The Laws Governing the Preparation of Fire before the Onset of Shabbos, 3 Seifim: It is forbidden to make a large fire – a medurah – using wood as fuel close to nightfall [on Friday] unless the fire catches hold to the extent that the flame burns on its own accord without the assistance of other wood (I.e., flaming wood from an outside source.) If [only] one log is [burning], the fire must take hold of the majority of its circumference and its width. If it is not kindled as such (like above) it is forbidden to make use of it on Shabbat. This is a decree, lest one stir [the wood] on Shabbos, moving [the pieces of] wood until the flame ascends on its own accord. If, however, [the wood] was kindled to the extent described [in the previous subsection] while it was still day, a person may warm himself by [that fire] and use its light, regardless of whether [the wood] was [burning] on the ground or on a holder for fire. And even substances that are forbidden to be used as wicks for [the Shabbos] lights.