Plasticizer: Any of a group of organic substances which are used in plastic to impart viscosity, flexibility, softness or other properties to the finished product.

Definition used with Permission, from
The New Clay : Techniques and Approaches to Jewelry Making
by Nan Roche, Chris Roche (Photographer), Sue Roche (Illustrator), Seymour Bress (Editor)
Paperback - 144 pages Reprint edition (March 1992)
Flower Valley Press, Inc.; ISBN: 0962054348