A study on the measurement of discoloration of paper cultural heritages using colorimeter

The autoxidation of cellulosic paper, even if stored in the museum storage with constant temperature and humidity, is inevitable. The oxidation causes a further decrease in the degree of polymerization of cellulose and causes discoloration of the paper. Therefore, color measurement of paper is an important factor for the assessment of the state of conservation. A colorimeter is generally used to measure the discoloration of paper. However, many paper cultural heritages are still difficult to measure according to international standard test methods. In particular, it is difficult to satisfy the standard conditions to be measured by increasing the number of sheets into a pad, at least 10 sheets of paper should be assembled, until the pad thickness does not change the color test readings. Therefore, this study is about the method of measuring the color of paper cultural heritages that does not satisfy this standard measurement condition. In order to improve the error caused when measuring in non-standard conditions, the lightness of paper was evaluated using the base plates with different lightness values. The lightness values under standard measurement conditions were estimated through correlations with these lightness values in each base plate. As a result, the error between the measured value using the base plate with white, gray and black in the non-standard condition and the measured value in the standard condition was maximum Δ 2.5, Δ 8.4, Δ 9.3 respectively. However, the result of calibrating the values evaluated using the three base plates improved the error below Δ 0.8. In addition, this method can be applied to not only paper lightness (L*) but also color correction (a*, b*).