Friday, February 22, 2008

Lead Sheet by Noel Knight

Lead has been found to predate Roman times, with some fragments being found in burial mounds. Its widespread occurrence and ease of smelting made it one of the earlier metals to be explored in Britain. The Romans favoured its usage to form pipes and cisterns, also for lining baths and roofing. Examples can still be found in Bath and Buxton, however, it fell into decline when the Romans left until the sixteenth century.

Lead mining and production in Britain peaked in the eighteenth and ninteenth centuries. Industry declined rapidly in the early nineteenth century due to importation. The last British mines closed about forty years ago.

Galena, lead ore, is particularly common in limestone, but can be found in any hard rock. Often it is found in veins along with other minerals such as quartz and at times has been found to contain silver. It is soft and easily worked yet durable and has a life in excess of 150 years.

Lead sheet can be produced in three basic ways:

Sand Casting. Possibly the original method of producing lead sheet with a few specialist firms today who can still undertake the process. molten lead is poured or run over prepared beds of sand into boxes of a set size with a gauging bar then being drawn across the surface to give a relatively constant thickness. Lead manufactured in this way has a particularly long life, due to its thickness and impurities. Old lead of this type is often melted down and recast.
Milled lead sheet. Produced by passing ingots of lead through a series of rolling mills. It can be supplied in a variety of thicknesses, which are encoded 3-8 and correspond roughly to its weight in pounds per square foot, and from 1/16th inches in thickness. It produced to a British Standard and can be cut to size or rolled sheets up to 2.4m wide and 12m long.
Cast lead. Also known as continuously cast lead or direct manufacture lead. In production a cooled roll is rotated on the surface of a bath of molten lead which solidifies as it passes over the roll to produce a continuous sheet. Speed of roll rotation determines the thickness of the sheet - slow rotation means its thicker, faster rotation produces thinner material. It is available in a variety of widths up to 1.4m and any required length.

Which product should one use?
sand cast lead is more or less reserved for elite structures and consequently very expensive. There is little between milled or cast lead, although there is much controversy around their differing molecular compositions. In cast lead the molecules are in their natural state, whereas in milled lead they are stretched and rearranged during the rolling process and choice will often lie with the individual.

Lead is highly resistant to corrosion and can be used in contact with most metals, however if used in a marine like environment contact with aluminum should be avoided. Care should be taken to avoid contact with chemically treated timbers until dry. Due to its softness and malleability lead naturally expands and contracts, this movement must not be restricted. Newly laid lead will at first release a white patina of lead carbonate which will cause staining and streaking of both the lead and any other material over which it passes. This can be prevented with the application of patination oil over the surface. Condensation could be a problem which can denature the lead causing it to flake to a fine white powder. Flat surfaces now incorporate underlays - a vapour barrier and a geotextile membrane. Lead sheet on historic buildings is mostly used to prevent rainwater ingress and to aid its drainage away from the roof. It is used in such places as ridges, hips, valleys and the sealing of abutments, also vertical cladding and both flat and pitched roofs.

When requiring any form of lead work to be undertaken, seek the services of a qualified plumber and ensure that all work undertaken complies with the relevant code of practice.

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