How does alum help in blood coagulation?

Question

I recently came across a test in my chemistry book which says that potash alum, $\\ce{K_2SO_4.Al_2(SO_4)_3.24H_2O}$ is used on cuts and bruises, because it coagulates the blood quickly. The explanation it gave, was that blood is a colloid of proteins in water and the $\\ce{K^+}$, $\\ce{Al^{3+}}$ and the $\\ce{SO_4^{2-}}$ ions coagulate(=precipitate) them, causing a blood clot (quickly). However, from biology, I know that blood coagulation(=clotting) is brought about by formation of fibrin network which traps the WBC and RBC in blood. The formation of fibrin from fibrinogen involves a series of reactions with many factors like $\\ce{Ca^{2+}}$ , but not $\\ce{K^+}$ or $\\ce{Al^{3+}}$. So, I was wondering what the actual explanation might be. 1. Do people actually use alum to stop bleeding, or only as a disinfectant? Does alum have any disinfectant effects? 2. Does precipitation of proteins in blood help clotting? 3. Does alum contribute in any way to the process of blood clotting?_Note: This is a question involving both chemistry and biology, but I posted the question here as I thought that this is more of biology than chemistry._

Answer 1

Alum is used as both a disinfectant and in blood coagulation. The ions from alum neutralize the charges on plasma proteins, causing the plasma proteins to clump together. Overall, this process is referred to coagulation. As a disinfectant, alum neutralizes the ions on proteins on microbes, causing precipitation. 

The process of blood clotting is a natural process where a fibrin meshwork is made. Red Blood Cells would still get trapped in this meshwork. However, with alum, the RBCs would precipitate faster. Therefore, in this way, alum does help blood clot.