In the paper, I examine the concept of “cruelty” (kaṭumai) of physical substances in the Tamil siddha alchemical literature, by taking as the starting point an analysis of the selected texts ascribed to Siddhar Yākōpu alias Rāmatēvar (ca. 17th–18th c.), a prominent author of the Tamil siddha alchemical tradition. The alchemical works of Yākōpu repeatedly describe certain alchemical operations, the names of which allude to acts of violence, such as “binding” (kaṭṭutal) and “killing” (kollutal). Such operations, according to the texts, should be mastered and performed by alchemists on various material substances. In the paper, I analyze passages that shed light on the purpose and character of such operations in the alchemical tradition of the Siddhars. I also address the issue of the non-human material agency related to the perspective of material ecocriticism as presented in the Tamil siddha texts.