[Histonet] Von Kossa staining on PMMA sections

[Monfils, Paul]

The standard Von Kossa silver stain for calcium calls for 20 minutes in the silver nitrate solution under UV light.  There is a modified Von Kossa for plastic embedded bone sections, which is identical except it calls for a minimum of 6 hours in the silver nitrate solution under UV.  Does anyone know why such a long staining time is recommended?  Visually the calcium in the bone sections turns black within 20 minutes, so why is so much additional time needed?  Thanks.