In this article, the impact of conventional carbon sources, alongside potential carbon bio‐sources, on slag‐foaming behavior is investigated. It highlights the complex relationship between these carbon sources and their properties, such as fixed carbon (FC), volatile matters (VMs), mineral composition in ash, reactivity, and wetting, which ultimately influence the slag foaming efficiency. The challenges associated with biochar and the significant differences in foaming behavior are addressed. For biochar to achieve effective slag foaming, it is essential that it contains an FC of at least 60 wt% and ash of less than 5 wt%. Though less impactful than CO generation from iron (II) oxide reduction, VMs from carbon sources, especially with high‐VM biochar, show secondary effects on reaction courses. The disadvantages associated with the high reactivity of biochar can be overcome by improving its physicomechanical and physicochemical properties. Despite the potential of biochar–coke mixtures to benefit slag foaming without enhancing biochar properties directly, challenges such as biochar floatation on the liquid slag surface and rapid burn‐off exist. Biocoke offers foaming results comparable to those of conventional sources. Despite the benefits of biocoke over other carbon sources, the review underscores its relatively unexplored status in the context of slag‐foaming applications.