Lead


Contents

  1. Introduction
  2. Properties of Lead
  3. The Chemistry of Lead
  4. Lead as a Poison
  5. Mineralogy and Production of Lead
  6. The Lead-Acid Storage Battery
  7. Cupellation
  8. Miscellaneous Uses of Lead
  9. References

Introduction

The symbol in the title is the alchemical symbol for lead, the sign of Saturn.

Lead is a shiny, blue-white soft metal when its surface is fresh. On exposure to the air, it becomes covered by a dull, gray layer of basic carbonate that adheres closely and protects it from further alteration. It resembles aluminium in this respect, which is protected by a dull, gray layer of oxide. Otherwise, lead would react rapidly with the oxygen and carbon dioxide in the air. When placed in sulphuric acid, lead is protected by a similar layer of PbSO4 that adheres strongly. For these reasons, lead is often used to sheathe cables for burial, to protect roofs from the atmosphere, and as tanks and pipes for sulphuric acid. Lead has many and varied uses in technology. This article mentions a very large number of uses of lead and its compounds, from cathedrals to crystal sets, from batteries to sailing ships.

Lead and its compounds were known and widely used in antiquity. Its metallurgy was well-developed even then. The uses of lead depended on its corrosion resistance, its softness and ease of working, and its low melting point. Metal pipes were made of lead, starting with a plate that was rolled into a cylinder and fusion welded down the seam. Buildings were roofed with sheets of lead, and glazed with lead mullions. Molten lead was poured into holes in stone to hold fasteners. Boxes of lead were used as protective containers, and as coffins. Lead was used in the metallurgy of other metals, as we shall see in some detail. Lead was, indeed, used in machines of all kinds, not as a structural material, but where its fusibility and workability were an advantage, and for many small items of daily use.

At the current time, the greatest quantity of lead is used in lead-acid storage batteries. Until recently, next in importance was the use in paint pigments. However, environmental considerations have largely removed lead from paints. Similarly, the once considerable usage of lead in tetraethyl lead antiknock compound has also disappeared for a similar reason. Lead is still used for sheathing cables, in bearing alloys, in artistic pigments and glazes, for decorative glass, in the chemical industry, and, of course, for bullets, which have always been in special demand in the United States.

This article gives some general information on lead, and discusses a few processes and devices in which lead has played a central role, both recently and historically. The banned uses of lead are also described, to satisfy curiosity that might otherwise be frustrated. As much of the curious lore of lead has been mentioned as I could find.

Properties of Lead

The chemical symbol for lead is Pb, from Latin plumbum. The name "lead" is cognate with the Dutch "lood." The high-German "Blei" has no echoes in English or Dutch. English, however, uses the word "plumber" for a worker in lead. The older term is preserved in the name "Ledbetter." Dutch still uses the word "loodgieter" for a plumber, which means "lead-pourer." In Greek, lead is molubdos, a name now used for molybdenum. This meant "lead pencil" in Greek, which describes the soft MoS2 well.

The atomic number of lead is Z = 82, and its stable isotopes are of mass numbers A = 204, 206, 207 and 208, of which the last is most abundant (52.3%). Each isotope is the end of a natural radioactive series, except for 204, which is the rarest at 1.48% abundance. The isotopic abundance, and so the atomic weight, varies in lead from different locations. The average atomic weight is 207.22. The series beginning with Th232 (half-life 1.39 x 1010 y) ends with Pb208. The series beginning with U238 (half-life 4.5 x 109 y), the most abundant natural isotope of uranium, ends with Pb206, and includes the natural isotope of radium, Ra226. The famous fissionable U235 (half-life 7.1 x 108 y) ends with Pb207. Because of the large number of electrons it contains, and its high density, lead is the most practical absorber of gamma rays.

The density of lead is 11.3437 g/cm3, higher than that of common metals like iron (7.86) and copper (8.933), but less than that of mercury (13.6), and not in the league with the really dense metals like gold (19.3), tungsten (19.3), platinum (21.45), iridium (22.42) or osmium (22.48), which are about twice as dense. Lead is, nevertheless, the densest of the common and inexpensive metals, and some uses depend on this property. The fishing sinker and the sailor's lead are humble examples. Lead crystallizes in the face-centered cubic structure, with lattice constant a = 0.494 nm.

The melting point of lead is 327.35°C, and the boiling point is 1515°C. The only common metals with lower melting points are tin and bismuth, while the boiling point is high enough to allow processes in liquid lead over a wide temperature range. The coefficient of linear expansion is 29.5 x 10-4 per °C. The bulk modulus is 0.44 x 106 Mbar. The heat conductivity is 0.081 cal/cm-s-°C, and the specific heat is 0.03046 cal/g-K (all properties are room-temperature values). The heat of fusion is 6.26 cal/g. The electrical resistivity is 20.648 μΩ-cm (compare to copper, 1.683). The hardness of pure lead is 1.5 on the Mohs scale (between talc and gypsum: it can be scratched by the fingernail), and its tensile strength is only 2000 psi. The Young's modulus is 2.56 x 106 psi. Its crystalline form is face-centered cubic, with lattice constant 0.4939 nm. Lead alloys considerably with bismuth and tin, to a smaller degree with antimony and silver. Lead anneals itself (rerystallizes) at room temperature, so cold work does not harden it.

The Chemistry of Lead

The electron configuration is 6s26p2 over a filled 5d subshell, similar to that of C, Si, Ge and Sn, the other elements in its column in the periodic table. The spectroscopic ground state is 3P0, and the resonance line is at 283.39 nm. Because of this, and the high boiling point, lead does not color the gas flame. The first ionization potential is 7.415V, second 15.04V, third 32.1V and fourth 38.97V. The lead spectrum is a good example of jj-coupling, which has effects even in the ground state.

Lead exhibits the oxidation state +2, corresponding to loss of the two p electrons, in most of its common compounds. In this oxidation state, lead is generally basic. The oxidation state +4 also occurs, and in it lead is more acidic. Lead is generally amphoteric, like aluminium, especially in the +4 state, like tin.

Lead forms a series of oxides, which are important compounds. The principal ones are PbO, a yellow oxide called litharge, and PbO2, a reddish-brown substance called lead dioxide, or, erroneously in technical practice, lead peroxide. It is, of course, not a peroxide. Orange-yellow Pb2O3 is PbO·PbO2, while red Pb3O4, red lead or minium, is 2PbO·PbO2. In addition to these, there is Pb2O, lead suboxide, a black, amorphous substance that is PbO·Pb. If metallic (liquid) lead is heated in air below 545°C, red lead forms; if above 545°C, litharge forms.

Basic lead carbonate, 2PbCO3·Pb(OH)2, when pure, is a brilliant white substance that makes an excellent paint pigment, called white lead. It reacts with H2S to produce black PbS, so should not be used in chemistry laboratories. It was generally made by the "Dutch Process" where perforated plates were treated with acetic acid, air and CO2. The lead for this process was called "corroding lead" in commerce, and had to be very low in antimony, which caused the white lead to darken.

The pigment "chrome yellow" was PbCrO4. Mixed with prussian blue, it made a very good green (this is not always the case, but it worked here). "Chrome red" was PbCrO4·PbO. Lead gave a full spectrum of bright colors, used by artists as well as by house painters. Red lead made a paint that protected iron and steel from corrosion; the famous Forth Bridge in Scotland is usually seen covered in red lead. The painters work continuously on it.

Colorless lead nitrate, Pb(NO3)2 is the most soluble lead compound. Lead acetate, Pb(CH3COO)2 is also soluble. It has a sweet taste, and for this reason is called "sugar of lead." When it was discovered, in Roman times, it was used in certain baked treats, which proved fatal. Any soluble lead salt dried on paper can offer an easy test for H2S. When moistened and waved around, it will turn black. Your nose, actually, is not a good detector, since the rotten-egg odor rapidly disappears by fatiguing your sense of smell. The halides of lead are rather insoluble, while the sulphate, carbonate and sulphide are quite insoluble. As we have mentioned, this insolubility is what protects lead from corrosion. Lead arsenate, Pb(AsO4)2 is a double poison, from the Pb and from the As. It was used for dusting cotton plants to fight the weevil.

In qualitative analysis, lead is identified by first adding dilute HCl to the sample, which precipitates the chlorides of mercury, silver and lead, the only insoluble chlorides of the common cations. After this precipitate is separated, it is treated with hot water (still in acid solution). Lead chloride is much more soluble in hot water than in cold, so it is separated by pouring off the supernatant. Then the addition of a little sulphuric acid will produce a precipitate of PbSO4, proving the presence of lead.

A powdered sample, mixed with a little Na2CO3 as flux on a charcoal block, and then heated in the reducing blowpipe flame (the yellow part), will make a drop of liquid Pb and some PbO. This is a mineralogist's test.

One of the most famous lead compounds was tetraethyl lead, Pb(C2H5)4. Here the lead has made four tetrahedral covalent bonds, like carbon in CH4, to fool the motor fuel into letting it dissolve. In the cylinder, the heat knocks the ethyl radicals off and the lead forms a cloud of PbO. The ethyl stops any explosion front in its tracks. Therefore, the fuel charge does not detonate and burns smoothly. This permitted the use of cheaper straight-chain hydrocarbons in motor fuel, instead of the branched and aromatic hydrocarbons of greater octane rating. The lead is only there to carry the ethylene into the combustion zone; it would evaporate if added to the fuel directly.

Tetraethyl lead was made by treating ethyl bromide or ethyl chloride with an alloy of sodium and lead, NaPb. The sodium grabbed the halogen, the lead latched on to the ethyl radical, and some lead was left over, which could be reused. Some extra ethyl bromide was added to the fuel, so that in the heat of the combustion, it would combine with the lead that had initially been released, and carry it out in the exhaust. Otherwise, the lead would stay around and foul the spark plugs and valves (which it did anyway). Some people thought the lead lubricated the valves and was necessary for the engine, but this is false.

Lead as a Poison

It is quite clear from what has been mentioned above that lead was all around us, principally in lead paint and in tetraethyl lead. Now, lead is a systemic poison, but its use in paint and motor fuel was thought safe because nobody ate paint or scraped it into their coffee, and there was lots of air to dilute the lead coming out of exhaust pipes. However, it was finally realized that children did eat paint, and were extra sensitive to lead in the air, and while this had no serious health effects, it seemed to make them stupider than they would otherwise be. This was an excellent reason to reduce the lead in the environment, and that has been done. Paints now use other pigments, such as titanium dioxide, and tetraethyl lead has been banished from motor fuel, which would be a good idea anyway, as easier on engines. This may have saved several lives a year. Now, if the cars were banished instead, we would save 40,000 lives a year.

Acute lead poisoning results from ingesting soluble lead compounds. The symptoms were called "painter's colic" since painters, covered with white lead, were at risk. The damage appears to be mainly to the nervous system, and the effects not as acute as those of mercury poisoning. Lead is an accumulative poison, building up until it reaches a toxic level. An antidote after swallowing a soluble lead salt is a stiff drink of epsom salts, MgSO4, which precipitates insoluble PbSO4.

Lead pipes were once used for household water. Lead pipe was easy to form by casting or extrusion, and easy to join by fusion. A mixture of litharge, glycerine and linseed oil made "plumber's cement" that could be used for joining pipes without heat. With hard water, a layer of sulphate or carbonate forms on the lead, and lead does not enter the water. With soft water, as from cisterns, this protective layer does not form, and a dangerous amount of lead can dissolve in the water. Since lead pipes were widely used in Roman times, some experts have concluded that they poisoned the populace rather generally. In fact, Vitruvius shows that the hazard was appreciated, and steps taken to reduce it.

Copper tubing is joined with solder, which usually contains lead. The amount of lead that comes from soldered tubing connections is vanishingly small, and no hazard, but nevertheless it is deprecated. Even fishing sinkers of lead have been banned, which shows an excellent lack of appreciation of the degree of hazard. Actually, what gave the most worry was simply lost and discarded lead bits. Just good sense would show that it is necessary to be wary of unusual concentrations in the environment and to correct them when discovered.

There is no risk at all in handling lead metal. It cannot be absorbed through the skin or the respiratory tract. Dilute hydrochloric acid has little effect on it, so the lead would pass through the stomach before any damage was done. Eating lead is probably safe, but not encouraged. Carbonated water dissolves lead to some degree. Food and drink should never touch lead, since organic acids, such as acetic acid, may dissolve lead. Lead is, on the whole, very much less a hazard than mercury. It was made dangerous by its widespread use in paint and motor fuel, and that is now past.

The Mineralogy and Production of Lead

Wherever mineralized fluids have percolated through rocks, and Pb++ ions have encountered S-- ions, the highly insoluble mineral galena, PbS, has precipitated. Galena is a cubic crystal with the same structure as NaCl, with P replacing Na and S replacing Cl. A galena cube is shown at the right (image © Amethyst Galleries). It is a semiconductor, with the small band gap of 0.37V, so it possesses good conductivity at room temperature. This causes its shiny lead-gray metallic lustre, but it is not a metal, and is brittle. The "crystal" of a crystal set is a galena crystal. The "whisker" makes a point-contact diode with rectifying properties. Its specific gravity is 7.4-7.6, so it is nearly as dense as iron. Only exceptionally does it occur as large, well-formed crystals, usually cuboids, suitable for a mineral collection, much more often disseminated in small bits in rock and other minerals.

In the Mississippi valley, the limestone rocks have in many places been eaten away by ground waters, and the voids have been filled with minerals, among them galena. Galena gave its name to the town in northwest Illinois, and large deposits were found in southwestern Wisconsin. Southeastern Missouri also has large galena deposits, that are still mined, and the Tri-State area around Joplin, including parts of Missouri, Oklahoma and Kansas, was once one of the great lead-producing areas of the world. This Mississippi valley galena is unusual in not being associated with silver, which usually accompanies lead.

Leadville, Colorado is named after the lead that accompanied the silver that was once mined there. In England, Derbyshire, Cumberland and Cornwall all had extensive galena deposits that have been worked from Roman times on. The mining area of the Harz Mountains in Germany was notable for galena, which was found in the Erzgebirge as well. The western United States and Australia also have important galena deposits. Lead is not a rare metal, as mercury is, but is found at many locations.

Galena deposits may have been altered by water filtering down from above, oxidizing and enriching the ore. This supergene enrichment has changed the sulphide into anglesite, PbSO4, and cerussite, PbCO3. This occurred notably on the island of Anglesey, where there were early lead workings. These "oxide" ores are much easier to treat than the sulphide, and were worked out long ago. The only ore of lead that need be considered is galena.

The problem of winning lead is complicated by the low concentration of lead in the available ore, sometimes only a few percent, and by the presence of numerous impurities. To make the process clearer, we'll assume we have relatively pure galena, and not many impurities. First of all, because of the sulphur, carbon will not reduce lead from galena. The galena must first be roasted in air to oxidize the sulphur and change the galena to oxide. A typical reaction is 2PbS + 3O2 → 2PbO + 2SO2. Now we can add some raw ore to the results of roasting, and get 2PbO + PbS → 3Pb + SO2 by further heating. The elemental lead then runs to the bottom of the furnace, where it can be tapped off and cast into pigs. This simply shows the general theory of the pyrometallurgy of lead. Ore of such purity and concentration to make this simple scheme works does not exist.

First of all, the ore must be concentrated to separate the lead ore from the zinc ore, for example. It turns out to be possible to do this with flotation. A substance is added that wets the zinc ore, allowing it to sink to the bottom, but does not wet the galena, allowing it to be caught up in a foam that floats on the surface of the water. The result is not just lead sulphide, but also the sulphides of copper, iron, zinc, antimony and arsenic. The enriched ore is then roasted in ovens to drive off as much of the sulphur as possible. The roasted ore must be ground and sintered to put it in the form of porous chunks that allow gases to pass through freely, and will not collapse into a thick, impervious layer in the blast furnace. Lead ores are such that these two operations are best combined into one simultaneous roasting-sintering process that produces a sinter ready for the lead blast furnace.

The second stage of smelting can take place in an ore hearth, or a larger blast furnace. The sintered ore is charged, with coke, limestone flux, and other additives depending on the impurities present. The products that accumulate at the bottom are lead, matte (containing iron and copper), speiss (containing iron and arsenic), and slag (containing the silicates, zinc, iron and calcium). Cold air is blown in at the bottom, and flue dust and gases come out the top. The lead bullion from Mississippi valley ores is called "soft" lead and is pure enough for most uses without further treatment. The other by-produts are treated to separate their valuable constituents. Zinc, incidentally, does not dissolve in molten lead, and can be added to extract impurities by differential solubility.

Lead is sold as soft lead, 99.90% pure, common lead (lead that has been desilvered), 99.85% pure, and corroding lead (for paint), 99.94% pure. Hard lead is alloyed with 6%-18% antimony, which increases the strength of the lead. The addition of only 1% Sb or 3% Sn increases the strength by 50%. Hard lead is used for battery plates. Terne plate is heavy sheet steel coated with a lead-tin alloy. 75-25 and 50-50 Pb-Sn alloys are used.

The Lead-Acid Battery

Gaston Planté (1834-1889) was experimenting with electrolysis in the 1850's. This was before the introduction of efficient dynamos, so all direct current electricity came from primary cells. He put two lead plates in dilute sulphuric acid, and passed a current between them. At one plate, the one connected with the positive pole of his battery, oxygen was evolved, and at the other plate hydrogen was evolved. This was no surprise, since water had been electrolyzed for over half a century at the time. What was a surprise was that when he had allowed the current to pass for some time and then disconnected the battery, the lead plates acted like a battery on their own, something that had not been observed before. The plate that had been connected to the (+) pole of the battery, the anode, was at a higher potential than the plate that had been connected to the (-) pole of the battery, the cathode, and now current flowed in the opposite direction. The lead plates were observed to collect a layer of a white substance, lead sulphate, as this happened. When the current finally stopped, the cycle could be repeated by reconnecting the power source. Planté had discovered a storage battery. He announced his discovery in 1859, but it attracted little interest, since there was no demand for such a device.

By 1880, there was a demand. Edison and Swan had devised practical incandescent lamps, and Edison was promoting a complete system, from dynamos to lamps, based on direct current distribution. If there were a practical storage battery, then the battery could be floated across the lines to supply power when the dynamos were shut down, at times of low demand or for repairs. Power could be produced when there was excess generating capacity, and used to bridge over times of unusual demand. Camille Faure looked at the Planté battery, and overcame its principal faults--the lack of capacity, high internal impedance and the long "forming" process necessary to prepare the plates. The cathode was made of spongy lead for lower internal resistance, and the positive plate was consisted of pockets filled with red lead, which was converted more easily to the active ingredient, PbO2, in the forming process. This battery was much more practical, and soon commanded a good market.

Edison tried to improve the Faure battery, extending its life and reducing its weight, looking toward applications in electric vehicles. The result was the Edison alkaline cell, in which the life was extended, and the battery rendered much more rugged. However, the weight problem was not solved, and is not solved to the present day. The lead-acid battery is still the best secondary battery in most heavy-duty applications. Electric street railways operated by battery cars were actually put into service. The batteries were to be charged overnight, when the cars were not in service, and their power used during the day, obviating the need for overhead conductors. There were also battery cars and trucks for road service. The expense and weight of the batteries made such use uneconomic, especially after the invention of the self-starter made internal combustion engines available to all. Of course, at the same time this led to the use of a lead-acid battery in each vehicle for starting and for maintaining the electricity supply with the engine idling or stopped.

The present lead-acid cell consists, in a state of full charge, of a negative plate, or cathode, of spongy lead in a grid of hard lead, a positive plate, or anode, of PbO2 paste in a grid of hard lead, and an electrolyte of dilute sulphuric acid of specific gravity 1.28. This is a 37% solution, with 472.5 g/l of H2SO4. At full discharge, the electrolyte is of specific gravity 1.05, an 8% solution containing 84.18 g/l of acid. Both plates are coated with PbSO4. Approximately 4 moles of acid are used per litre, which corresponds to 213 A-h of charge (an ampere-hour is a current of one ampere flowing for one hour, or 3600 coulomb). Assuming that 4 moles of Pb are reacted at the cathode, and 4 moles of PbO2 at the anode, the total weight of active materials is about 3 kg. This gives a weight-to-capacity ratio of 14 g/A-h. Of course, this is much lower than is required for a practical battery, with case, electrode grids and other necessities. However, a limit of perhaps 25 g/A-h represents the maximum that can be expected of a lead-acid battery, and a limit of about 200 A-h per litre of electrolyte volume.

The cathode reaction is Pb + SO4++ → PbSO4 + 2e-. For each atom of lead, two electrons pass through the external circuit when the cell is delivering current. At the anode, the reaction is PbO2 + 4H+ + 2SO4++ + 2e- → PbSO4 + 2H2O. This reaction uses the two electrons sent by the cathode through the external circuit. For each two electrons, two molecules of acid are turned into two molecules of water and two molecules of lead sulphate. The electrode potential of the cathode reaction is -0.355V, and the electrode potential of the anode reaction is 1.685V, at standard concentrations. The net potential difference is 1.685 - (-0.355) = 2.040V. At the concentrations in a fully charged battery, the potential difference is closer to 2.2V, decreasing to 2.0V for a fully discharged battery.

It is easy to measure the specific gravity of the electrolyte with a hydrometer, and this gives an accurate estimate of the state of charge of the battery. This is one of the great advantages of the lead-acid cell. Note that the electrode reactions do not show any evolution of gases. With open cells, there is in fact some emission of H2 and O2, so the water lost in this way must be replenished regularly. This was once a regular duty in servicing a car, but modern batteries require very little care, and some are sealed, venting gas only when necessary. Also, ventilation was necessary to prevent the hydrogen from becoming an explosion hazard. When ordinary car batteries are charged rapidly, water is electrolyzed.

The lead in batteries is easily reclaimed, and secondary lead is an important source of the metal. There is even a special blast furnace that accepts a charge of scrap batteries, from which metallic lead can then be tapped. It is necessary to handle the impurities involved, coming from the battery cases and the wooden separators between the plates. These components may be burned to provide the fuel for the process. The contrast with tires, that just pile up in unattractive mountains with little value or use, and sometimes smokily catch fire, is notable.

Cupellation

In the ancient world, a very important application of metallurgy was to the winning of gold and silver from their ores, and the testing of objects made from gold and silver to determine their purity. This was made much more difficult than it is today by the unavailability of strong mineral acids. A particular problem was the "parting" of gold and silver; that is, separating the two metals. Often, whatever alloy happened to result from the smelting process was used without further treatment. Lead played an important role in these matters, both in recovering gold and silver from their ores, assaying ore quality, and testing objects for purity.

The usual way of testing alloys of gold and silver, gold and copper and copper and silver was the use of the touchstone. This word is used today, normally without any understanding of its significance. The touchstone was a particular type of black stone that was abrasive enough to rub off some metal when the object to be tested was scraped along it, leaving a colored streak. This streak was then compared with the streaks left by test strips of metal of known composition to determine its composition.

A cupel was a refractory crucible made from bone or wood ash. In later times, beech ashes were preferred. The carefully purified ashes were moistened and molded into the desired form. The cupel was heated to dry it thoroughly. If the cupel was not properly made, it could break in use with the loss of the test sample. The ore sample or material to be tested was reduced to a powder, and some form of lead was added, together with certain additions that handled known impurities. The cupel was heated in a furnace to fuse the lead, and the sample was stirred with a wooden stick, which added some carbon. In this step, any gold or silver in the sample dissolved in the liquid metal, separating it from matter that would not dissolve. The solubility of gold and silver in lead, and the insolubility of iron, copper and zinc is the basis of the process. This rejected matter was then discarded, and the metallic mass brought to a higher temperature by blowing the fire with bellows. This raised the temperature to the point where litharge, PbO, formed rapidly. The litharge was pulled out as it was formed, and finally only a metallic drop remained, which was the purified gold and silver.

This assay process was called cupellation, which separated the gold and silver from the usual adulterants. Unfortunately, it did not part the gold from the silver, which required other means. It did, however, reveal the content of precious metals, and the touchstone could then determine the relative amounts of gold and silver that were present, and through this the value of the sample. Evidence of cupellation was found in excavations of the Athenian silver mines at Laurium, so it dated from at least the 6th century BCE. These mines produced silver-bearing lead.

The same process was applied in mining, carried out in a much larger cupellation furnace, to separate gold and silver from the ore. It is still used to desilver lead, which can be economically done because of the value of the by-product silver. Pure lead is easily obtained by reducing the litharge with carbon, as described above.

Other processes with the same end effect are amalgamation, dissolving in mercury followed by distillation, described in the article on mercury, and cyanidation, leaching with dilute cyanide solution to dissolve the gold as a soluble cyanide complex, followed by precipitation. Cyanidation is the currently favored method of gold recovery, since it works with very lean ores.

Miscellaneous Uses of Lead

The uses covered in this section are: sounding, glass, pottery glazes, pewter, fulminates, type, bearings, soldering, fusible alloys and bullets.

An interesting use of lead that depends only on its density was the determination of the depth of water at sea by means of a sounding with a lead line. The leadsman stood on the chains, a platform on the side of the ship, on the weather side (that is, the side from which the wind was blowing), with a hand lead line, 25 fathoms (150 ft) long. This consisted of a hemp line with a lead plummet, weighing about 7 lb, attached to one end by a rope or leather device. Certain fathom distances were marked by cloth or leather markers that could be individually distinguished. The leadsman cast the plummet ahead, so it would be vertically beneath him when it reached the bottom, and called out the mark that was observed in a traditional manner. The first mark was at 2 fathoms, so if it appeared at the surface, the leadsman would sing out "By the mark, twain!" This wasn't much water for an ocean-going ship, but on the Mississippi it was enough, and Samuel Clemens took it for a pen name.

There were marks only at 2, 3, 5, 7, 10, 13, 15, 17 and 20 fathoms. If the lead bottomed with the water in between marks, say at six fathoms, the leadsman would call instead "Deep, six" to let the helmsman know that it was not an exact mark, but an estimated one. The plummet had a recess in the bottom that was filled with tallow. When it struck bottom, it would pick up any loose material so the nature of the bottom would be known (for anchoring purposes). If it came up clean, the bottom was rock. In these last two paragraphs, "lead" is always pronounced like the metal.

Glass is a fused mixture of silica with some addition,a flux, to lower the melting point of the mixture. When it is cooled, the substance finds it very difficult to attain the correct structure for crystalline silica, which is complicated, in view of the tangled mass of long molecules that results from the typical formation of long chains of silica atoms. If the added material is Na2CO3, the result is soda or crown glass with an index of refraction of about 1.5. A glass made from K2CO3, PbO and ground flints is called flint glass, and in extreme cases can have an index of refraction as high as 1.890. This glass is used in decorative cut glass. Too much lead makes the glass unstable to crystallization. In the making of clear glass, impurities in the raw materials must be eliminated.

Pottery is made from clay that is moistened and shaped. It is then fired in a kiln to a stage called biscuit, in which the clay has undergone a chemical reaction so that it is rigid and brittle, and no longer affected by moisture. Now a glaze can be applied on the surface in the form of a liquid, decoration added, and the item given a finish firing, that fuses the glaze into a smooth coating for the piece. The glaze is essentially a coating of glass. Early glazes used sand, lime, powdered galena and salt. Salt melts at 801°C, but the glaze would seem to be soluble. The Romans introduced lead glazes, that incorporated PbO and other lead compounds. One lead glaze gave the red terra sigillata ware, another a popular blue-green glaze. The technique was lost in the west, but preserved in Constantinople, and was re-established in the 11th century. A tin glaze from Mesopotamia was popular in medieval times, which consisted of PbO, SnO, NaCl and potassium glass. In Staffordshire in England, glazes, many of which used lead, played an important role in the evolution of tableware.

Pewter is a soft alloy of tin that was used for pitchers, platters and pots, a sort of inexpensive silver that was easily shaped and worked. A traditional alloy was 80 Sn, 15 Pb, 5 Sb. The lead was added to reduce the cost of the metal, the antimony to harden it. If the lead content is objectionable, as it is if food is to be in contact with the pewter, lead-free pewter of 85 Sn, 15 Sb or 85 Sn, 6.8 Cu, 6 Bi, 1.7 Sb can be used.

The first fulminating compound seems to be mercury fulminate, discovered by Howard in 1799. "Fulminating" comes from the Latin fulminis, "lightning," and describes compounds that explode violently when shocked or heated. They are used to detonate high explosives like dynamite or TNT that require a detonating shock wave to explode. Gunpowder in a restricted volume can also be set off by fulminates. A small percussion cap containing a fulminating compound at the end of a cartridge explodes when struck by the firing pin, igniting the main propellant charge.

Lead azide, Pb(N3)2, is another useful fulminate that is an alternative to mercury fulminate. It is a salt of hydrazoic acid, HN3, which freezes at -80°C and boils at 35.7°C, becoming an explosive gas. The azide ion has the structure -N=N+=N-, where the (+) represents a partial electron deficiency, and the (-) a partal electron excess. The radical is actually a resonance structure, and cannot be properly represented by a single structural formula. When connected to an unsaturated carbon, the added latitude for the electrons lowers the energy and the radical is not as unstable. When connected to a metal atom, the radical is unstable and can decompose to nitrogen. For example, 2HN3 → 3N2 + 2H2 + 113,200 cal. Sodium azide can be made from sodamide and nitrous oxide, and hydrazoic acid then obtained by treatment with sulphuric acid. Lead azide is the Pb++ salt of hydrazoic acid. The colorless needles are insoluble in cold water, slightly soluble in hot water, and very soluble in acetic acid. When heated to 350°C, lead azide explodes, giving lead, nitrogen and heat. The root "azo" comes from the French for nitrogen, azote. KN3, NaN3, AgN3 and TlN3 are also known. The potassium and sodium azides are probably not as unstable as the last two. While we are on the subject of fulminates, NCl3 and NI3 are very sensitive, the latter exquisitely so, since touching it with a feather is enough to set it off. Such fulminates are not practically useful, of course.

Lead styphnate, the lead salt of 2,4,6-trinitro,1,3-dihydroxybenzene, or lead 2,4,6-trinitroresorcinol, is now used, in combination with other ingredients (to give it more bang), in electrical detonators, replacing mercuric fulminate. It is non-hygroscopic and stable, having a positive heat of formation, but very sensitive to flame or spark. It is unusually sensitive to static electricity, so it is dangerous to handle. It is insensitive to nuclear radiations.

It is popularly believed that Johann Gutenberg (1400-1468) of Mainz invented the printing press and produced the first printed book, an edition of the Bible, in 1456. Of course, this is true in essence, but the full story is more complicated and more interesting. The printing press, paper and ink were all known and used at the time. Books and illustrations were already produced. Paper was what made printing possible. Parchment and similar media, which had been used in the West since papyrus became unavailable, are not suitable for printing, because they are too uneven to receive a good impression, and besides were much too expensive for mass use. Paper made from textile fibres was invented in China, and was perfect for the press. This is perhaps the most important Chinese invention that actually reached the West and found actual use without essential modification.

Gutenberg's contribution was the process of making type of good quality and sufficient amount to be used in general printing, so that a plate could be assembled from fonts of type, and the type re-used after printing, a much more practical process than creating individual blocks for each page. The method he chose was to cast the type in metal, using a carefully made die to stamp out the molds. The problem here is that most metals shrink on freezing, a problem that would be severe with the small castings. The solution was type metal, an alloy of 62 Pb, 24 Sb and 14 Sn, that not only melted at a low temperature, but also expanded slightly on cooling, so that it would fill the molds accurately. It was also a hard alloy, so the type would stand up to repeated use. Linotype machines used an 85 Cu, 15 Sb, 5 Sn (or 82, 15, 3) alloy. These machines, as the name indicates, cast a complete line of type at once after the operator had typed in the letters. Gutenberg's invention was the method of manufacturing type. From there it was an easy step to the printing that made books available to the general public.

The design of bearings, where one metal slides on another, has always been important in technology. The governing principles are that the two metals in contact must be different, and that the interface must be lubricated in such a way that the two metals are kept from actual contact. There is another class of bearings in which the contact is a rolling one, where different problems arise. We'll treat only bearings with sliding motion here. Such bearings have the advantage of being able to carry heavy loads, of being of simple construction, and are well adapted to carry away excess heat by conduction.

A lightly loaded bearing is loaded up to 1000 psi, moderately loaded up to 2000 psi, and a heavily loaded above 3000 psi. A low surface speed is less than 10 fps, moderate 15-20 psi, and high above 30 fps. Resistance to seizure is called "score resistance." A high score resistance is necessary at high surface speed, or if the bearing is started and stopped frequently, when the lubricating film may be squeezed out. Score resistance and compressive strength are inversely correlated.

An excellent form of lubrication is hydrodynamic, where the oil film is drawn into the bearing by the movement. The oil may be supplied by a pressure system, or by a wick dipping into a reservoir. There is no metal-to-metal contact at all, and the friction is very low. With this kind of lubrication, the oil supply must not be interrrupted, and must be kept very clean. Such bearings are used in automotive engines (usually with soft-metal bearings), and were originally used for the journals of railway axles, where the load was heavy. In this latter use, they have been replaced by roller bearings, not to reduce the friction, but to overcome the dangers of poor maintenance. A roller bearing requires no regular maintenance and can largely be forgotten (a valuable feature on American railways), while the oil reservoirs for a hydrodynamic bearing must be kept filled and clean, and the wick in place. A hydrodynamic bearing has larger friction before the oil film is established, but when running is as antifriction as a roller bearing.

A simple, traditional bearing is 60 Cu, 40 Zn brass on steel, lubricated by grease that sticks to the surfaces because of its viscosity. Before the petroleum age, tallow was often used as the lubricant. A few percent of lead improves the anti-scoring property. The lead does not dissolve, but forms small drops of soft material. Bronzes with high lead content, say 70 Cu, 10 Sn, 20 Pb, have good score resistance, but less compressive strength. These are called leaded bronze bearings.

For low loads, the tin-based bearing alloys called "babbitt" are superior. Isaac Babbitt was the original patentee. A typical alloy is SAE10: 90 Sn, 5 Sb, 5 Cu. These alloys depend on the formation of an intermetallic compound SbSn which forms hard, antifriction "cuboids." Similar alloys are lead-based, such as SAE14: 80 Pb, 10 Sn, 10 Sb, or "white metal," 75 Pb, 19 Sb, 5 Sn, 1 Cu. The popular and inexpensive Magnolia Metal was 80 Pb, 15 Sb, 5 Sn, or 90 Pb, 10 Sb. The lead alloys can carry less load than tin babbitt, because they are softer. Tin is expensive compared to lead, so lead alloys are cheaper. These soft bearing alloys are often backed with a steel shell to support them and to prevent them from extruding. There are also copper-lead bearing alloys, such as 60 Cu, 40 Pb. The lead does not dissolve in the copper, and the soft crystallites smear out to make a good bearing surface.

Lead forms the intermetallic compound Pb3Ca. It dissoves in lead at 316°C (just below the melting point) and precipitates out at about 20°C. The precipitate hardens the lead from Brinell 7 to Brinell 19. This is called precipitation hardening, and makes a stronger bearing material.

Soldering is a valuable method of joining metals, that can be done with solders that melt at a conveniently low temperature, in the region of 200°C to 300°C, so that an electric soldering tool can be used. These are sometimes called "soldering irons" though always of copper, not iron. Soldering depends on the liquid solder's wetting the two surfaces to be joined, which requires that the surfaces be clean and free of corrosion. A flux is used to maintain the surfaces clean, and is absolutely necessary to a successful solder joint. Lead on its own does not wet metals well, but when tin is added the alloy wets copper, silver, gold and tin very well. Besides, it melts at a lower temperature and so is easier to use. The frequently-used "60-40" solder is 60 Sn, 40 Pb, which begins to melt at 183°C and is fully melted at 190°C. The 50-50 alloy was called "common solder" and is a compromise. A 40 Sn, 60 Pb low-tin plumbing solder begins to melt at 183°C and is fully melted at 238°C. The wide temperature interval in which the solder is "pasty" makes a "wiped" joint easy. An alloy 62 Sn, 38 Pb melts and freezes sharply at 183°C, and is called the eutectic alloy. Eutectic solder is excellent for electronic work.

A schematic equilibrium diagram of the tin-lead system is shown at the right. The vertical scale is temperature, the horizontal is composition, from 100% Sn on the left to 100% Pb on the right. The component α is a solid solution of a little Pb in Sn, while β is a solid solution of Sn in Pb. In the liquid state, Pb and Sn mix uniformly in any proportion. In other regions of the diagram, there is a mixture of two phases of different composition and structure. The mixtures of Sn and Pb melt at lower temperatures than either pure component, a common happening. The melting point is a minimum, 183°C for a composition of 38% Pb, 72% Sn, which we have noted is called the eutectic. This is from Greek, and means "easy melting." If we start with a homogeneous liquid of this composition and cool it, it freezes abruptly at 183°C, and the temperature remains constant until all the liquid has solidified. The result, the eutectic solid, is a finely layered strudel of α and β components.

If the liquid is richer in lead than the eutectic, some α phase begins to crystallize out when the line joining E and the melting point of Pb is reached. As it cools further, the α phase is joined by some eutectic material, and when it reaches the horizontal line the last to solidify gives eutectic. The same thing happens if the liquid is richer in tin, but now the β component is mixed with eutectic. At the far right and left of the diagram, a pure α or β phase crystallizes out. An understanding of this diagram will explain the behavior of solder well.

Rosin, or a solution of rosin in alcohol, is a noncorrosive flux that can be used with copper, silver, gold or tin. It will not remove oxides, but protects the surfaces when heated and causes the solder to flow easily. There is an activated rosin that is somewhat more effective, and only slightly corrosive. Rosin flux, in general, does not have to be removed. For soldering iron and steel, a more active flux that removes oxides is necessary, and zinc or ammonium chlorides are satisfactory. They are corrosive, and must be removed after use, best by washing with weak hydrochloric acid and then water. One popular flux is mixed in petroleum jelly so it will stick, and called "Nokorode" although it will, of course, and should certainly be avoided for electronic work. The solder wire generally used in electronics includes a rosin core, so it generally does not need added flux.

Hard, or "silver" solders melt at around 700°C, and contain no lead. A typical alloy is 45 Ag, 30 Cu, 25 Zn. Soldering with these alloys is called brazing, and is generally done with a gas torch and a special flux.

Lead is also found in fusible or low-melting alloys. One use for these was for fusible plugs in the crown sheets of boilers. The crown sheet is the sheet directly above the fire. If it is not kept covered by water, the metal can overheat, distort and fail disastrously. A fusible plug melts if not kept below its fusing temperature by water, and allows a jet of water and steam to extinguish the fire, besides warning the crew. Fusible elements are also used in automatic fire sprinkler systems, and, of course, in fuses that melt when an excessive electric current passes through them.

Low-melting alloys are about 50% bismuth, with various amounts of lead, tin and cadmium, depending on the melting point required. Rose's alloy, which melts at 100°C, is 50 Bi, 28 Pb, 22 Sn. Wood's alloy, melting at 71°C, is 50 Bi, 24 Pb, 14 Sn, 12 Cd, or 50 Bi, 25 Pb, 12.5 Sn, 12.5 Cd. Wood's alloy is used for casting teaspoons for entertainment purposes. These alloys can also be used as patterns for investment casting, and for hobby and modelling purposes. Note that the compositions of such alloys vary a little, but all will have similar properties.

Lead is used for bullets and shot, chiefly because of its high density. Shot alloys have been given as 99.8 Pb, 0.2 As, or 94 Pb, 6 Sb. The latter is the common antimonial hard lead, also used for battery plates. Lead was too expensive for use in cannonballs. A smooth-bore hand weapon uses a spherical bullet. Any other shape would tumble excessively and produce an inaccurate, short range shot. A musketeer often carried his own lead, crucible and bullet mold and made his own bullets over his campfire. The mold was often furnished with the weapon, so the bullets would fit. Rifle bullets are made to rotate by the helical lands or rifling in the barrel, so they can be made longer and heavier. Originally, however, they were round like any other bullet, and were usually wrapped in a lubricated patch that engaged the rifling. A "round" consists of bullet, wad, powder and cap (for a percussion round). These were packaged for convenience in a "cartridge" made of cardboard. Only later do we find a brass cartridge case containing the powder, with the bullet crimped at the end, and a percussion cap in the base, that can be inserted in the chamber and fired with no other complications.

Shotgun cartridges, incorrectly called "shells," are still cardboard, with a brass piece containing the percussion cap crimped on one end. The spherical shot was at one time (and maybe still is) made in shot towers, one of which is to be seen beside the Thames today, converted to some other use. Molten lead was poured into a sieve at the top of the tower, and the falling drops assumed an accurate spherical shape and cooled by the time they reached the bottom, where they only had to be picked up and sorted by screening.

References

Properties of lead and its compounds are passim in handbooks such as the Handbook of Chemistry and Physics, Lange's Handbook of Chemistry, and Kent's Mechanical Engineer's Handbook.

G. Agricola, De Re Metallica (1556). H. C. Hoover and L. H. Hoover, transl. (New York: Dover, 1950). Description of cupellation.

J. L. Bray, Non-Ferrous Production Metallurgy, 2nd ed. (New York: John Wiley & Sons, 1947), Chapter 14.

R. A. Higgins, Engineering Metallurgy, 3rd ed. (London: The English Universities Press, 1971). Equilibrium diagrams.

W. N. Jones, Inorganic Chemistry (Philadelphia: Blakiston, 1949), Chapter 31.

P. Kemp, ed., The Oxford Companion to Ships and the Sea (London: Oxford University Press, 1976). Sounding.

Image of galena kindly furnished by Amethyst Galleries, Inc.. This is an excellent website and specimens can be purchased online. This is probably the best mineral website, and the company should be supported for making it available.


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Composed by J. B. Calvert
Created 10 November 2002
Last revised 29 May 2004