Shulchan Aruch: Chapter 255 - Laws Governing Preparation of Fire before Onset of Shabbos

SECTION 255 The Laws Governing the Preparation of Fire before the Onset of Shabbos (1-4)

1 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,1 without the assistance of other wood.2 This is a decree, lest one stir [the wood] on Shabbos, moving [the pieces of] wood until the flame ascends on its own accord.3

If [only] one log is [burning], the fire must take hold of the majority of its circumference and its width4 while it is still day [on Friday], i.e., the fire must penetrate more than half the width of the log and must cover half of its circumference from the outside. Under such circumstances, [the log] will burn on its own accord on Shabbos and it will not need to be stirred.

2 When wood was not kindled while it was still day [on Friday] to the extent described [in the previous subsection], even if later, on Shabbos, in and of itself, the fire took hold to the extent that there is no concern that one will stir it,5 it is nevertheless forbidden to benefit from [this fire] on Shabbos, since it was kindled in a forbidden manner while it was still day.

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. Even substances that are forbidden to be used as wicks for [the Shabbos] lights – because the flame will flutter around them6 and will not catch well onto them, as will be explained in sec. 264[:1] – are permitted to be used [as fuel] for a fire in any situation.7 [The rationale:] Since a medurah is a large fire [and contains a large quantity of fuel], one [piece of wood] will cause another to burn.

3 There are authorities who maintain that if coals [are used for kindling], even though the fire only started burning minimally on its own while it was still day [on Friday], it is permitted [to benefit from it], because they will continue to burn on their own. There are other authorities who differ with this [ruling. Nevertheless,] with regard to a Rabbinic decree,8 one may follow the more lenient opinion.

4 When tar, sulfur, straw, or stubble [are used as kindling for] a medurah, even though the fire only started burning minimally [while it was still day on Friday], it is permitted [to benefit from it], because these substances will continue to burn on their own.9

The same [rule applies when] reeds or date pits are used as kindling when they are spread out.10 If, however, the reeds are bound [in a bundle] and the pits are [collected] in a basket, then the fire must take hold of them to the extent that it ascends on its own accord, because [when they are collected in such a manner], the flame cannot penetrate them on its own accord.