The polishing process is the final smoothing out of the mirror. In this stage, the mirror goes from its foggy gray look to being smooth and shiny. The fine polishing rouge, cerium oxide removes the small pits left over from fine grinding, but it also makes corrections to the glass by a few wavelengths (a few hundred nanometers).
The two pieces of glass (the mirror and tool) do not establish
sufficient contact with each other to properly polish the mirror
blank. Another substance, pitch, is needed to form a tight fit
on the mirror. Pitch is a real work of the devil: it sticks to
everything and never lets go. It is solid when one needs it liquid.
It is liquid when one needs it solid. It gets on a workers clothes,
hands, shoes, furniture, and tools. When one tries to remove it,
it gloops into stringy globs, and makes a bigger mess. About half-an-hour
is required to clean hands after using pitch, and rarely does
this succeed in cleaning them. The best way around this is to
use gloves.
Despite its recalcitrant nature, pitch is essential in polishing.
It is melted in a pot, poured onto the glass of the tool and channeled
with a knife. The channels help the pitch conform to the shape
of the mirror. Cutting the channels with a knife can take up to
an hour, so it is not always the preferred method. Instead, individual
pieces can be made and then placed on the mirror. Either way,
making this pitch lap is sticky business. When this lap is made,
it must be pressed against the mirror to conform to the proper
shape. Occasionaly, when polishing, the channels close up from
the pressure. They must be re-cut to continue polishing.
With a properly made lap, the polishing may begin. It is a similar
procedure to fine grinding, but the pitch makes the strokes a
little slower and stickier.
