PVOH was selected as a binder in the coating formulation because of its high binding power and high water absorption capacity. The solution of PVOH was prepared at 20% solids by adding the required amount of dry PVOH power to cold water under agitation and heating the mixture to 85 °C. A de-foamer was added before the PVOH addition. The solution was held at this temperature for 35-40 min to assure complete dissolution and hydration of PVOH. The PVOH solution was cooled to 25°C before addition to the slurred diatomite particles at a pigment to binder ratio of 5:1 (pigment: 100 parts and binder: 20 parts) under slow agitation. Finally, pH sensitive dye is added into the coating formulation between 0.05 parts and 1 part. The coating were mixed in a laboratory mixer for 20-30 min under low shear to ensure that no air is entrapped into the coating and cause foaming. Application of the coatings on acidic paper was made with standard industry coating thickness quickly and easily. The coat weights were varied by rod number and the coating solids content. The coat weights applied were between 6 and 20 g/m2.