Meaning of "plastic value"

What is the meaning of plastic value below? I read it in the book The art of painting.

A work of art may, incidentally, tell a story, but error arises when we try to judge it by the narrative, or the moral pointed, instead of by the manner in which the artist has used his materials to produce a work of plastic art; when, in other words, a literary or moral value is mistaken for a plastic value.

It’s acting as a somewhat technical term, I think. Plastic here is surely calling back to plastic art earlier in the sentence; so I would understand plastic value here as meaning value as a work of plastic art.

Plastic art, in turn, is a somewhat old-fashioned technical term for art forms involving moulded materials (clay, ceramics…), and by extension sculpture in general, as Wikipedia, the OED and various other googlable sources confirm. (This comes from the original meaning of plastic, as quoted in @mgkrebbs’ answer.)

This use of plastic value is analogous to how one might talk about the harmonic significance of a chord in a Beethoven symphony. We’re not saying that the significance harmonises nicely; we’re talking about the significance of the chord, as part of the harmony. Similarly, the author isn’t (as I understand them) talking about whether the value is plastic, can be reshaped, etc; they’re talking about the value of the work, as a piece of plastic art.

The term 'plastic value' has a specific, technical meaning in art. I am not sure of the full meaning, however I recommend searching through google books and reading up on it.

One of the sources (Art Fundamentals: Theory and Practice) defines it as:

These lights and darks that create the appearance of depth are referred to as plastic value.

However, I have a sense that this definition is actually very narrow and deals only with one aspect of plasticity in arts.

From dictionary

ADJECTIVE:

Capable of being shaped or formed: plastic material such as clay. See Synonyms at malleable.
Relating to or dealing with shaping or modeling: the plastic art of sculpture.
Having the qualities of sculpture; well-formed: "the astonishing plastic beauty of the chorus girls" (Frank Harris).

Although I can not find a direct reference in English, I know some languages use the word in a sense of realistic, life-like - probably related to the appearance of depth, but not only limited to contrast, more in the sense of any attribute that makes the work life-like.

The first definition in the American Heritage dictionary for the adjective plastic is:

Capable of being shaped or formed: plastic material such as clay. See Synonyms at malleable.

In "a literary or moral value is mistaken for a plastic value" it is saying that someone mistakenly thinks that these values can be reshaped as desired (implying that the values are really fixed things).