Purple ink was used in tests in the 1970s for canceling purposes because the existing black ink often jammed machines and slowed down operations. Purple ink was used in 146 cities primarily from 1980 thru 1983. It was criticized because it was hard to read and would run and discolor stamps and the cancel could be washed off stamps. It was phased out when a new black ink was developed in 1982.
A few examples of purple machine cancels are known in the latter part of the 1980's and as late as 1998. This probably is the result of those offices using up old supplies of the purple ink.
Initial tests were in the Northern Virginia and Prince Georges Maryland offices, and the first known purple machine cancel was on April 29, 1972. The last known usage was on July 8, 1998.
(Information from the introduction to "The Purple Machine Cancel Handbook" by Robert M. Washburn and Carl R. Ditsch, and updated by Rob Washburn.)
All PMCs on Letter size envelopes have a circular dial which shows the place and date of cancellation, plus 6 or 7 wavy or straight lines or a slogan. Cancels on Flats are different than the dial/wavy lines/slogan cancels, and in particular the Type B purple flats cancel does not have a circular dial.
There are several articles dealing with Purple Machine Cancels in the Coil Line Archives that may be of interest to those who wish to learn more about Purple Machine Cancels. The archives are in Adobe Acrobat PDF format.
1. May 1990 (page 9). "Classic PNC Commercial Covers" by Rob Washburn
2. August 1991 (pages 6-7). "A Look At History of Purple Machine Cancels" by Ken Lawrence
3. September 1991 (page 8). "Purple Machine Cancels Part Two" by Ken Lawrence
4. February 1992 (page 25-26). "PNC's With Purple Machine Cancels" by Rob Washburn