Scientists presently concerned with the conservation of paper warn that acidity is a most damaging factor, causing the gradual and irreversible breakdown of cellulose molecules. Many a work of art on paper has been injured by being placed in prolonged contact with acid materials, notably mats and containers made of impure wood-pulp cardboard. Since acidity migrates from one material to another, the valuable paper, which may not originally have been acid, is eventually affected with consequent weakness and embrittlement. One of the reasons why all-rag cardboard is used for mats and for the lining of solander boxes is because the general belief is that it is nearly neutral. However, only too often all-rag board is acid and its acidity may exceed the limits considered tolerable for the safekeeping of the paper it encloses. Acidity may also be introduced by secondary mounting materials--slip sheets, adhesives, tapes, backing boards, etc. Therefore it would be a great benefit to the curator if there were a reasonably sure yet simple method, not involving elaborate equipment, for gauging the acidity or alkalinity of the materials used with the valuable papers in his care.

Acidity or alkalinity is measured by chemists in term of the concentration of the hydrogen ion. Thus pH (literally potential of Hydrogen) is a symbol representing quantitatively the hydrogen ion concentration. Thus pure water at normal temperature has a value of 7. pH values from 0 to 7 indicate a decreasing sequence of acidity, pH values from 7 to 14 an increasing sequence of alkalinity.