High tin bronze has been used for mirrors by various cultures from a
very early date, but such high levels of arsenic are uncommon.
However, 18-19th century speculum telescope mirrors often contained
several percent of arsenic, both as an antioxidant and to increase the
whiteness and brilliance of the metal. James Nasmyth (b.1808), in his
autobiography, quotes such a formulation:
"Nevertheless I set to work with all earnestness, and began by
compounding the requisite alloy for casting a speculum of 8 inches
diameter. This alloy consisted of 32 parts of copper, 15 parts of grain tin,
and 1 part of white arsenic. These ingredients, when melted together,
yielded a compound metal which possessed a high degree of brilliancy."
I have seen an early 17th century speculum mirror (not astronomical)
from Cornwall with about 3% arsenic. This may not be an intentional
addition, but merely reflect the very high arsenic content of the local
copper ores (no pun intended).
Nigel Seeley.
>>> Benoit Mille <[log in to unmask]> 25/October/2000 12:47pm
>>>
Dear members,
we have recently analysed a mirror which is fixed in an ivory box. This
ivory box is decorated in a french gothic style (XIV AD). Results show
that
the mirror is made of a bronze which contain 30 % Sn and 3 % As. Does
anybody know other examples of medieval high-tin bronzes mirrors ?
Thanks in advance
---
Benoit Mille et David Bourgarit
Groupe metallurgie
Centre de recherche et de restauration des musees de France
LRMF - CNRS UMR 171
Palais du Louvre
6, rue des Pyramides
75041 Paris Cedex 01 - France
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