Lead roofing

Corrosion of Lead Roofing, Interim Report for Ecclesiastical Architects and Surveyors Association (1986)... 

The first reports on the use of nuclear methods for analyzing archaeological materials were those of Ambrosino et al. and Emoto dealing with coins, the detection of silver in a lead roof tile, and the proof of gold in a foil attached to a Korean... 

In a normal atmospheric exposure, a Iresh lead surface will slowly be oxidized into a thin, protective lead oxide, which halts further oxidation of the metal. The rate of formation of lead oxide is determined by the absorption of oxygen and water vapor into the lead. Such factors as industrial and marine pollution, humidity, temperature, and rainfall profoundly affect the aggressiveness of the atmosphere, and most metals suffer accordingly. However, the protective films formed of lead are so effective that corrosion is insignificant in most natural atmospheres. The extent of this protection is demonstrated by the survival of lead roofing and auxiliary products after hundreds of years of atmospheric exposure that may continue for a much longer time if these films are not damaged [2]. 

Fluorspar occurs in two distinct types of formation in the fluorspar district of southern Illinois and Kentucky in vertical fissure veins and in horizontal bedded replacement deposits. A 61-m bed of sandstone and shale serves as a cap rock for ascending fluorine-containing solutions and gases. Mineralizing solutions come up the faults and form vein ore bodies where the larger faults are plugged by shale. Bedded deposits occur under the thick sandstone and shale roofs. Other elements of value associated with fluorspar ore bodies are zinc, lead, cadmium, silver, germanium, iron, and thorium. Ore has been mined as deep as 300 m in this district. 

Applications. The principal use for rigid polyurethane foams is for iasulation ia various forms utilized by a variety of iadustries. Lamiaates for resideatial sheatiag (1.2 to 2.5 cm thick with aluminum skins) and roofing board (2.5 to 10.0 cm thick with roofing paper skins) are the leading products with about 45 metric tons of Hquid spray systems also ia use. Metal doors iasulated by a pour-ia-place process coastitute another substantial use. 

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

Lead has a good general resistance to various atmospheres. As a coating, it has had its greatest application in the production of terne-plate, which is used as a roofing, cornicing, and spouting material. 

A note of caution about roof fasteners. A common mistake is to fix a galvanised or aluminium roof in place with nails or screws of a different metal copper or brass, for instance. The copper acts as cathode, and the zinc or aluminium corrodes away rapidly near to the fastening. A similar sort of goof has been known to occur when copper roofing sheet has been secured with steel nails. As Fig. 24.6 shows, this sort of situation leads to catastrophically rapid corrosion not only because the iron is anodic, but because it is so easy for the electrons generated by the anodic corrosion to get away to the large copper cathode. 

Tin finds widespread use beeause of its resistanee to eorrosion, or as foil or to provide proteetive eoats/plates for other metals. Properties of lead whieh make industrial applieation attraetive surround its soft, plastie nature permitting it to be rolled into sheets or extruded through dies. In the finely-divided state lead powder is pyrophorie in bulk form the rapidly-formed proteetive oxide layer inhibits further reaetion. It dissolves slowly in mineral aeids. Industrial uses inelude roofing material, piping, and vessel linings, e.g. for aeid storage. 

Timber roof with zinc or lead covering and 0.96... 

For some purposes where the strength and ductility of steel are not prerequisites, other metals or materials may be used to advantage, particularly when the component or article is not a load-bearing one. Some of the non-ferrous metals and plastics materials are extremely useful in this respect, especially the latter with their excellent corrosion-resistant properties and ease of formability. Non-ferrous metals in sheet form are often used as roof covering. In such situations they could well become subject to condensation. Condensation could be the result of thermal pumping or internal conditions. Under conditions in which condensation can occur, copper is not normally attacked, but lead, zinc and aluminium may be attacked and corrode from the inside of the building outwards. 

These alloy coatings have advantages over tin in atmospheric exposure where there is heavy pollution by oxides of sulphur. They are cathodic to steel and anodic to copper. In industrial atmospheres, however, formation of a layer of lead sulphate seals pores and produces a generally stable surface and terne-plate has been used extensively as roofing sheet, especially in the USA. It is easily and effectively painted when additional protection is required. Copper heat exchangers in gas-fired water-heaters may be coated by hot dipping in 20% tin alloy . 

No shiny notice boards here, no competent line-drawing reconstructions that remind me of my childhood history books. Here are only fields and fences and crumbling stone. What s left is just that leavings. After the lead s been stripped from the roof, the timbers taken to build a barn or burn Guy Fawkes, the good dressed stone carted away for some squire s new house, what remains is only what s too stubborn to be easily used again. 

---