Bronze & Brass
As the ancients toyed with fire they created glasses and ceramics and discovered several metals. In antiquity only about 7 elements (gold, copper, silver, lead, tin, iron and mercury) were recognized as being metals. Although their ores were used to create alloys – arsenic, antimony, zinc and bismuth were not determined to be unique metals until the 13th or 14th centuries AD. With the addition of the discovery of platinum in the 16th century (ignoring the Incas who were apparently smelting it earlier) that makes a total of only 12 unique metals (out of 86 known today) mankind recognized before the 18th century AD. Gold and copper were the first metals to be widely used. Because of their low melting points the first metals to be smelted however might have been tin and lead. Lead beads unearthed in Turkey have been dated to about 6,500 B.C. Galena (lead ore) is fairly common and widely dispersed whereas tin oxide (as in cassiterite SnO2) is relatively rare. Neither gold, copper, tin nor lead however were as influential in the course of human events as was bronze; a hard alloy which upon occasion has been mixed from all four simultaneously.
The progression of mankind’s technological advancement is broadly categorized into the Stone, Bronze and Iron Ages, the Renaissance, the Industrial Age and presently what some are pleased to refer to as the “Information Age”. Erudite scholars from some future millennium however may look back and call ours simply – the “Plastic Age”. The “Bronze Age” is subjective term depending upon location or culture but it implies the mining and smelting of copper that has been hardened with an alloy to make bronze weapons and tools. The Bronze Age occurred around 3,150 BC (or BCE) in Egypt, 3,000 BC in the Aegean area, 2,900 BC in Mesopotamia and about 1,700 BC in China. Gold, copper, lead (and silver to a lesser degree because it is found in the same galena ore which produces lead) metalworking dates from about 6,000 BC onward and so actually predates the so called ‘4th millennium BC – Bronze Age’. The first gold and silver coinage comes much later in the 7th century BC in Lydian, Persian and Phoenician cultures. The first exchangeable copper coinage might have appeared in Greek and Roman societies.
* Both copper and silver ions are germicidal and can inhibit or kill bacteria in water. The ancient Egyptians employed copper medicinally. Some modern hospitals incorporate copper in the form of water faucets and doorknobs.
We usually think of bronze as being an alloy of copper and tin and brass as being merely an alloy of copper and zinc but as usual the real picture is more complicated than that. There are many alloys of copper that are called bronze or brass and occasionally the distinction is not clear. There is tin bronze, leaded tin bronze, manganese bronze, silicone bronze, phosphor bronze, aluminum bronze, arsenic bronze and beryllium copper. ‘Red brass’ copper alloy historically used for casting cannons actually contains about 5 times more tin than it does zinc. Many modern coins (like the American dime, quarter and half dollar) have a copper core sandwiched between two layers of cupronickel (an alloy of 75% copper & 25% nickel). The Swiss franc and American nickel (5 cent piece) are actually solid homogenous cupronickel. There are a large number of distinctly recognized mixtures of brass as well. ‘German silver’ (or nickel silver or nickel brass) is similar to afore mentioned cupronickel but contains about 2% zinc. ‘Muntz metal’ (copper, zinc and a trace of iron) is an alloy thought up a couple of centuries ago to provide a cheaper ship hull protective sheathing than the copper one which it replaced. ‘Nordic gold’ is an alloy of 89% copper, 5% aluminum, 5% zinc, and 1% tin – that is used in several Euro coins.
The first bronzes were arsenic bronzes. While this might be attributable to the fact that the copper ores then smelted usually contained some indigenous arsenic, at some point early metalworkers deliberately added more arsenic to make a harder alloy. Arsenic bronze is much harder than the original – excessively ductile copper and allows for the creation of useful tools, weapons, body armor and sculptures that will stand up under their own weight. At some later point in history, tin gradually supplanted arsenic as the preferred alloy for bronze. Tin bronze is not harder or mechanically superior to arsenical bronze. It is likely that tin ore (which was scarce and required many civilizations to acquire it by trade) produced an alloy that required less work hardening to produce a sharp sword or an alloy that would fill a casting faithfully without leaving voids. Arsenic sublimates (does not melt) at a temperature lower than molten copper and some arsenic oxide could be lost during casting. Arsenic vapors are unhealthy and can attack the eyes, lungs and skin. The use of tin probably afforded more control over the forging and casting processes.
* Arsenic (atomic element #33) is a metalloid that is used to harden both copper and lead. Modern lead/acid car batteries usually feature some arsenic as well as some antimony within their lead components. Long called the “Poison of Kings and the King of Poisons” arsenic is also a common and widespread groundwater contaminant. Arsenic compounds were used as a vesicant (blistering agent) and or vomiting agent in Lewisite and Adamsite gasses used after WWI. Arsenic is also used in the green pressure treated wood preservative known as CCA (Chromated Copper Arsenate). Within CCA copper acts to slow the decay caused by fungus and bacteria, arsenic kills insects and chrome just helps bind or fix the other two to the wood. When used as a discreet poison the “poudre de succession” is not deadly in small amounts but can stay in the body and accumulate before it becomes lethal. Two early developed analytical test in forensic toxicology were concerned with determining the presence of arsenic, namely the Marsh test and the Reinsch test. Significant concentrations of arsenic in ground water are found in parts of New England, Michigan, Wisconsin, Minnesota, both Dakotas, Bangladesh, Vietnam, Cambodia, and China.
* In ancient times lead was too pliable or ductile a material to make useful tools but that very characteristic allowed the Greeks and Romans to hammer and roll plumbing pipes to conduct water. In Rome lead was used to line water cisterns or to pipe water to public drinking fountains or into the homes of the very rich. In Rome the possibility of lead poisoning would seem to have been greatly reduced due to calcium buildup within the pipes. Rome sits upon or near large limestone and travertine deposits. A better source of lead poisoning if one were to be elected would be from lead dinnerware and acidic foods. Wine for instance can easily leach toxic lead from goblets and cups.
* Incidentally, bronze swords were often preferable to wrought iron swords. Even in Roman times, officers commonly carried bronze weapons while the rank and file carried iron weapons. Perhaps bronze swords were superior or simply more prestigious than their iron counterparts because they looked less crude and did not rust. The Hittites (c.2000 – 1200 BC) are generally regarded as being the first iron-smiths. Although their iron weapons were less brittle than hardened bronze weapons, these still had to be beaten or wrought from a bloom of roasted, not melted ore. The inability to cast iron with a socket complicated the attachment of spear and arrow heads to their shafts. It took about 3,000 years for furnace smelting technology to progress from copper melting temperatures to iron melting temperatures. The first iron ore to be exploited was probably “bog ore” – a precipitate of iron oxide found in marshy areas, created by bacterial action and the decomposition of iron minerals. Gradually the mining of rich hematite and magnetite ores occurred. The Greeks used wrought iron beams in the construction of the Parthenon (between 447 – 432 BC). The Romans occasional used T-shaped wrought iron girders in construction (as in the Baths of Caracalla). Eventually it was realized that the carbon from charcoal created a stronger iron (steel).
The Greeks were masters of bronze casting. Although the Greeks probably cast as many bronze sculptures as they chiseled from stone, the stone statues have remained where the bronze statues have not. Every time a new war came along, bronze statues were hacked to pieces to provide the valuable metal needed to forge new weapons and body armor. Very few Greek bronze statues have survived the ages and those that have been discovered in modern times (as in the image above – excavated in 1928) have been found underwater. The Greek “Riace bronzes” date from about 460-450 BC and were discovered by a snorkeler just offshore of Riace, Italy in 1972. A link is provided to this website because its fine pictures of the Riace bronzes can be enlarged.
Bronze alloys used for casting have the innate ability to expand slightly before they set and therefore all the fine nooks, crannies and scratches inside a mold are filled in with detail. Life-sized hollow sculptures were made by a process known as the “lost wax process” (or as “investment casting” in jewelry-making or industrial vernacular).
It has not been determined if the horses of the “Triumphal Quadriga” are of Greek or Roman origin. They are presumed to date to the 4th century BC and were stolen from the hippodrome in Constantinople where they had long resided, by Venetian troops following the Fourth Crusade (1202 -1204). Napoleon stole the horses from the Venetians in 1797 and took them to Paris but they were returned to St Mark’s Basilica in Venice following the Battle of Waterloo in 1815.
The iconic Greek Corinthian bronze helmet is an enigma to modern intellectuals because they can’t determine how it was constructed. Our understandings of the construction of other helmet designs of the period are less controversial. Neither the process of casting nor forging by hammer stroke alone adequately explain how the classical Corinthian helmet was built. The best explanation seems to be that it was a product of both.
Most bronze or brass alloys are denser and heavier than iron or steel. In the 17th century almost all naval guns and terrestrial cannon were cast in bronze. Bronze was the best material for the purpose but while being more durable than iron, it was also more expensive. With the beginning of the 18th century, technology allowed the displacement of bronze artillery with more affordable cast iron pieces needed to supply growing armies and navies. The weight of cannon and their placement aboard fighting ships (heaviest at the bottom) was an important consideration but weight was perhaps a more important concern for armies that had to tote them over hill and dale, streams and rivers. Early French cast iron naval guns were notoriously dangerous and exploded with frequency while British, American, Swedish and Russian cast iron naval cannon were usually much superior.
The famous or iconic American Liberty Bell is an interesting story in bronze failure. The metal in the Liberty Bell was cast not once, but three times: first in an English foundry when the bell was commissioned by the Pennsylvania Assembly in 1751, and then twice again by American foundry workers John Pass and John Stow. The bell has a diameter of 12’ around its rim and weighs 2,080 lbs. The bronze composition consist of 70% copper, 25% tin and a remainder a mixture of lead, zinc, gold, silver and arsenic. The bell gained its moniker “Liberty Bell” from zealous abolitionist in the 1830’s, not from association with the Revolutionary War. The one ton bell traveled or toured a lot considering its weight and there is disagreement over when its crack began. Vigorous ringing encouraged a hairline fracture in the brittle alloy to grow into a wide crack. The Liberty Bell rang last on Washington’s birthday in 1846, its sound after that no longer being acceptable.
The first brasses seem to have appeared somewhere around 500 BC and are sometimes referred to as calamine brass (calamine is zinc ore containing zinc carbonate or zinc silicate). In early brasses calamine ore was introduced to molten copper and the zinc was readily absorbed, producing an attractive and useful alloy. Zinc melts at 787 °F which is a temperature not much greater than that required to melt lead and which can be produced by a simple campfire. Zinc boils and turns to vapor at 1,665 °F (907 °C) which is still lower than the 1,984 °F temperature needed to turn copper into a liquid. For a long time not appreciated as a metal because heat caused it to escape as a vaporous gas, zinc production did not begin until about the12th AD century in India, the 16th century in China and (in large scale production) after 1738 in Europe. In modern day zinc smelting, zinc sulfide is first roasted into an oxide called ‘zinc calcine’. From there either electrolysis or any one of several complicated processes involving sintering (the electrothermic fusing of powders) or even the distillation of zinc fumes might be employed to retrieve the metal.
What usually distinguishes a brass from a bronze is the presence of zinc and a brighter or attractive golden color. Brass is a softer, more malleable alloy than bronze and has some properties that make it uniquely desirable for some applications. Brass is used in bearings, gears, valves, locks, keys, doorknobs and clothing zippers because it has a low friction coefficient. Brass does not spark as other metals might when struck. Because of its desirable acoustic qualities and malleability, brass is the favored material for several musical instruments – especially horns. Brass is the favored material in ammunition cartridge casings for a couple of reasons. First, brass has the capacity to expand and contract quickly. When a cartridge is fired in a firearm, the brass expands to fill the breach and prevents hot gasses from escaping rearward. The brass then contracts to allow the casing to be ejected. This action occurs quickly enough to allow for high cyclic rates of fire in machine guns. Also brass’s softness and low friction attributes work more fluidly and cause less wear in the firearm’s steel mechanism than would any other metal. While lead might be added to bronze to improve cast-ability, lead is added to brass to improve machine-ability. California mandates that manufacturers of brass keys employ no more than 1.5% lead within keys sold in California, or otherwise label the product as potentially hazardous.
Do it Yourself
If the ancients were able to smelt copper, iron, gold and silver gold eons ago it seems reasonable that a lone individual should be able to duplicate that feat today. Someone attempting to melt copper for instance will soon realize that it is not a simple task and that it takes concentrated energy to accomplish. Above is an image of the bottom half of a homemade crucible furnace, the top has been temporarily removed. In the center of a charcoal fire sits a crucible made from a scrap of square steel tubing that has had a bottom and two links of chain (for lifting) welded to it. On the right side a rusty steel pipe conducts forced air from a hair drier into the bottom of the fire.
Above is the mold for the same bottom section of this crucible furnace, made from a plastic flowerpot and some tin cans. A refractory mix was poured or tamped into the bottom 2” of the mold and allowed to dry, anchoring the wire reinforcement. The tin cans were placed and now the mold is ready to receive more refractory cement between the large can and the plastic flowerpot. Refractory is simply a building material that retains its integrity at high temperatures. The refractory used was a mix of sand, Portland cement, fireclay and Perlite. The ratios of the constituents used closely resembled this recipe.
Above is a downward image of the mold and a view of the finished result. Note that on the left a bolt passes through a small hole in the set (or dried) bottom layer of refractory. Presumably if the crucible were to leak during a cook, the molten metal should be able to run out the bottom and not be stuck in the bottom of the furnace.
A few notes about this furnace:
– Even while the crucible and metal it held were white hot inside, the lid could be removed with bare hands – if done quickly.
– Fire at this heat combined with the forced air is very destructive of steel crucibles, both inside and outside. Big flakes of iron oxide are almost guaranteed to sluf off and fall into and contaminate your precious metal. The best crucibles are made of porcelain or graphite.
– The forced air should enter the furnace at an angle to encourage a whirl or vortex within the fire.
– Although stoneware ceramics are fired and glazed at temperatures exceeding the melting point of aluminum, brass and copper, such ceramics cannot withstand such a rigorous acceleration in temperature. You can expect a stoneware coffee cup crucible to shatter in a matter of minutes in such a furnace.
– The interior dimensions of this furnace are a bit too small to acquire a useful copper-melting heat from coal or charcoal alone. There is simply not enough room for a crucible and enough charcoal at the same time. Propane or waste oil would be better fuels for a furnace of this interior dimension. These fuels can be introduced into the air pipe before it enters the furnace. In the case of waste oil (any used automotive oil, diesel fuel or vegetable oil), it can be gravity fed, its viscosity potentially reduced by a lighter volatile or fraction, regulated by a simple valve and / or forced by a little additional air pressure.
With a little effort an interested reader can find a wealth of information and instructables about crucible furnaces on the Internet. Here are a few links to help such a reader get started.
In this video the furnace is constructed of stacked firebricks. Brick furnace in snow.
This guy provides a good 3 part series on the construction of a backyard foundry. In this video however he constructs his own graphite crucible. Most people might simply purchase a graphite or porcelain crucible. It is not necessary for a novice to go through all this trouble, but the information presented is useful. “Making a Graphite Crucible“.
This video features a rather large furnace, requiring two men to handle the crucible. “A Brass Casting Demonstration“.
If there is a Hot Stuff part 3 to come in the future it will discuss ceramics and glass.