  Maya blue is an organo-clay artificial pigment composed of indigo and
palygorskite. It was invented and frequently used in Mesoamerica in ancient
times (eighth to 16th centuries). We analyse in this paper one of the
characteristics of Maya blue that has attracted the attention of scientists
since its rediscovery in 1931: its high stability against chemical aggression
(acids, alkalis, solvents, etc.) and biodegradation, which has permitted the
survival of many works of art for centuries in hostile environments, such as
the tropical forest. We have reproduced the different methods proposed to
produce a synthetic pigment with the characteristics of the ancient Maya blue.
The stability of the pigments produced using either palygorskite or sepiolite
has been analysed by performing acid attacks of different intensities. The
results are analysed in terms of pigment decolouration and destruction of the
clay lattice, revealed by X-ray diffraction. Palygorskite pigments are much
more resistant than sepiolite pigments. It is shown that indigo does not
protect the clay lattice against acid aggression. We show that Maya blue is an
extremely resistant pigment, but it can be destroyed using very intense acid
treatment under reflux.
