Abstract: The production processes for Si and FeSi have traditionally been considered slag-free. However, recent excavations have revealed significant accumulation of CaO–SiO₂–Al₂O₃ slag within the furnaces. This accumulation can obstruct the flow of materials and gases, resulting in lower metal yield and higher energy consumption. The main objective of the current work is to enhance our understanding of slag formation during Si and FeSi production. We investigate slag formation through the dissolution of limestone and iron oxide in quartz and condensate, focusing on the reactions between these materials at a gram scale. Our findings indicate that most slag reaches equilibrium relatively quickly at temperatures starting from 1673 K. Notably, slag formation starts at lower temperature when the iron source is present (1573 K) compared to when only CaO is involved (1673 K). The minor elements tend to accumulate at quartz grain boundaries prior to slag formation. Furthermore, the slag produced from condensate contains less SiO₂ than that generated from quartz with limestone. The type of quartz source and SiO₂ phase appears to have little influence on slag formation. Good wettability is a significant factor in reaction between quartz and slag. FactSage calculations indicates that the viscosity of the slag ranges from 0.02 to 14.4 Pa∙s under furnace conditions, comparable to the viscosity of honey or motor oil at room temperature.