A very fine-grained soil that is plastic when moist but hard when fired WordNet 3.0 & Open English WordNet

Clay is a type of fine-grained natural soil material containing clay minerals. Wikipedia

Material primarily composed of a grouping of clay minerals, and can be used as an art medium. Wikipedia Disambiguation

Soft rock based compound often used for sculpture and tools Wikidata

A loose, earthy, extremely fine-grained, natural sediment or soft rock composed primarily of clay-size or colloidal particles and characterized by high plasticity and by a considerable content of clay mineral and subordinate amounts of finely divided quartz, decomposed feldspar, carbonates, ferruginous matter, and other impurities; it forms a plastic, moldable mass when finely ground and mixed with water, retains its shape on drying, and becomes firm, rocklike and permanently hard on heating or firing. OmegaWiki