I say, with Allah's guidance: It is strictly disliked to use bones and dung for istinja, and it is mildly disliked to use pottery, coal, and baked clay (which is the brick used for building). It is also disliked to use anything of respected value: such as paper, silk cloth, and human food like wheat and barley, as this constitutes waste of resources without necessity, and it is established in the two Sahihs (authentic collections of Hadith) that wasting wealth is prohibited.

It is also disliked to use animal fodder — which is grass — for istinja, as it defiles something pure without necessity. Using iron, copper, and lead is also disliked. However, if one uses these things, it is permissible with dislike, because the main consideration is cleanliness, which is achieved; and because the prohibition is due to a reason outside of it, it does not prevent achieving purification: like using someone else's cloth or water for istinja. See: Bada'i al-Sana'i, 1/18, Radd al-Muhtar, 1/339, Al-Ikhtiyar, 1/48-49, Al-Lubab, 1/46, and Al-Hadiyah Al-Alaiyah, p. 45, and Allah knows best.