An interesting little article  found on the web by our local CSI guy, Ken C :

http://www.crime-scene-investigator.net/agingofshoes.html

Aging of Shoes and its Effect on
Shoeprint Impressions
From the Journal of Forensic Identification
Vol. 55, No. 2, March/April 2005*
by
J. Matney Wyatt
Katie Duncan
Michael A. Trimpe
Hamilton County Coroner�s Laboratory
Cincinnati, OH