## 2 Theoretical Considerations  
To address the aim, the means required to synthesize the iron silicate slag system were reviewed. Three challenges were as follows: (i) controlling the partial pressure of oxygen to avoid oxidation of ferrous iron; (ii) choosing a crucible system that allows for controlled variations of the slag's components; and (iii) choosing a crucible system that allows simple handling of water granulation in laboratory scale experiments. Concerning the first point, zinc fuming processes designed to remove zinc from iron silicate copper slags drive the reduction close to the precipitation of iron if the process aims for maximum zinc removal [33]. This information offers a basis for the partial pressure of oxygen required to synthesize a slag with the correct oxidation state of iron. Pyrometallurgical experiments with controlled oxygen partial pressures are commonly performed in gas-tight tube furnaces where calculated mixtures of carbon monoxide and carbon dioxide at fixed temperatures are used [34; 35]. Although this approach is suitable in other experimental designs, the setup offers challenges when water granulation of the synthesized slag is desired. Most importantly, breaking the atmosphere would cause poisonous carbon monoxide to exit the gas-tight equipment and spread to the vicinity of the furnace. Another approach to control the partial pressure of oxygen is to use an iron crucible. According to [PERSON] and [PERSON] [36], using an iron crucible will provide an oxygen buffer to the system. The benefit of this is evident when considering fumed slags that are driven close to the reduction of ferrous iron. Figure 1 illustrates the 1300 \({}^{\circ}\)C isothermal section (a reasonable experimental temperature in accordance with Section 3.2) of the FeO-Fe\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) phase diagram. Assuming that the FeO/SiO\({}_{2}\) ratio is designed to avoid cristobalite and monoxide, the iron crucible will buffer the oxidation state of iron within the phase field of iron and slag. Naturally, an inert atmosphere in combination with the crucible must be employed to mitigate the excessive introduction of iron atoms into the slag system. Regarding the second challenge, i.e., finding a crucible system that minimizes the slag-crucible interactions and, thus, allows for controlled variations in the chemical composition, considerations of phase equilibria must be taken into account. No crucible-slag system is inert at high temperatures, which was highlighted by the careful selection of crucibles based on the origin of different steel slags in the study by [PERSON] et al. [40]. A common approach to avoid extensive slag-crucible reactions is to use platinum. This approach was adopted by [PERSON] et al. [27] and [PERSON] [28] in the syntheses of blast furnace slags and calcium aluminosilicate glasses, respectively. For iron silicate slags, using platinum is impossible due to the iron-platinum solubility and the formation of intermediate compounds [36]. Another method to minimize interactions with the crucible is to use a ceramic system and saturate the slag with the component representing the crucible. This method was used for basic oxygen furnace (BOF) slags saturated in magnesia by [PERSON] and [PERSON] [41]. In BOF steelmaking, this approach has industrial relevance since magnesia-saturated slags minimize refractory wear [41]. For iron silicate slags, the solubility of fayalite and forsterite, in combination with the progressively higher melting points of olivine toward higher magnesium contents [42], makes magnesia crucibles unsuitable. Although industrial slags are more complex, the phase diagram of the three-component system, including magnesia and the two major components of the slag, i.e., FeO-SiO\({}_{2}\), can be used to demonstrate the slag crucible interaction. Figure 2 illustrates the isothermal section at 1300 \({}^{\circ}\)C of the FeO-SiO\({}_{2}\)-MgO system. The phase diagram shows that the slag phase is never in a two-phase equilibrium with magnesia.