modelling clay certainly is unctuous. I don't normally associate clay with being unctuous, but immediately understood, (and could associate clay with both the word unctous and plastic!)--most mothers have once in their lifetime made the mistake of buying molding clay for children, --who leave it every where--and it leaves oily, permenent stains!
meaning 3, well, how can one use the word unctuous with out thinking of Uriah Heep, and i don't think of him as being 'assumed' or 'exagerated', but rather, ingratiating, false, and (i will accept)superficial earnestness of language or manner--definately 'plastic'. he attemped to mold himself into what ever he thought his company desired or needed: a fitting companion, a loyal employee, and so on.
plastic, in what i guess is the first sense (i haven't checked out merriman, but guess it is bakelite, etc, ) vies with plastic [?] the quality of being mold-able for me. I USE plastic most often to describe something, (a plastic toy) but in my minds eye is a liquid, being molded; a huge vat that spits out goo that is transformed into any number of things.(and these things ending their life on a conveyor belt heading into the vat!)
(where did i read, just today--about the man (Bakelike is an english 'version' of his german name).. he was attempting to make a durable varish for bowling ball alley's, and ended up with creating bakelite?) Aaaugh! senior moments are becoming the norm!
i think plasic became associated with artificial, because bakelite was used extensively to copy (quite successfully) materials like tortoise shell and ivory. by the 1930's, bakelite was fashioned into costume jewelry, --and no attempt was made to copy any known materials.. it was plastic pure and simple!
but the idea of plastic being artificial, and of unctuous being false, are similar.
the plasticity of words
