He was asked:

A man purchases soot from bathhouses, paying silver in advance for the entire year’s soot. Before witnesses, he specifies a fixed quantity in irdabbs (measures of capacity), although no measuring actually takes place; he does this only to make the advance-payment sale valid. The customary practice is that whatever accumulates is placed in its container, sealed, and sold in its entirety. Is this valid or not?

He replied:

This question rests on two fundamental issues:

“The first”: Is this deposit formed from smoke ritually pure or impure? This requires distinctions, and there is scholarly disagreement concerning it. If the fuel is pure, such as the fuel used in ovens or pure fuel used to heat a bathhouse, then the resulting deposit is pure. If, however, an impure substance is used as fuel, is the resulting deposit pure? Scholars hold two opinions on this. The same disagreement applies to every impure substance that undergoes transformation, as with ashes, qasarmal, jarsīf, and similar materials.

If it results from the transformation of an impure substance, it is impure according to the school of al-Shāfiʿī and one of the two opinions in the schools of Mālik and Ahmad; this is also an apparent view within the school of Abū Hanīfah. The other opinion in the schools of Mālik and Ahmad is that it is pure, and this view