## 2 Theoretical Considerations
Assuming that the mass of the crucible exceeds that of the slag, equilibrating the phases means that the slag will form olivine and, subsequently, move into the phase field of olivine and monoxide, i.e., magnesiowustite. On the other hand, if the slag phase exists in excess, the magnesia crucible could be consumed by the reaction with the slag. The remelting and modification of iron silicate slags, performed by [PERSON] et al. [19], was achieved using alumina crucibles. Figure 3 illustrates that alumina is readily soluble in iron silicate slags. However, as opposed to using magnesia crucibles, iron silicate slags can be saturated in alumina, as shown by the phase fields where both corundum and slag are present. Based on the results presented by [PERSON] et al. [19], the remelting of the industrial slag did not cause severe changes in the alumina content, which highlights that kinetic limitations favor the use of this ceramic system. Nonetheless, synthesizing slags from chemical reagents using a crucible made of a component that, in fact, is a desired variable is not attractive when aiming to evaluate the effect of chemical composition. In  
Figure 1: 1300 \({}^{\circ}\)C isothermal section of the FeO-Fe\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) phase diagram with activity of Fe = 1. Calculated using the Phase Diagram module of FactSage 8.2 with the GTOx and FactPS databases [37; 38; 39]. addition, Figure 3 suggests that different starting conditions regarding the FeO/SiO\({}_{2}\) ratios will translate to different possible alumina contents when moving toward equilibrium. Another ceramic system is zirconia, which has been used by [PERSON] et al. [43] for iron silicate slag. Based on the three-component diagram, Figure 4, the solubility of zirconia in the slag is significantly lower than alumina. Nonetheless, the application of this system was found to be inappropriate for the experiments performed by [PERSON] et al. [43], who reported catastrophic failure of the crucible. Based on the present authors' experience, yttria-stabilized zirconia functions well for industrial iron silicate slags in experiments where remelting is desired. However, to allow for simple handling and water granulation of the slag, ceramic crucibles, such as zirconia and alumina, are less attractive due to their brittleness at high temperatures. Figure 3: The 1300 \({}^{\circ}\)C isothermal section of the FeO-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\) phase diagram with activity of Fe = 1. Calculated using the Phase Diagram module of FactSage 8.2 with the GTOx and FactPS databases [37; 38; 39]. Figure 2: The 1300 \({}^{\circ}\)C isothermal section of the FeO-SiO\({}_{2}\)-MgO phase diagram with activity of Fe = 1. Calculated using the Phase Diagram module of FactSage 8.2 with the FToxid and FactPS databases [39]. The challenge of granulating iron silicate slag from ceramic crucibles on a laboratory scale, i.e., the third challenge identified above, is exemplified by the experimental setup used by [PERSON] et al. [19]. In that study, using alumina crucibles necessitated an experimental setup using a tiltable induction furnace and intricate design to maintain the crucible from falling during tapping [19]. The third challenge can be addressed using iron crucibles since they are not brittle nor subjected to thermal shock. Therefore, these crucibles can be handled manually at temperatures relevant to iron silicate slags. Furthermore, the pure oxides of common components in iron silicate slags, e.g., calcium oxide, magnesia, and alumina, are more stable than ferrous oxide (Figure 5). Naturally, the partial pressure required for reducing the oxides in the solution is different. However, Figure 5 illustrates that iron is not a strong reducing agent in comparison to silicon, aluminum, magnesium, and calcium.