November 2007

Metallurgy

History of metal casting (Part 3)

By F. Habashi

Casting of Cannons

With the discovery of gun powder around 1250 AD, European bell founders turned to guns. During the sixteenth century, the production of cannons increased as armies came to appreciate their destructive power. As a result, monarchs became interested in casting cannons. For example, Maximilian I, the Holy Roman Emperor, established an arsenal in Innsbruck in 1505, now a museum, while Henry VIII (1491-1547) established an Ordinance Depot (later Royal Arsenal) at Woolwich in 1518. The inadequacies of early iron resulted in the use of bronze as a material for cannons. Early cannons were cast hollow, using cores to create a rough but serviceable barrel that could be finished and smoothed by hand. However, the bores were often not round, which caused wide variations in range and accuracy because of the difficulty in aligning the core to the barrel during casting.

It was Johan Maritz, Master Founder at Burgdorf, Switzerland, who, in 1713, designed a machine tool that was capable of boring cannon barrels from a solid casting. The method was time-consuming; however, it produced cannons with round, smooth, and parallel bores. When the Dutch Ordinance decided to adopt his technique in 1747, Maritz moved to the Netherlands State Gun Foundry in The Hague, Europe’s leading gun producing facility at the time. Maritz’s sons later introduced the technique to France and Spain.

Monge and casting of cannons

In the early days of the French Revolution, the serviceable artillery pieces were very small. In 1793, Napoleon appointed his friend, the mathematician Gaspard Monge (1746-1818), to lead a special commission to oversee the production of artillery. Monge abandoned the use of clay in favour of sand to decrease cost and improve the quality of the casting. He established gun foundries in churches and on farms throughout the French countryside, and instituted training programs intended to familiarize workers with the techniques and skills needed to implement the new methods to be used in making cannons. France produced 7,000 pieces for the army and navy in 1793-1794. Monge’s contribution to the advancement of cannon production was recognized on a French stamp issued in 1990.

Iron cannons

Bronze possessed greater tensile strength than iron and could withstand bore pressures more readily when the weapon was fired. On the other hand, it was much more expensive than iron. This drove the research into improving the production of iron. In 1795, John Wilkinson (1728-1808), the iron master of Soho Works in Stoke-on-Trent in England, designed a small shaft furnace that became known as a cupola in which he melted pig iron and other material to produce cast iron of better quality. With these improvements in the quality of iron, bronze was gradually replaced by iron. His high-quality cannons were the reason for the British Navy’s superiority in battles.

The first cannons were the bombard type but these were later replaced by barrel-type cannons that were bigger in size. The early projectiles used were stone balls. Then, in 1373, iron shot came into use, but only to a small extent due to the high cost; they became widely used around 1600. Explosive projectiles were later used.

Casting of hollow cannons

In casting the early cannons, an octagonal piece of timber known as arbor was used, around which straw rope was wound. Loam was pressed into the straw and smoothed by a strickle board, forming the outside of the cannon. Trunnios were then applied and more loam added. The entire mould was then bound with iron bands and baked over a fire; the whole assembly turned on its arbor until completely dry. After cooling for a few days, the arbor was removed. A chaplet was used to hold the core in place. The breech was usually moulded separately and the whole job was assembled, breech down, in a pit before the furnace.

Boring of solid cast cannons

A solid cannon was firmly secured horizontaly in a water-powered machine designed specifically for boring. An iron boring bar with a steel cutting tool was advanced into the bore of the piece as the gun blank was turned by the machinery. A series of cutting heads were used; the first was small and subsequent heads increased incrementally in size until the desired bore diameter was achieved. Boring typically lasted for a period of days.

Boring cannons and the theory of heat

In 1798, while manufacturing cannons for the Bavarian military, Count Rumford (1753-1814) observed that grinding used to hollow out the barrel produced huge amounts of heat, which continued to flow with the borings as long as the grinding was maintained. According to the theory at that time, the stress of rubbing surfaces together forced some caloric fluid to be pushed out from between the atoms, and it appeared as heat. Rumford, however, noted that the piece of metal must have contained an apparently infinite amount of caloric fluid. He therefore came to the conclusion that the friction of grinding set the internal invisible microscopic particles in the metal in motion, resulting in heat emitted as atoms came into contact. Rumford’s work did not however kill the caloric theory. It was the physicist James Prescott Joule (1818-1889) who in 1847 conclusively supported Rumford’s views—a turning point in the history of science.

Continuous Casting

Iron from the blast furnace was allowed to flow in sand moulds prepared on the ground and left to cool. When solidified, the pigs were then removed and the moulds reused. This process is no longer used because it involves extensive manpower. Continuously moving casting machines were then introduced; by the time the molten pigs were moved from one end to the other, they were solidified and dropped away from the moulds in the form of pigs, which were then used to make cast iron in the cupola.

The bulk of the pig iron is transferred in the molten state to the steelmaking plant. Steel was usually cast in ingots and when solidified, it was removed and put in a furnace to be heated to a determined temperature before transporting it to the fabricating mills. This meant handling a batch often during the cooling step. Introducing continuous casting solved this problem in 1960s. In this process, the molten metal is continuously fed from a reservoir and is allowed to solidify rapidly in a mould so that at any given time, there is only a small pool of molten metal present at the top of the ingot. As the solidified ingot emerges, it is grasped by a set of rolls which regulate its downward progress. The contraction of the freezing metal causes it to pull away from the mould walls. Beneath the pinch rolls is an oxyacetylene flame, which cuts the emerging ingot into convenient lengths. A few years later, the same technique was introduced in the copper and aluminum industries.

Epilogue

Metals were cast by ancient people to produce ornaments, primitive agricultural tools, or arrow heads for hunting. The introduction of copper, bronze, and later iron was so important in the history of man that epochs are named Bronze Age and Iron Age to emphasize the shift from the Stone Age. The close ties between casting metals and pottery indicate that the two arts must have developed simultaneously. It was the potter’s art of handling suitable clays and their proper firing that gave the foundry man the crucible for holding molten metal.

Centuries later, when gun powder was discovered, casting changed hands from monks and church officials, who were casting bells, to monarchs, who became interested in casting cannons. In times of war, bells were usually confiscated and cast into cannons. The artistic ornaments and statues conserved in to museums, the gigantic bells, and the monstrous cannons that have been cast throughout history are a testament to the skill of the metal founder. The new technology of continuous casting reflects the response of industry to the need for a fast and reliable method to satisfy the requirements of a developing society.


Suggested Readings

Aitchison, L. (1960). A History of Metals. New York: Interscience.

Berenguer Rodriguez, J., & González, L.A. (2004). Copper Art in the Andean World. Santiago: Museo Chileno de Arte Precolombino.

Biringuccio, V. (1943). De La Pirotechnia (published in 1540; English translation by C.S. Smith and M.T. Gnudi). New York: American Institute of Mining and Metallurgy.

Derry, T.K., & Williams, T.I. (1960). A Short History of Technology from the Earliest Times to AD 1900. New York: Dover Publications.

Johnson, R.E. (1993). The changing technology of artillery manufacture. CIM Bulletin 86, 156-161.

Habashi, F., editor (1994). A History of Metallurgy. Québec City: Métallurgie Extractive Québec/Laval University Bookstore.

Knauth, P. (1974). The Metalsmiths. New York: Time-Life Books.

Leibbrandt, A. (2001). Civilization and Copper—The Codelco Collection. Santiago: Corporación Nacional del Cobre.

Simpson, B.L. (1948). History of the Metal-Casting Industry. Des Plains: American Foundrymen’s Society.

Tylecote, R.F. (1976). A History of Metallurgy. London: Metals Society.

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