Joy of Mirror Making
The goals of fine grinding are to prepare the
curved mirror face so that it is ready for polishing and figuring and
make any adjustments of the focal length.
Previously in rough
grinding we deepened the center of the mirror's face either by grinding
grit and a tool, slumping in a kiln over a mold, or purchasing the
During fine grinding, we'll grind the mirror face against a tool with a
series of ever finer grits. I like to use
the fewest possible number of grit sizes, using professional quality
grits that are accurately sized. This gives me the best confidence that
the mirror surface is free of major defects and can be quickly polished
in preparation for figuring.
The three grit sequence for fine grinding:
In microns, grit sizes are:
- 220 silicon carbide grit
- 500 silicon carbide grit
- 9 micron aluminum oxide (MicroGrit WCA-9T or similar)
60-90 silicon carbide grit for rough grinding: 200 microns
220 silicon carbide grit for initial fine grinding: 60 microns (varies from 100 down to 20)
500 silicon carbide grit for smooth grinding: 20 microns (varies from 40 down to 10)
9 micron aluminum oxide for final grinding: 9 microns (varies from 15 down to 5)
Keep in mind that silicon carbide leaves pits about three times deeper than aluminum oxide.
more grit sizes is largely an exercise in wasting time. The difference
in grit size between 400 and 500 grit is very little. Simply grinding a
tad longer with 500 grit means avoiding 400 grit completely. Also,
professional quality grits are better sized, meaning that the
percentage of abnormally large and small particles is very small.
Cheaper grits have a higher percentage of extra large and extra small
particles. The anomalously large particles make the grit act like a
coarser grit and the anomalously smaller particles slow down the
grinding action. The cleanup time between grits is significant.
Everything must be cleaned and cleaned again. Favor fewer cleanup changeovers
Keep in mind that the way that silicon carbide attacks glass. The sharp
pointed edges of the silicon carbide particles
chip away the glass as they are caught, tumbling between mirror and
tool. The glass
sustains fracture damage up to a depth of three times the grit size.
By contrast, the
aluminum oxide is a much gentler sliding of aluminum oxide particles in
the shape of plates between the glass and tool. Using aluminum oxide as
the final grit size greatly speeds polishing time since there are no
making a separate fine grinding tool from plaster and tiles. Strive to
make the tool the same size as the mirror. Tools larger than 20 inches
are difficult to lift gently onto the mirror face. Tools act roughly
similar as long as they are at least 50% of the primary mirror
diameter. Experiments during a recent
mirror making class show that there is a distinctly superior recipe to
creating the tool. The recipe goes like this.
220 grit, begin fine grinding. The webbing will be quickly rubbed away.
When it has disappeared, periodically use a metal or stone tool of your
choice to slightly open up the plaster channels between the tiles
sufficient to allow air, grit and water to flow across the tool.
- Use Hydrostone
(a dental stone) or casting plaster, mixed with a power mixer, using the precise mixture
of water to plaster. Use large measuring cups. Hydrostone proved
superior to casting plaster, concrete and other materials.
- Place the mirror on its back, face up.
- Cover the mirror face with thin kitchen plastic wrap.
the 1 inch square unglazed ceramic tiles, still in their webbing, either face
up or face down onto the plastic. It's
not necessary to chip every edge tile to fit within the paper
a thick paper dam around the edge of the mirror, taping it into place.
1 1/2 inches tall is fine for mirrors up to 12 inches in size; beyond
that make a 2 inch thick tool (large tools that are too thick are very
heavy and dangerous near glass because it's too easy to drop it the
short distance to the glass, chipping the glass and ruining the blank).
- Mix up the Hydrostone and pour onto the tiles without
delay. The Hydrostone will flow into the spaces between tiles.
- After a few minutes, the Hydrostone has set enough to
remove the paper dam. It will feel warm and moist to the touch.
- Slide the tool off the mirror.
- Let the tool dry overnight.
of Jerry Oltion's tiled tools. The first shows a tool immediately after
it has cured enough to slide off the mirror face. The second shows a
tool with freshly opened channels between the tiles.
how I made tiled tools. First, I place the webbed tiles face up against
a plastic sheet. Then I pour in the Hydrostone obtained from local
hardware or ceramics store. Then I spend a few minutes grinding it into
shape with 220 grit.The tool does not have to be pristinely
a mirror is critical to avoid small chips and flaking at the mirror's
edge. A rounded bevel can help prevent scratches streaking in from the
edge with very fine aluminum oxide in the final stages that
caused by particles clumping together. I like to maintain a 1/10 inch
diamond belt or cloth will round the mirror edge in minutes; a wetstone
will take longer. Stroke down and across to avoid lifting
off the mirror's face. Use water if necessary.
bevel periodically if it becomes too small.
the tool's tiles is not necessary - sharply angled tile edges do not
break off and cause scratches. Instead, sources of scratches include:
Scratches are hard to come by with 220 silicon carbide grit; they are
very easy with the finer 9 micron aluminum oxide.
large grit particles that are not broken down in the initial strokes by
reduced pressure (holding the tool up in such a way to reduce its
weight on the mirror) and cautious strokes (if a scritching sound is
heard, movement must be stopped immediately and the tool lifted
straight up, followed by a washing of the mirror and tool and a new
charge of grit).
- Continuing a wet too long which leaves broken down grit
the wet dry out causing part of the tool to be pulled in against the
mirror and which can cause grit particles to clump together, acting
like a much larger particle.
- Too fast of stroke which can plow the edge of the tool into
the mirror and cause grit particles to pile up against each other.
- Flexing of thin mirror or tool resulting in the tool's edge
being forced into the mirror's surface.
- Too thick of mixture, causing clumping.
- Too thin or watery of mixture, allowing the tool's edge to
ride into the mirror's surface.
- Abrupt starting and stopping of the grinding strokes.
- Contamination, either a previous grit not being washed out
of the tool or washed away from the mirror's edge, or a previous
grit being drug onto the mirror's face by your shirt sleeve or hands,
or debris being knocked down from the ceiling or even dust in the air
(I've seen the former, experienced the latter).
- Bad grit.
- Forgetting to remove your wedding ring, scratching the
mirror's surface when you mix up the grit and water with your fingers.
- Dirty or contaminated water or water bottle.
- A tile getting lose, often caused by dropping the tool onto
- Pausing the tool's motion on the mirror where the grit
compound can dry out and clump up.
- Too sharp of mirror edge, resulting in glass flakes
breaking off and getting caught between tool and mirror.
- Tool warping.
Fine grinding is done with tool on top (TOT). The grit lasts longer
because it's not pushed down into the channels. Grinding also proceeds
quicker because the tool spends more time working the mirror's edge,
where most of the glass is located and where pits tend to grind out
Tool on top tends to lengthen the radius of curvature
while mirror on top tends to shorten the radius of curvature. The
sagitta should be checked every hour.
Initially the mirror may
not be that spherical as it emerges from rough grinding, or has been
slumped. If it's been generated, then it can be counted to be
spherical; only the generating marks need be removed. Depending on the
mirror shape after rough grinding, the sagitta may undergo unexpected
leaps, either deeper or shallower as the mirror is brought to a
spherical shape. Once the mirror becomes spherical, the sagitta can be
adjusted by prolonged TOT or MOT with 220 grit. Remember that TOT will
lengthen the focal length and MOT will shorten the focal length.
the sagitta is far from desired, a return to rough grinding is
indicated. If that's not practical, then 120 grit or even 80 grit can
be used. The tiles will grind down if pursued too long, resulting in
the need for a second layer of tiles to be glued on top of the thinned
tiles. The new layer of tiles will need to be ground to shape with an
hour to several hours of 220 grit, depending on glass size and type.
Run each grit size for about two hours; longer for large mirrors larger
than 12 inches diameter.
fine grinding session is called a 'wet' and runs five to ten minutes
long. Start with a thoroughly washed mirror and tool, placing the
the bottom. Make sure the mirror's face is clean and dried. Sprinkle
some grit on the surface. You
will quickly see how much grit is necessary: too much results in lots
of grit being pushed off the mirror's edge, too little results in the
wet ending prematurely. Using a water bottle add an equal volume of
Again, you will quickly gauge how much water to add: too little results
in a sluggish muddy mess, too much results in the wet ending
prematurely and possible scratching. Using your fingers, mix up the
grit and water mixture, spreading it thinly across the mirror face.
Gently lower the tool on top, and holding the tool upward such that
there is as little pressure as possible, gingerly move the tool in a
few slow strokes, listening carefully for any scratching sounds. If
they are heard, remove the tool, wash the tool and mirror and start
over. After a half dozen short stokes, relax the upward pressure and
commence with the intended
strokes. As the mixture thins and dries out, spritz a few drops of
water and continue. Eventually, the sound level will drop off
indicating that the grit particles are exhausted, broken down in a
muddy mixture. Lift the tool off. Wash, dry and begin a new wet.
the tool across the mirror face, going past the mirror edge about 1/6
the tool's diameter. Vary a little bit from side to side (no more than
1/10 the tool's diameter). Take ten or so strokes, then take a step to
the right, spin the tool a partial turn clockwise, and continue
stroking. It's important to keep the tool moving at all times - don't
let it pause on top of the mirror, especially with finer grits because
it can seize. Placing the water bottle
on top of the tool is a good way to ensure smooth tool action. Don't
rotate the tool during the strokes. Don't press down on the tool any
more than is necessary to grip it. Keep your hands spread out across
the middle of the tool; avoiding pressing down on the tool's
edge. From time to time, swirl the tool to evenly distribute the grit
and water mixture. After a few minutes, add a couple of drops of water
to prevent drying. The tool and mirror can suddenly seize together if
the grit becomes too dry and thin. Adding a dash of water also clears
the "mud" or broken down grit. Don't go for too long and let the
mixture become too thin. This also carries a high risk of sudden
seizing. Swirl the tool by spinning it (not too fast!)
clockwise while making figure eights across the mirror's face. After
every wet, rotate the mirror clockwise about 30 degrees.
called, "walking the barrel". If you can't walk entirely around the
mirror, then work from a counter corner, walking 90 degrees before
a little bit the stroke length and step size and spin of
the tool along with spin of the mirror. This variation is
not random: in fact it is very regular. Random would be using the throw
of a die to determine stroke length, step size and so forth. It is this
regularity that drives the tool and mirror to spherical shapes. A truly
random approach results in a 'random walk' towards an astigmatic,
non-spheroidal figure. It is this regularity that is the key to
particularly in large thin mirrors.
Supporting the mirror in order to avoid astigmatism is a
particular concern. Options include:
mirror back must be flat, or at least regular in shape. Grind a few
minutes with 220 grit and inspect for low unground spots. Grind until
they disappear. It's OK for the back to be ever so slightly spherical.
pitch. This is very hard pitch, produced in half inch thick by 2 inch
square squares, stuck onto a rigid base such as a granite block. The
mirror is allowed to settle onto the pitch over several days. Once the
mirror is evenly supported, it is never moved from the pitch until
figuring is finished.
- Soft cloth like a bed sheet. Use cleats
to keep the mirror from sliding more than 1/8 inch. Rotate the mirror
15-30 degrees once or two every wet. This is the approach I've used for
- Rubber non-stick pads, found in the kitchen section of big
box stores. Not suitable for very thin work. Rotate as above.
large thin blanks, check the wedge. Wedge is the difference in
thickness from one side of the blank to the other. Take a series of
measurements around the mirror's edge, using a permanent marker pen to
write the results on the mirror back. Preferentially grind the mirror
back where the wedge is too thick. Then re-grind the mirror back flat.
Measure again and repeat until the wedge has disappeared. Wedge can
cause astigmatism; the thinner side of the blank flexes more and adopts
a different radius of curvature. I keep wedge under 0.002 inch.
to spend two to three hours per grit size. This is a relatively
constant time regardless of size of mirror, as long as the tool is as
large as the mirror. Using a magnifying lens, mark the largest pit you
can find by circling it from the back side of the mirror using a
permanent marker. Grind until it's gone. Once again, mark the largest
pit. Eventually you'll notice that no large pits are to be found, just
slightly larger and smaller pits. Circle the largest to remove for the
next grit size. Plate glass grinds a third quicker than Pyrex but can
show more damage than Pyrex if the fine grinding tool is pressed too
hard while grinding.
When you finish the 9 micron aluminum oxide, the mirror will be ready
for polishing and subsequent parabolizing.
6 inch [15cm] fine ground and a 10.5 inch [27cm] fine ground, reading
for polishing and parabolizing.
(end of fine grinding)
For more see