Color Change Rates of a Rock (Oya Tuff) by Spectro-Colorimetry

In order to evaluate degradation rates of rocks, color changes of Oya tuff samples were monitored by a spectro-colorimeter at room temperature (24–25°C) with relative humidity around 30%. Under ambient condition permitting drying of the rock, water was lost gradually during 8 days. a* (green) and b* (yellow) values increased rapidly with the increase of L* (lightness) value during the first 3 days. On the other hand, b* value increased exponentially after 6 days until 42 days. Under wet condition avoiding drying of the rock, L*, a* and b* values increased slowly quasi-linearly with time during 130 days. b* value increased exponentially for the first 10 days. The first order rate constants for b* increases under these ambient and wet conditions are in the order of 1 × 10^–6 s^–1. These color change rates can be possibly related to oxidation of Fe²⁺ to Fe³⁺, occurring at the interface of mineral with water film present under normal humidity conditions.