One of the difficulties of coldworking is when to change
grits. Checking the completeness of one grit grinding visually is difficult. It
is difficult to see the effect while wet. Even when dry it can be difficult to
see that all the previous marks have been ground out. This is the background to
the recommendation that each successive grind should be right angles to the
previous. It is easier to see marks that are in a different direction than
marks that are just wider or deeper than others.
However, there have been aids developed by cold workers in
the past that are relevant today. Use a witness at each stage of grinding. The
best at present are paint markers. These are the kinds used by metal workers to
identify the pieces, their dimensions, etc.
The paint needs to be dry, or it simply washes off the glass
surface as soon as it is placed on the grinding surface. So, there is a process
to ensure this witness works as it should.
On a dry surface, run the marker at random across the
surface to be ground. Let it dry before putting on the grinding surface. You
can test by putting your finger on the paint. If some comes off on your finger,
it is not yet dry. When dry, grind the surface. When all the colour has been
ground away, it is time to change grits.
Dry the glass surface again and paint it. While the paint is
drying, change grits, or discs and do any other cleaning up between grits that
is required. Test for dryness. Grind at right angles to the first grind. Make
sure absolutely all pinpoints of colour are removed. If they are gone, dry, and
paint. While letting it dry, change grits, clean up, and do the other miscellaneous
tasks. Then test paint for dryness and grind.
Repeat this process with each grit until finished. If on
occasion you find you have gone to a finer grit earlier than you should have,
go back to the coarser grit with you painted glass and repeat the progress back
from the immediately previous grit through to the finer grits.
This process of using a witness works whether hand or
machine cold working.
