Making a polisher using Kevin
McCarthy's LM-3 "Perfect Pitch" lap
mold--my first and second tries
Ross Sackett's amateur telescope making
My telescopes and ATM Projects
Here's the 6" hydrostone tool,
trimmed in tape so the pitch doesn't
make too much of a mess
Wiped down with acetone to degrease,
then primed with a little melted pitch
dissolved in turpentine. This might
help with adhesion (but who
knows--like many things, I do it as a
Gugolz 73 on the hot plate. I like to
use a disposable aluminum loaf pan
for this--lots of surface area and easy
to pour, and if the pitch gets
contaminated it is a lot easier to throw
it out than to clean a saucepan.
The lap mold on the mirror,
squares-up. It is important at this point
to position it carefully so that (1) the
pattern is not centered on the tool, but
also (2) you don't have too many
orphaned partial squares on the sides.
I did (1) fine, but should have thought
a bit more about (2).
The sticks are so that the edges of the
mold don't droop too far. This isn't
really necessary, it turns out.
Starting the pour. It's easy, like
making waffles. Just overpour the
squares a bit so that the ribs are
covered by a couple of millimeters of
pitch. It's almost automatic.
The tape-protected tool set down in
the runny pitch. Now wait twenty
minutes or so for the pitch to chill.
The waffle sandwich inverted, with the
mirror on top. The next challenge is to
get the mirror off the lap. I didn't follow
Kevin's advice here and vaseline the
mirror or separate the mirror and mold
with a wet paper towel. I really should
have--it was tough to separate,
especially since the mirror already had
a pad-polish giving it a nice vacuum
seal to the smooth mold. This was the
only scary part of the operation,
imagining the mirror flying off onto the
cement floor. Ten minutes of careful
peeling broke enough of the air-seal to
release the mirror. Gonna work more
on this for the next lap.
Stripping the mat is easy, especially if
you used the optional PVA mold
release spray. The mold is seriously
stained with pitch however, so
afterwards I cleaned it up with
gasoline. You need to be more careful
than I was when you strip--I broke the
corners off several squares of pitch.
Rough trimming the lap the easy way,
using a putty knife and light taps of a
mallet. The bits go back into the pitch
The rough-trimmed lap. You can see
on the left where some orphaned
squares broke free as I stripped the
mold. It probably didn't matter since I
would have melted them off in the next
step anyway since they were too
narrow to hold up.
I'm using a soldering pencil with a large
broad tip to melt off the edge to form a
nice bevel. This helps seal the edge
between the squares and the tool, too.
I like to do the final trimming and lap
maintenance with the soldering pencil
since it saves the splintery mess of
shaving the pitch off. However, it does
leave a lip of melted pitch on the
squares so some warm pressing is
called for just afterwards.
You can see some CeO left from a
quick check for contact--it was pretty
close without any pressing at all. It's
probably better to do the final trimming
before you press, though, because the
residual water causes some sputtering
Stripping the tape. Protecting the tool
was totally unnecessary, but it sure
looks prettier. And see that nicely
In the clean kitchen now, with some
mesh to microfacet the pitch.
Yes, I like to press. 20 minutes and
the contact looks pretty good and the
squares are microfaceted.
Ta-dahhh! Wanna bet that little
orphaned square at 1 o'clock breaks
off the first time I use the polisher?
This was so much fun, I think I will
make another lap in the morning using
a second hydrostone tool I happen to
have around. I love Kevin's mold!
Gonna buy a couple more so I can cut
them up for different size mirrors.
I hate channeling pitch. Really hate it. At RTMC 2009 California ATM Gil McFarland
mentioned that he likes to use a lap mold, so I ordered one from Kevin McCarthy. Here is
my first attempt to use it. Like every method it has a learning curve, but even so I think I
did pretty well on the first attempt. I think this is a very nice product! Note: Kevin's
website has specific instructions on how to use it. I only partially followed these because I
wanted to develop a feel for how it works. No doubt results would have been perfect had
I followed them exactly. But I had fun! ...and at the end I have a picture of my second
polisher, which is close to perfect.
CAVEAT: Before you run out and buy a mat, read the last section! I ran into problems
that need to be addresses before I can recommend the mat.
Whoa! It's the next day and here's my second polisher. Damn, I love short learning curves. I
would never claim that I ever did anything perfectly, but this is about as near-perfect as I can
imagine. I love this mold! I made the polisher the same way as described above, except that I
trimmed the mold down so that the edges didn't flop around so much. I also inverted the
tool-pitch-mold-mirror stack half-way through the cooling so that the outer squares had less
tendency to peel away from the tool. Like Kevin suggested on his website I used a wet paper
towel as a separation layer between mirror and mold. And, of course, I was a bit more careful
about the orphaned squares on the edge. In this photo I have hot-trimmed and beveled the
lap and microfaceted the squares (maybe an hour or so pressing, though clearly this was too
much because it pulled little chips of pitch with the mesh). A quick check shows good contact
on all squares; a little actual polishing should whip it into prefect contact in no time.
WARNING: First appearances can be deceiving!
I posted my initial findings on the Cloudy Nights ATM forum, and received both positive and negative feedback. I did
some experiments to check these, and in consequence I must withdraw my initial appraisal of this method of
channeling a lap. Don't get me wrong, the laps are gorgeous and very quick and easy to make. However, they may
cause the polisher to become gummy and potentially useless. I haven't yet figured out how to consistently correct for
this tendency, so I don't at this time recommend closed-square rubber molds mats made by any manufacturer.
Here are some excerpts from my Cloudy Nights posts:
"Contrary to my initial glowing assessment, on day two I ran into some serious performance problems with laps molded
with the mat. While the polishers made with the mold look gorgeous, several discussants below note that they often
lead to a gummy pitch surface. On the day after the original posting I conducted an experiment (described below) that
unintentionally recreated this problem. I should note that I WAS NOT strictly following the recommendations of the
manufacturer, but rather had washed the mats in acetone, a non-approved solvent, which seems to have greatly
exacerbated and accelerated the gumminess problem. As I have not yet figured an effective work-around, I must
withdraw my recommendation of this approach.
"[Today (the next day)] my first polisher has sticky brown streaks on the surface of the squares. These dissolve off
easily with a quick turpentine wipe, but that leaves the surface tacky and a little softer than the nominal hardness of
the pitch (Gugolz 73, in this case). I am going to let this lap sit and we'll see if the surface improves.
"The second lap doesn't show any brown streaks. After the first use I had washed the mold in gasoline to remove the
pitch stains and little chips; perhaps this removed some surface contaminant.
"This morning I am doing an experiment: I have cut some pieces of the mold material, and given then a wash in
acetone and hot soapy water. I am going to cast some squares of pitch with different mold preparations: one plain and
untreated, one with the PVA release sold with the mold, and one powdered with cerium oxide (like the talc used in
foundry casting). I'll play with these a bit and report back.
"Ok, here are the preliminary results, and they are not encouraging. In fact, I was able to rapidly accelerate the
gumminess problem: no longer do we have to polish for thirty minutes to obtain a gummy, useless lap!
"As I mentioned, I washed some pieces of mat in acetone then in water, in the hope of leaching out some of the original
mold release in the rubber. Apparently, however, the rubber is a hydrocarbon sponge, and absorbed enough of the
acetone to immediately attack the hot pitch.
"None of the three release conditions mattered--the untreated, PVA-treated, and CeO powdered molds all released
easily. HOWEVER, in all three cases the outer layer of pitch was very sticky (quite unlike the pitch that had hardened
in contact with air) as though it had been brushed with strong solvent. The surface was also covered in 1/2 to 3
millimeter pits. These hadn't been there the first time I used the mold.
"Here is my guess as to what is going on. When the hot pitch hit the rubber, it released some of the volatile acetone
which both dissolved into the pitch, making it gummy, and formed bubbles of vaporized solvent, making the pits.
"I should note that the acetone wash was not recommended by the manufacturer of the mat. He suggested using
gasoline or Coleman fuel to lift the pitch stains from the mat. I did use gasoline after the first pour, but noticed it held
strongly onto the smell of the gasoline, suggesting that a significant amount had diffused into the rubber.
"So here is where I stand now. I have to withdraw my recommendation of this style of mat. While the acetone wash was
in retrospect a bad idea, I think it did reveal an inherent problem with this rubber, one that to a less dramatic degree
seems to be common to other users (that were presumably using the mat as directed). I have edited my original post to
reflect this, but ask the moderators not to delete this thread because I think the results are important.
"I have ordered another mat and will continue to experiment. Perhaps washing the mats with hot soapy water and
perhaps a degassing in a hot oven will be all that is necessary to flush the volatiles from the mat. We'll see, and I'll