Brass coatings

Reply to Alan Hawk's on brass coatings.

The two resins you mention are covered in a book by J. Gauld Bearn, THE
CHEMISTRY OF PAINTS, PIGMENTS & VARNISHES, London, Ernest Benn Ltd,
1925, p. 172ff:

    Coloured Resins

    For the sake of completion it is perhaps desirable to give here a
    brief description of some of the very many naturally-occurring
    coloured resins and woody fibres, such as gamboge, dragon's blood,
    grass tree gum, red and yellow sanders wood, and others, which were
    formerly largely used for the purpose of colouring spirit varnishes.

    These natural coloured resins have now been largely replaced owing
    to their lack of permanency by the synthetic coal tar dye-stuffs,
    and are now only used in special cases.

    Red Sanders Wood
    (Red Sandal Wood, PTEROCARPI LIGNUM)

    Red sanders wood is the heart wood of PTEROCARPUS SANTALINUS.  It is
    imported in large heavy logs of a dark reddish-brown colour, the
    colouring matter of which is readily soluble in alcohol, but only
    sparingly soluble in water.  For use in spirit varnishes the logs
    are cut up into chips, or, more often, rasped into a fine powder.
    ...

    (p. 173) Dragon's Blood
    This comes from various species of plants, but chiefly from the
    fruit of the CALAMUS DRAGO of Sumatra, Indo-China and Molucca....

    It was formerly much used for colouring spirit varnishes, but is
    being gradually superseded for this purpose by the soluble spirit
    red aniline colours....

We have at the Freer a fair collection of older books on varnishes and
varnish resins, as well as three samples of Dragon's Blood.  I can't
find any sandalwood here, though.

These varnishes (which I assumed to be boiled oil varnishes with
colorants) can be quite stable.  I'd use restraint in removing them, and
try to leave the original where possible.  The final treatment of the
metal substrate (polishing, scratch- brushing, etc.) is an important
factor in the surface appearance, as well.

Some articles have been published on treatment of old scientific
instruments, but I don't have them to hand at the moment.

Anyway, if you want to see some Dragon's Blood, The Department of
Conservation and Scientific Research of the Freer Gallery of Art and
Arthur M. Sackler Gallery, Smithsonian Institution, Washington, DC has
some!  (since we're the national museum of Asian art, who could be a
better source?)

Tom Chase
Head Conservator
DCSR
FGA/AMSG, SI

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                  Conservation DistList Instance 8:49
                Distributed: Tuesday, December 27, 1994
                        Message Id: cdl-8-49-003
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