## 2 Metallurgical Background of Research
### Cast Iron  
A blast furnace had been used for the production of cast iron in China since 500 BCE [16]. Blast furnaces, fed by hot air blown into the lower part of the furnace, could reach temperatures about 1450 \({}^{\circ}\)C for smelting. The partially molten cast iron was fed into sand moulds, generating 'pigs' that could be re-melted [14]. The first blast furnace arrived in Britain from continental Europe [14] towards the end of the fifteenth century; during the sixteenth century, blast furnaces spread to most of western Europe. The first documented serious effort to use pig iron as ballast in the Royal Navy's warships was in the late 1720s [27; 28]. The modern European blast furnace was a large construction, consuming a considerable amount of charcoal, and it needed a continuous water power supply for operating the bellows [29]. Charcoal was replaced by coke as the fuel in blast furnaces from 1750 to 1770, and by 1770, coke furnaces were already common [30].  
Grey cast iron (ternary iron-carbon-silicon alloys) is characterised by the content of more than 2 wt% C as graphite particles, the increased carbon content reducing the melting temperature of the alloy [28; 31; 32]. Cast iron objects are often cast in a sand mould containing a cavity of the desired shape, consisting of cope and drag (top and bottom parts) with an upper pouring space (cup and sprue), as well as a vent and riser [33] (p. 102, Figure 6). At the end of the casting procedure, the sand mould was broken, and the cast item was retrieved. To reduce shrinkage porosity during the process, a feeder was frequently used, and a thermal sleeve was commonly added around the feeder for adding molten metal during the solidification of the metal [33; 34].  
The main difference between white cast iron and grey cast iron is in the silicon content. White cast iron contains less than 1 wt% Si, whereas grey cast iron normally contains 1-3 wt%. Si is a graphite stabiliser that causes carbon to precipitate as dark graphite flakes, surrounded by ferrite and pearlite grains [28; 31; 35]. It also contains smaller concentrations of other elements, including P, sulphur (S), and Mn. The high concentration of manganese in cast objects (>0.5 wt% Mn) results in high-quality products with only a small amount of porosity defects attributable to the sand casting. The presence of manganese in cast iron products, to prevent the formation of gas holes and porosity, indicates a post-1839 manufacturing date [31; 32; 33].  
Cast iron has good fluidity and castability, as well as good hardness; however, it tends to be brittle (strong in compression and brittle in tension). Grey cast iron has good machinability and wear resistance; however, the graphite flakes cause low strength and low ductility [31; 36; 37]. The primary motivation for cast iron production was probably the military, such as the production of cannons and cannonballs [31]. The production of cast iron increased gradually from the sixteenth century onwards, reaching a yearly production of 250,000 tons in England alone at the beginning of the nineteenth century [26] (p. 124). The production of modern grey cast iron with metallurgical control started in Europe between 1810 and 1815 following the development of new equipment and improved manufacturing processes [33].