Lead burning

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Lead rainwater goods, formed by casting and lead burning Drain-pipe-stone-wall.jpg
Lead rainwater goods, formed by casting and lead burning

Lead burning is a welding process used to join lead sheet. It is a manual process carried out by gas welding, usually oxy-acetylene.

Contents

Uses

Lead burning is carried out for roofing work in sheet lead, or for the formation of custom-made rainwater goods: gutters, downspouts and decorative hoppers. Decorative leadworking may also use lead burning, particularly where a waterproof joint is required as for planters. Lead burning is thus part of traditional plumber's work, in its original sense of a worker in lead (Latin: plumbum ). Although rare and specialised, this work is still carried out today and not just for restoration of historical buildings. Most lead sheet work is formed and sealed by bossing, a mechanical fold or crimp. This is adequate for roofing that sheds water, but is insufficiently watertight when standing water sits upon it and so an impermeable burned joint is needed.

Lead burning is not used as part of plumbing work for installed pipework. Lead piping has long been considered obsolete, owing to the health aspects. Even where lead piping, or lead-sheathed cable, still needs to be jointed, this is carried out with a wiped joint, rather than a burned joint. Wiping a lead joint is a soldering process, using plumber's solder (80% lead / 20% tin) and is carried out at low temperature, with a natural-draught propane blowtorch. Today, even wiped joints are rare and where an existing lead pipe must be connected to, a proprietary mechanical joint is more likely to be used.

In some rare cases within the chemical industry, lead burning is used for pipework, where acid-resistant tanks and pipes are required to be made of lead rather than steel. Niche uses for lead burning include the manufacture of lead plates for lead-acid batteries and for electro-plating electrodes. [1]

Process

Lead burning is an autogenous welding process. Two sheets of lead are formed mechanically to lie close against each other. They are then heated with the torch flame and flow together. No filler rod is required, the sheets form their own filler (autogenous welding). Neither is a flux used. [2] Soldering, by contrast, uses a solder alloy that is some compatible alloy showing eutectic behaviour. This gives a melting point lower than the base metal, allowing a soldering process rather than welding. A filler rod may be needed for some welds, if there is no convenient way to form sufficient close overlap at a sheet edge. Offcuts of the same lead sheet are used as this filler. Excessive use of a filler, rather than an initial close fit, is considered a sign of poor technique.

The torch used for lead burning is a small, hot, gas flame. Oxy-acetylene is most commonly used, as it is easily portable. A small size #0 nozzle is usually used, sometimes with a miniature torch body, but the torch is otherwise the same as that used for steel or copper work. A variety of fuel gases may be used, but to achieve the high temperature needed, an oxygen supply is always used. Fuel gases may be acetylene, natural gas or hydrogen. Oxy-hydrogen is considered to be the best, but is not easily portable. Oxy-natural gas is cheapest and is often used on fixed workbenches. As it is less hot, it cannot be used for some awkward positional (overhead) welding. Oxy-acetylene is the most common, as much leadwork is carried out on site and this is easily portable.

A neutral flame is used. A reducing flame (fuel rich) gives trouble with soot deposits in the weld. An oxidising flame burns the lead and creates lead oxide dross, leading to poor welds with low malleability.

History

Lead burning requires a gas torch as autogenous processes require an intense, controllable flame that can be applied to a small area. It was first developed along with the early growth of the bulk chemical industry, as acid manufacture required leakproof lead vessels and flow process plumbing to be made. At the same time, coal gas was increasingly available for domestic lighting. By using a mouth-blown blowpipe, a gas flame could reach temperatures adequate for lead burning. Larger equipment could use mechanical fans.

Before this, leadworking used either manual bossing or wiped soldered joints to seal it.

Safety

Fire risk

Lead burning, and lead soldering, are some of the few building processes which still requires the on-site use of a naked flame. This has obvious safety hazards and lead working has been implicated in some fires during restoration work on historical buildings.

Health

Metallic lead is relatively safe to work with, although lead oxide dross formed on the surface of lead is more easily absorbed by the body, thus much more of a hazard. As lead burning is a high temperature process, it creates a significant hazard from such dross. Safety precautions are relatively simple: goggles to protect the eyes from molten metal splash, overalls or dustcoat kept in the lead workshop to stop contamination spreading, and dedicated workbenches equipped with air extraction.

Regular lead burners should be screened for accumulated lead exposure. Industrially this is done by weekly checks for blue lines around the gums, a simple indicator for heavy metal poisoning, and by regular urine testing.

See also

Related Research Articles

Acetylene Unsaturated hydrocarbon with formula C2H2

Acetylene (systematic name: ethyne) is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas (lower hydrocarbons are generally gaseous in nature) is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities.

Soldering iron

A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.

Welding Fabrication or sculptural process for joining materials

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature metal-joining techniques such as brazing and soldering, which do not melt the base metal.

Metalworking Process of making items from metal

Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.

Brazing High-temperature soldering; metal-joining technique by high-temperature molten metal filling

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

MAPP gas A fuel gas based on a stabilized mixture of methylacetylene and propadiene

MAPP gas was a trademarked name, belonging to The Linde Group, and previously belonging to the Dow Chemical Company, for a fuel gas based on a stabilized mixture of methylacetylene (propyne) and propadiene. The name comes from the original chemical composition, methylacetylene-propadiene propane. "MAPP gas" is also widely used as a generic name for UN 1060 stabilised methylacetylene-propadiene. MAPP gas is widely regarded as a safer and easier-to-use substitute for acetylene. In early 2008, true MAPP gas production ended in North America when production was discontinued at the only remaining plant in North America that still manufactured it. However, many current products labeled "MAPP" are, in fact, MAPP substitutes. These versions are composed almost entirely of propylene with minuscule impurities of propane (<0.5%).

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas. A filler metal is normally used, though some welds, known as autogenous welds, or fusion welds do not require it. When helium is used, this is known as heliarc welding. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

Plasma arc welding

Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode and the workpiece. The key difference from GTAW is that in PAW, the electrode is positioned within the body of the torch, so the plasma arc is separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities and a temperature approaching 28,000 °C (50,000 °F) or higher.

A filler metal is a metal added in the making of a joint through welding, brazing, or soldering.

Oxyhydrogen Explosive mixture of hydrogen and oxygen gases

Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases. This gaseous mixture is used for torches to process refractory materials and was the first gaseous mixture used for welding. Theoretically, a ratio of 2:1 hydrogen:oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is needed to avoid an oxidizing flame.

Atomic hydrogen welding

Atomic hydrogen welding (AHW) is an arc welding process that uses an arc between two tungsten electrodes in a shielding atmosphere of hydrogen. The process was invented by Irving Langmuir in the course of his studies of atomic hydrogen. The electric arc efficiently breaks up the hydrogen molecules, which later recombine with tremendous release of heat, reaching temperatures from 3400 to 4000 °C. Without the arc, an oxyhydrogen torch can only reach 2800 °C. This is the third-hottest flame after dicyanoacetylene at 4987 °C and cyanogen at 4525 °C. An acetylene torch merely reaches 3300 °C. This device may be called an atomic hydrogen torch, nascent hydrogen torch or Langmuir torch. The process was also known as arc-atom welding.

Pipe (fluid conveyance)

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.

Piping and plumbing fitting

A fitting or adapter is used in pipe systems to connect straight sections of pipe or tube, adapt to different sizes or shapes, and for other purposes such as regulating fluid flow. These fittings are used in plumbing to manipulate the conveyance of water, gas, or liquid waste in domestic or commercial environments, within a system of pipes or tubes.

Pipefitter

A pipefitter or steamfitter is a tradesperson who installs, assembles, fabricates, maintains and repairs mechanical piping systems. Pipefitters usually begin as helpers or apprentices. Journeyman pipefitters deal with industrial/commercial/marine piping and heating/cooling systems. Typical industrial process pipe is under high pressure, which requires metals such as carbon steel, stainless steel, and many different alloy metals fused together through precise cutting, threading, grooving, bending and welding. A plumber concentrates on lower pressure piping systems for sewage and potable water, in the industrial, commercial, institutional, or residential atmosphere. Utility piping typically consists of copper, PVC, CPVC, polyethylene, and galvanized pipe, which is typically glued, soldered, or threaded. Other types of piping systems include steam, ventilation, hydraulics, chemicals, fuel, and oil.

Oxy-fuel welding and cutting Metalworking technique using a gaseous fuel and oxygen

Oxy-fuel welding and oxy-fuel cutting are processes that use fuel gases and oxygen to weld or cut metals. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material in a room environment. A common propane/air flame burns at about 2,250 K, a propane/oxygen flame burns at about 2,526 K, an oxyhydrogen flame burns at 3,073 K and an acetylene/oxygen flame burns at about 3,773 K.

Soldering Process of joining metal pieces with heated filler metal

Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained lead, but environmental and health concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

Blowtorch

A blowtorch, is a fuel-burning tool used for applying flame and heat to various applications, usually metalworking.

Weld pool

Weld pool commonly refers to the dime-sized workable portion of a weld where the base metal has reached its melting point and is ready to be infused with filler material. The weld pool is central to the success of the welding process. It was first observed in oxy-fuel welding by Fouché & Picard in 1903, after the discovery of acetylene by Edmund Davy in 1836.

Wiped joint

A wiped joint is a form of soldered joint used to join lead pipework.

An autogenous weld is a form of welding, where the filler material is either supplied by melting the base material, or is of identical composition. The weld may be formed entirely by melting parts of the base metal and no additional filler rod is used.

References

  1. "Lead Burning Procedure". Plating.com.
  2. Untracht, Oppi (1969). Metal Techniques for Craftsmen. Robert Hale. p. 22. ISBN   0-7091-0723-4.CS1 maint: ref=harv (link)