then recommending that
the box be kept in a humidity-controlled environment or even making
a little humidity-controlled enclosure for the box.

One of the problems here may be central heating and excessive
dryness during the winter.  In Scotland, Blair Castle (where there
is no central heating) has marquetry furniture with the inlays
remarkably well preserved.  Some control of environment may be
necessary here.  If the client wants to use the box, you might
suggest that it be kept in a humidor (or a humidor you design to ca,
55% RH or so) while he is not using it.  In this case, it is
important both to moderate short-term humidity changes and to make
sure that the average ambient relative humidity (the year-round
average) is adequately high.

Of course, we don't know how old this horn box is or whether the
cracking is due to seasoning of the horn rather than relative
humidity changes.

Tom Chase

                                  ***
                  Conservation DistList Instance 13:28
                 Distributed: Tuesday, November 2, 1999
                       Message Id: cdl-13-28-001
                                  ***