Nora,
Are you sure you don't mean maximum thickness?  Originally devised
for calcium deposits in kidney (and for which it was not terribly
specific), this can be a great method in bone (can hardly wait for
Gayle's comments) and can be speeded up considerably by exposing to
an ultra-violet light source.  Of course, if you live in sunny Ca or
Fl, just put it in the window! (watch out for the guy who will want
to tell me window glass filters out the UV)
If you really want to know about the von Kossa, read Susan Meloan and
Holde Putchler's article, ingeniously and appropriately labelled
"What von Kossa really said......"  Not the only publication of
theirs titled "What so & so REALLY said."  Anyone who has tried to
look up original methods will testify to how modern desriptions have
drifted from the original.  (including mine!!! their papers actually
record what xyx really WROTE!!!).

 von Kossa's paper is all about renal disease and a real grind to
read if you don't do German!  Susan and mentors paper is:
Meloan SN & Putchler H. Chemical Mechanisms of Staining Methods.  J
Histotechnology. 1985;8:11-13.

While you are at it, you may also like to find the same authors'
"Harris Haematoxylin: What Harris really wrote and the Mechanism of
Haemalum Stains." J Histotechnology. 1987;10:257-261.

She really deserved her award!!

OK, That's enough preaching.
Russ Allison, Wales