Lead
came is available in several hardnesses. One (soft) is almost pure
lead, another is half hard and contains up to 5% antimony, and the
third is hard, containing up to 10% antimony. The difference between
these is hardness, or resistance to creep, not resistance to
corrosion.
 |
| elemental lead |
Lead
with antimony as an alloy is subject to the same corrosion rate in
atmospheric environments as chemical lead (99.9% commercial-purity
lead). However the greater hardness, strength and resistance to
creep of antimonial lead often makes it more desirable for use in
specific chemical and architectural applications.
The
ability of some antimonal leads to retain this greater mechanical
strength in atmospheric environments has been demonstrated in
exposure tests in which sheets containing 4% Sb [antimony] and
smaller amounts of arsenic and tin were placed in semi-restricted
positions for 3 years. They showed less tendency to buckle than
chemical lead, indicating that their greater resistance to creep had
been retained.
Handbook
of Corrosion Data, by Bruce D Craig, p89ff
 |
| Antimony crystals |
Thus,
the use of softer leads in conservation or restoration, because they
were used in earlier periods, is not indicated. It is known that
lead came up to sometime in the early 19th century was melted and
re-formed into came, incorporating tin from solder and other trace
elements which made the lead “stiffer” than the more pure lead
that began to be produced commercially and used widely at that time.
This may be the reason that so many 19th
century windows contain failing leads, while many earlier ones remain
sound.
