The State of Rio Grande do Sul in Brazil is one of the main world producers of agates. The gray type of agates found in Salto do Jacuí is specially suited to be stained in various colours. The purpose of this work was to evaluate the colour loss of artificially stained agates, considering different exposure environments. Studies were carried out with agates stained by classical procedures (green, red, blue, and black) and with organic dyes (green, red, pink, and purple). The experimental procedure consisted of displaying agate plates in three places with distinct luminosity: dark, indoor/domestic, and outdoor. The color variation was analyzed by differential colorimetry, using the CIE L*a*b* colorimetrical system. The results show that the classical procedures provided permanent colours, while the colour given by organic dyes was unstable. Depending on the luminosity and exposure time, the colour loss can be practically complete.
Agate; coloring; colour; photodegradation
