Alum stops bleeding in minor cuts because of

Coagulation is the process by which blood forms clots. It is an important part of homeostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel. Disorders of coagulation can lead to an increased risk of bleeding (hemorrhage) or obstructive clotting (thrombosis). Alum in block form (usually potassium alum) is used as a blood coagulant. Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts. For example, alum blocks are commonly sold in pharmacies in India.