Making the Pitch Lap
For the final polish, we shifted to using a pitch lap tool. The best
way to describe this is "insanity". The reason for this is that
the pitch (basically boiled down tree sap) is extremely sticky (especially
when warm or melted), and although brown pitch looks like caramel candy,
at room temperature it is more brittle than peanut brittle (minus the peanuts).
Jim had taken some wood and built a jig to allow him to pour the melted
pitch into strips.
Between room temperature and it's liquid state, it will turn into an almost
taffy consistency and stickiness! Cutting it into its final shape
is difficult. You need to warm it enough that it can be cut without
shattering, but not warm enough to stick to your instruments. The
compromise that was made was to warm it enough to just barely cut it. This
resulted in a bit of a mess that needed to be cleaned up.
The concrete base of the tool was now covered with a thin layer of pitch.
This helped make sure that no particles of concrete would come loose and
scratch the mirror. It also provided the base that the squares of
pitch would stick to.
A small amount of turpentine was applied to the back of the squares to
help them adhere to the tool.
At this point, the lap is almost done. The remaining corners will be fitted
with properly cut pitch. Note the channels visible between the tiles.
The bright light is from a heat lamp that was used to soften the squares
so that they would deform slightly to the curved shape of the tool and
make them stickier for good contact.
John and Jim are applying a bit of correction to the tiles. Some
of them had a bubble hole or two in them that needed to be filled.
The channel edges are where the polishing actually occurs. The holes
could cause uneven polishing of the surface.
After the basic lap was finished, a bee's wax mold is used to impress a
pattern onto the surface of the tool. This will speed up the polishing
because it will form an edge at each ridge. The edges are where the
polishing occurs most. (We used this only in the early stages of
polishing as the small facets and rapid polishing are thought to produce
small-scale ripple and roughness in the surface.)
Remember with the rough grinding that the grit acted like boulders that
smashed their way along the glass surface. We went to finer and finer
grit. Since we are after a surface with an accuracy measured in fractions
of a micron, we cannot find a grit (or even powder) that would be fine
enough for the original boulder approach. Rather, the pitch will
be covered in polishing rouge and the rouge will be used to scrape the
surface smooth. Some theories suggest that the heat from the constant
rubbing and scraping will cause the glass to flow at the molecular level
and have the material from the peaks fill the valleys on the surface.
Finally we are ready to start polishing...