Following the reference I gave your yesterday, here is the procedure in use 
in my lab:
Primary amyloid (AL) is not affected by an oxydation by potassium 
permanganate, but secondary amyloid (AA) is.
Prior to staining with Congo Red, the slides are oxydised in a solution  of 
aquous potassium permanganate 0.5%(w/V) mixed in equal part with an aquous 
solution of sulphuric acid 0.3%(v/v) for 2.5 minutes. The slides are then 
washed thoroughly, and bleached in an aquous solution of oxalic acid 
0.5%(w/v).
Primary amyloid will still stain with Congo Red, but secondary won't.

Ref: John D. Bancroft, Harry C. Cook, Manual of histological techniques and 
their diagnostical applications.

Hope this will help you.

Richard Blais
blaisrj@hotmail.com
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