Alum is essentially aluminium sulphate with empirical formula Al2 (SO4)3.18HO. It is of two types (i) Ferric and (ii) Non Ferric.
Ferric alum is a less pure form of aluminium sulphate. It is used in lower grades of paper for loading and sizing purposes. Ferric alum uses bauxite as the raw material. The raw material is cooked with sulphuric acid under suitable conditions. The resultant product is sold in liquid form.
Ferric alum imparts better brightness to paper when used with rosin for sizing. It forms a complex resinate that spreads evenly over the surface of paper. It finds other uses in pharmaceutical and mordanting industry. In water purification the highly positively charged aluminium precipitates negative suspensions as sludge.
Benefits It is the most versatile and easily used material for sizing paper with rosin.