Wednesday, 16 October 2013

Lead Came with Alloys

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.