Clay is not one material
The word "clay" covers a family of minerals with very different structures. Idol makers across India work primarily with two: natural river clay, which is largely kaolinite, and processed clay bodies that contain montmorillonite (the dominant mineral in what potters call "black cotton soil"). Kaolinite has a rigid, two-layer crystal structure. Montmorillonite has a three-layer structure that absorbs water between its sheets and swells. When a kaolinite idol enters a river, the water penetrates slowly along grain boundaries. When a montmorillonite idol enters, the mineral lattice begins to expand almost immediately, the idol softens from within before it crumbles at the surface.
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