A Result of Over-Dipping
A prune must be dipped in lye, but the lye must not be too strong, nor can the fruit be left too long in the bath. Too much dipping may be disastrous, producing a condition of the skin that makes proper curing impossible. The dipping of the prune is, therefore, the only part of the process of prune production that requires expert attention.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 8