Two wood block prints printed 90 years ago were investigated. In white parts of the prints and their lining papers, calcium carbonate mixed with plaster of Paris (calcium sulfate) was identified by nondestructive x-ray diffraction analysis; alum was not found. The existence of plaster of Paris and absence of alum was considered to be the result of the reaction between calcium carbonate and alum. Tensile strength, folding endurance, elongation, and pH of the brown and white parts of the lining paper were measured. The white parts showed higher values than the brown parts in all measurements. The pH of the white areas was about 6, whereas it is about 5 in the brown parts. The authors speculated that the brown parts of the lining paper resulted from the interaction with alum which lowered its pH, promoting the degradation of the paper. They thus conclude that calcium carbonate prevents the degradation of paper. -- AATA