Support material for bronze

Zoran Milic <zoran.milic<-a t->narmuz-lj< . >si> asked about a suitable support
for an archaeological object.

We usually use Perspex (the name used in England for acrylic sheet
such as Plexiglas), because it is inert, transparent, smooth,
strong, pleasant to work with and easy to adapt. Depending on the
size and weight of the object we would use acrylic sheet which is 2
or 3 mm thick. Heavier acrylic sheet is sometimes needed for
structural support, sometimes just for specific areas. In order to
mould it to support unevenly shaped objects we use the following
process:

    1.  The object is placed face down on a temporary support. For
        an item such as an uneven copper alloy sheet a bed of sand
        covered with clingfilm could prove suitable. The bed is very
        carefully shaped to make sure that it supports the object at
        every point.

    2.  The back of the object is protected. Again, a clingfilm
        barrier can be used, since this is only in contact with the
        object for a few minutes. Please make sure that you test the
        clingfilm (we use one with no plasticiser) to make sure that
        it does not affect the object or any protective coating.

    3.  Plaster of Paris is applied to the protected back of the
        object and built up to form a thick bed (at least 2 - 3 cm
        thick) with a flat upper surface. Take great care that the
        plaster cannot penetrate through holes in the protection,
        around the edges of the protection and that the dampness of
        the plaster cannot permeate through to the object. We have
        used this technique on thin wood panels of great antiquity
        and fragility with no problems, but you must be careful.

    4.  When the plaster is sufficiently set, remove it carefully
        (it will probably carry the clingfilm with it) and allow it
        to set thoroughly. Turn it over. You now have a mould of the
        back of object. You now need to make a cast replica of the
        back of the object.

    5.  Make sure that this plaster mould is fully protected with
        clingfilm and cover it with plaster of Paris to a depth of 2
        - 3 cm. This upper layer of plaster will form the replica of
        the back of the object.

    6.  Remove the new plaster when it is set and allow it to harden
        thoroughly. Turn it over and remove the clingfilm. Trim any
        edges to remove excess plaster.

    7.  Dry the new plaster thoroughly in a slow oven (120 degrees
    Centigrade).

    8.  Cut a piece of acrylic sheet of a suitable thickness and
        slightly larger than the finished shape.

    7.  Lay the piece of acrylic sheet on the replica of the back of
        the object which you have made out of the plaster and place
        the original plaster mould on top, forming a sandwich. Place
        all three items in an oven at 120 degrees Centigrade.  Watch
        the acrylic sheet. As it warms it will slowly soften and
        start to become floppy, conforming to the shape of the
        plaster.

    8.  When it is at the right stage it is as floppy as a piece of
        leather. Remove the sandwich from the oven with oven gloves
        on and gently press down on the plaster. Put a weight on the
        upper piece of plaster to keep pressure on the sandwich
        until it is cool. Do not put too much weight on the plaster
        or it will crack. As the acrylic sheet cools it will retain
        its new shape. It may not be a perfect fit at this stage and
        a few small areas may need further shaping.

    9.  When the acrylic sheet is cool test it against the back of
        the object. Mark any areas which require adjustment with a
        felt tip pen (these marks can later be removed with
        IMS--Industrial Methylated Spirits) and remove the acrylic
        sheet from the object. Heat just the areas which need more
        reshaping with a hot air gun (as used for paint-stripping)
        making sure you heat the area evenly (and not too much).
        While the areas are soft and malleable, reshape them with
        tools padded with cloth, shaped pieces of wood etc. 10. When
        the shape is right and fits the back of the object, trim
        away the excess acrylic sheet around the edges, leaving any
        projections which you need to turn up around the object (to
        prevent it sliding off the support) or to turn down to form
        feet for the support to display the object at the right
        angle.

    11. Smooth all cut edges of the acrylic sheet with wet and dry
        carborundum paper, using grades of increasing fineness (and
        washing the acrylic sheet well between grades). We start
        with 200 grit and move up to 400, 600, 1200 and 2400 grades.
        Finally polish the edges with a very fine abrasive such as
        Duraglit wadding (silver cleaning grade) or a chrome polish
        cream and rinse clean with IMS.

    12  Once the support is correctly shaped, all the edges are
        smoothed and the surface is clean and dry, mount the object
        on the support.

    13. Stand back and wait for grateful appreciation.

Practice with a small item first and gain confidence in handling
scraps of acrylic sheet. Heating it too much will cause it to bubble
(or even to smoke or burn). Bending it too fast when it is too cool
will cause it to crack and craze. When it is soft and hot it can be
easily indented and marked by tools such as pliers or even the
pattern of the fabric of the oven gloves if it is gripped too hard.

*Safety*: Remember to be aware of hazards and observe safety
procedures. Acrylic sheet is flammable and should not be used near
naked flames. Wear eye protection and protect hands against heat.
When using wet and dry carborundum paper we find that barrier cream
helps to protect the skin from the prolonged contact with water and
abrasive particles. Dry plaster of Paris powder can be a dust
hazard.

Good luck,

Richard and Helena Jaeschke
Archaeological conservators

                                  ***
                  Conservation DistList Instance 11:16
                 Distributed: Thursday, August 14, 1997
                       Message Id: cdl-11-16-003
                                  ***