You are bound by your state health code for retention of slides and blocks.
Does JCAH fault you if your state is shorter than their reg's. In NY its 20
years for blocks and biopsy slides, ten years for positive and non-gyn
cytology and five years for negative Paps). 
Marge- Periodic acid is used as the oxidizer in the Sigma Silver stain kit
and I use it to stain pneumocystis all the time. I think its 0.5% for 5
minutes. 
Jeff Silverman Southampton NY