The water effect on instrumental hardness and adhesiveness of clay mixtures for pelotherapy

• Water and clay mixtures are studied according to their instrumental texture.
• Four new parameters define appropriate hardness and adhesiveness with water content.
• Equations and parameters proposed can help to prepare tailored peloids.

This study was designed to assess several clay-water mixtures in terms of their instrumental texture to identify candidates for use as peloids in pelotherapy. Eight commercially available clays (including three from Spanish quarries) were characterized in terms of their mineralogical composition (X-ray diffraction). Using each clay, series of mixtures containing different proportions of distilled water were prepared and hardness and adhesiveness measurements made using a Brookfield® texture analyzer. The data obtained were then used to derive exponential equations describing the hardness and adhesiveness of the mixtures depending on their water content. Four new parameters were also defined: reference water content for adequate hardness and adhesiveness and water content required to reduce hardness or adhesiveness by half. The bentonites composed of Mg-smectite with interlayer Na, need the higher water contents to obtain hardness/adhesiveness values in the range of Spanish peloids, follow the bentonites composed of Mg-smectite with divalent interlayer cations and dioctahedral smectites (Al-smectite). Sepiolite and palygorskite samples also need high water contents. Kerolite-stevensite and kaolins showed the lower water contents. According to the equations and parameters defined, peloids similar to those currently used in spas could be tailored for pelotherapy.