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PUMICE. 
Pumice is a very porous variety of lava. In structure 
it resembles a sponge, containing innumerable small cavities 
or vesicles caused by the evolution of gases during cooling. 
The walls of these cavities, which are very thin, consist of glassy 
obsidian, in composition approximately that of feldspar. 
Owing to the hardness and sharpness of this material, it 
is very valuable as a polishing agent. For some purposes it is 
used in the lump form, while for others the pumice is reduced 
to a very fine powder. 
USES. 
It is as a polishing material that pumice finds practically 
all its uses. In the finishing of fine furniture, pianos, carriages, 
etc., pumice is employed to smooth and polish the varnished 
surfaces. The finely ground and bolted pumice is generally 
used for this class of work, though occasionally the lump is used. 
For the dressing of lithographic stones a small quantity 
of lump pumice is used. 
In the polishing of pearl and bone buttons, celluloid goods, 
jewelry, and other fine metal work, the powdered material 
is used. It should be very carefully graded as to size of grains, 
the grades ranging to an almost impalpable powder. 
Pumice powder is used in the polishing of plate glass, fol 
lowing beveling, etc. Freedom from large particles is essential. 
In the manufacturing of scouring soaps, metal polishes, 
etc., finely ground pumice is used, and small quantities are used 
in toilet preparations, such as tooth and nail powders. 
PRICES. 
The price of pumice varies greatly according to the quantity 
purchased and to the grade. 
Powdered pumice ranges in price from $27 to $60 when 
purchased in lots of one ton or over, the average price being 
about $40. The average price of selected lump in quarter ton 
lots is about 3 cents per pound.