Frequently
people who are grinding the edges of bowls, aperture drops and other vessels
that need to have a smooth rim find that they are getting small chips of glass
coming from the edge of the ground part of the glass.
There
is a way to prevent theses unwanted chips
The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit. This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.
You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle. The explanation is held in the angle. As the rim is ground down, the outer acute angle becomes very thin as well as sharp. At some point the glass is thinner than the grit used to grind the surface. This causes little chips of glass to break off the edge.
By
creating an oblique angle at the edge of the grinding surface, the glass will
remain thicker than the grit being used to grind the glass. If you feel you are taking off a lot of
glass, it is advisable to check that the arris is still in place. If not, give it a light grind to maintain the
arris while using that grit.
At
the end of each stage of grinding, you need to add an arris for the next
stage. The reason for doing it with the
coarser grit rather than the one you are about to proceed to, is that it maintains all
the grinding at the same stage, enabling the whole piece to be finished to the
same level of polish.
Wipe the surface dry and add marks with a paint marker. Allow this to dry while you change
grits. The purpose of the marker is to
assist you in determining when you have ground out all the previous marks, by
the elimination of the paint.
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