How should the Siwak stick itself be treated and what materials are prohibited?
Chapter on Siwak and the Sunnah of Wudu
Al-Mughni
Book of Purification
Primary text
The Siwak should be a soft stick that cleans the mouth without causing injury or disintegration, such as the wood of Arak or Arjoon. It must be washed with water after use to remove residue, as recorded in Aisha's narration. The prohibition extends to using the wood of pomegranate, myrtle (As), or aromatic sticks, based on the narration warning that using the basil or pomegranate stick moves the root of leprosy.
Supporting text
If one uses a finger or a rag instead of a stick, the correct view is that the Sunnah is achieved to the extent that purification is accomplished, as abandoning a small part of the Sunnah due to inability to perform the entirety is not justified. However, a view exists that the Sunnah is not achieved because the ritual was not specifically legislated for the finger or rag, nor does it achieve the same level of cleansing.