from a Han dynasty tomb near Luoyang were painted with a light blue pigment consisting of blue, purple, and colorless components. An Eastern Han-period tomb mural painting in the Xi'an area is one of the last examples of the use of synthetic barium copper silicate pigments (Han purple). Preservation Due to the instability of Han purple, it shows significant signs of weathering on archaeologically excavated artifacts. The copper(I) oxide formed in the decomposition of Han purple (see section on color) remains stable, but Han purple continues to deteriorate, and its purple color increases with time. Han purple fades in acid, so colorless particles found in pigments containing Han blue and Han purple may be particles which were originally purple, but which faded in acidic conditions in burial. In addition, Han blue has fungicidal properties, so preserves better. Han purple reacts with oxalic acid to form BaCu(C2O4)2. The light blue color of this coordination polymer may explain the light blue color of some of the Terracotta Warriors' trousers – the color resulting from the presence of oxalate-excreting lichens. Notes Two other synthetic blue barium copper silicate compounds have been found in trace amounts, but are as yet unnamed. They are BaCu2Si2O7 (blue color) Ba2CuSi2O7 (light blue color) See also Blue pigments References External links Raiders of the Lost Dimension (Magnet Lab, FSU) May 21, 2006 Microscopic image of Han Purple (credit: Marcelo Jaime of MST-NHMFL) Shades of blue Shades of violet Pigments Inorganic pigments Barium compounds Copper(II) compounds Oxides Silicates Ancient China