Cast iron is a form of iron alloy that contains about 2-4% of carbon and 1-3% of silicon, along with some other alloying materials. It is somewhat brittle and cannot be shaped by heating and hammering like wrought iron. However, it can be cast into shapes using sand molds. It also has good wear resistance and has some resistance to rusting as well.
Cast iron was known to the Chinese around 500 BC and was used by them to make pots, pans, farm tools etc. Almost 2000 years passed before it started becoming available to Western Europe in the 15th century.
Some blast furnaces can produce cast iron directly, simply by adjusting the amount of carbon absorbed by the iron ore. If not, cast iron is produced by heating pig iron back into a molten state, often along with a good quantity of scrap iron and steel (both of which have far less carbon content than pig iron). Limestone is also added as a flux to remove some of the other impurities, such as sulfur and phosphorus. The resulting alloy's carbon content is reduced to be about 2-4% and silicon to 1-3% and other alloying elements (such as manganese, nickel, chromium, copper, vanadium etc.) are added to change the properties of the cast iron as required. The molten cast iron is then poured into molds to solidify into the final shapes desired.
Since it is much cheaper to manufacture than bronze, many of England's navy cannons started to switch to use cast iron rather than bronze. This allowed ships to be fitted with more guns at a cheaper price. For instance, in 1570, the price of just the raw tin and copper needed for a single bronze gun cost about £60, whereas the cost of a complete cast iron gun (including raw materials and manufacturing cost) was about £20. The price of cast iron guns dropped even more, as the technologies for producing cast iron improved. By 1670, a ton of bronze cost about £150 in England, whereas a ton of cast iron cost £18. Another benefit of using cast iron was that the guns could generally be loaded with more gunpowder and therefore had more range.
However, early cannon were generally made of bronze or wrought iron rather than cast iron initially. In the first part of the 16th century, cast iron was not thought of as a suitable material to cast large guns from and bronze was preferred instead. The problem was that molding technology was somewhat simple in those days and the cast iron would often harden in the mold before all of it had been poured in. Also, bronze is more resistant to corrosion. Cast iron guns also had the problem that they would occasionally burst with no warning, whereas a bronze gun would slowly wear down. Therefore, most of the European navies used bronze guns initially.
Around 1625, during the reign of King Charles I of England, it was realized that if England's navy had to expand, there needed to be a cheaper way to make guns. At that time, the cost of bronze guns alone was about 35% of the cost of the entire ship. The Commissioners of the Royal Navy were directed to see if iron guns could be used instead. John Browne, who was then a royal gun maker, was one of the few people who showed interest in solving the problem and he delivered a set of six cast iron guns in 1626, which successfully passed the Royal Navy's specifications. Incidentally, John Browne came from a family of people who dealt with cast iron guns -- there are records of his father, Thomas Browne, obtaining a license to make cast iron cannon in 1589 and in 1609, he testified that he had delivered 469 tons of cast iron ordnance since 1591, which showed that he was a big armaments maker. In 1613, John Browne stated to a commission that he had four factories making cast iron cannon and employed 200 people and exported mostly to the Dutch. Later, he was granted a royal monopoly to provide iron guns to the Navy. His grandson George, was also a royal gun-founder for King Charles II.
As cast iron was a lot cheaper to make than bronze, therefore England's Royal Navy decided to go with cast iron cannon so that they could build more ships with their limited budget. Of course, there was the risk of cast iron guns exploding without warning, but the cost savings was determined to worth making the switch. Other countries followed them and soon, cast iron cannon became common around the world. For smaller firearms though, wrought iron or steel were still the materials of choice. In our next post, we will study how raw pig iron was converted to wrought iron or steel.