model in clay, sculpt, plastics
塐 centers on shaping a material into a form: modeling in clay, sculpting, or working with plastics. The common thread is giving shape to a substance.
塐 combines 土 (earth, clay) with 素, which likely contributes the sound ソ. The structure suggests working with clay or earth to create a form.
The left side 土 is earth or clay, and the right side 素 suggests a plain material. Picture a sculptor taking raw clay and shaping it into a detailed model.
For ソ, imagine a sculptor working so intently that the clay seems to sew itself into shape: sew -> ソ, and the clay becomes a sculpture.