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Maya Blue, a pigment used by the Mayas in Yucatan, is remarkably stable: the color is not destroyed by hot concentrated mineral acids or by heating to about 250 degrees C. The principal constituent is the colorless mineral attapulgite. It is proposed that the pigment is an adsorption complex of attapulgite and natural indigo; a synthetic equivalent may be prepared from attapulgite and either indoxylester or indigo, or by applying the vat-dyeing technique, with reduced indigo.The low dye content of the pigment (less than 0.5 percent) indicates that the dye is absorbed only on the external surfaces of the attapulgite particles and not throughout the channels in their structures. The complex as such is not stable to acids, but the stability displayed by Maya Blue is achieved simply by heating the complex to from 75 degrees to 150 degrees C for several days. An analogous stable pigment can be prepared from sepiolite and indigo. No stable pigments could be prepared from clays with platelike structures or from zeolites.

The yield stress of a dialyzed system containing 3 yo of the low particle size fraction of sodium Wyoming bentonite decreases on the addition of a few mequiv./l. of a neutral salt like NaC1.On further addition of salt the yield stress increases and, contrary to the pure gel, the system displays a slow thixotropic stiffening. The structure formation in the pure gel is attributed to edge to flat surface association of the pla%e-like micelles due to the opposed charges of the double layers a t these surfaces. The structure of the thixotropic gel is seen as the result of the predominance of the van der Waals attraction forces between both lateral and flat surfaces of the different particles.

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