Fun with Pumice

Some fun products have been arriving on my doorstep this week.  I'm about to embark on a little research for an article I'm writing for The Artist's Magazine.  The article is on surfaces for pastel.  One of the products that arrived is a 2 lb bag of finely ground pumice.  This is FFFF (4F), which is about as fine as you can get it.  

Pumice is a product of vulcanism.  The rock is full of gas, and it can float on water.  When ground up, it makes an excellent abrasive for sanding.  It's been used for polishing furniture and, if I'm not mistaken, teeth.  (Your dentist may have a problem with that.)  But it's also great for making pastel surfaces.  When mixed with gesso and applied to a substrate, it becomes the "grit" that holds the pastel.

I wonder how far my 2 lb bag will go?  It looks like an awful lot of pumice to me.  And it would be easy to confuse it with stone-ground wheat flour.