24 
GLAZING. One of the pricipal reasons why whetstones 
glaze is because they do not wear away fast enough. ‘The wear 
on the stone should always be greater than the wear on the steel, 
or the stone will glaze. This may be due to the scratching points 
becoming worn and smoothed which frequently happens in coarse 
grained, strongly cemented stones; or it may be caused by the 
filling in of the spaces between the grains with particles of the 
metal so that the stone loses its scratching power, which is more 
likely to happen with hard, fine-grained stones; or it may be due 
to a combination of both causes. Hence it will be seen that slow 
wearing stones always have a tendency to glaze, while fast cut- 
ting stones, by constantly giving a renewed supply of scratching 
points, ought never to glaze. But all mechanics know that some 
of the best fast wearing stones also glaze. Whenever this has 
occurred it has almost always been due to the use of poor lubri- 
cants, or else to an imperfect knowledge of their use. There can 
be no question but that the dry surface of a whetstone cuts more 
keenly than does the same surface when covered with a fluid; still 
a dry stone heats the tool which should be diligently guarded 
against. With the coarser stones which are liable to heat tools 
water is used, but as all such stones are fast wearing plenty of 
water should be used, otherwise the particles worn off by the 
abrasion will unite with the water to form a paste which is slowly 
ground into the pores of the stone. This soon produces a glaze 
which will damage the stone if allowed to harden. Such glazed 
spots are sometimes of service in putting a finer edge on the tool 
than would the stone itself, but they should be carefully scraped 
off after finishing the edge of the tool. Water is also occasionally 
used with some fine grained stones but. as such stones do not 
heat much its main use is for carrying away the powder formed. 
Since but a small amount of heat is generated by the use of such 
stones, it might at first appear feasible to use them dry, thus 
securing more rapid work, but the pores of the dry stone would 
surely become filled with the fine dust which is always worn off, 
thus causing it to glaze. As glazing due to this cause would 
prove equally injurious to a coarse stone it forms an important 
