That piece of string is pretty long.  Here's what I've seen with CT, 16 mm blades:

small calcified ectopics (SQ or IM), mouse or rat bones, - greatest dimension ~ 1.0 cm -- 20 - 30 blocks on 1/2 of a blade.  

large calcified long bones (femurs, ulna etc) - depending on the depth needed to face off and reach area of interest, 2-3 blocks in the same spot on a knife.  

The same goes for the cryostat

Having said this, several factors will determine longevity of the knife - number of levels needed, bone density (cortical vs trabecular, high or low density), how well has the knife been sharpened (always check it and mark the areas that may still have a gauge leftover from before it was resharpened), hardness of the plastic, over processing (bones are dry and brittle), proper knife angle, technical ability, etc.  Also, how much rough cutting you have to do - and how thick your rough cuts are (10 microns causes less damage than if you advance quickly - taking 20-50 microns off - using control buttons.  You can cut down on the amount of facing off if you have a shop grinder (or isomet saws, etc) to remove some of the plastic and bone before you cut - becareful not to get too close to the AOI -- the grinder leaves pretty deep gouges in the block - about 100 + microns, depending on the roughness of the grinder.

Mary   

>>> RUSS ALLISON <Allison@cardiff.ac.uk> - 8/24/1999 5:14 PM >>>
You may expect me to know this

How frequently do you have to sharpen tungsten carbide knives.
Yes, I know, how long is a piece of string!

I appreciate comments on types of tissue cut - soft, calcified, 
horses' hooves, etc

How about using them in the cryostat for undecalcified bone?

All experiences gratefully received.
Russ Allison, Wales