POLISHING

Once the fine grinding with the finest abrasive is done, next step is polishing. Many people won’t believe if you show them the ground mirror and tell them that it will be polished to a mirror finish.

OK, you need following to start the polishing…

Pitch Lap (follow the link to see how the pitch lap was made)
Polishing Mix (cerium oxide)

Please see the Materials page to know about polishing pitch and cerium oxide and how to get them.

Once the pitch lap is ready, you can start polishing the mirror using 1/3 CoC stroke in MoT or ToT position. I suggest doing MoT for entire polishing unless you have a reason to do ToT.

Prepare Polishing Mix

Mix 1 part cerium oxide with 6 to 8 part distilled water (you can get it from a medical shop). It is to prevent any contamination of bigger size particles with fine polishing agent. Use filtered water or packaged drinking water if distilled water is not available.

If you doubt about any contamination in cerium oxide, then you can wash it using the below method.

Take 3 equally sized bottle. I find Bisleri 250 ml best for this purpose but any other type of bottle will also work.
Mix the cerium oxide in 1:10 ratio with water in the first bottle. Mix it rigorously and let it settle for 30 seconds. Heavy particles will be settled at the bottom.
Move the 90% content in the second bottle carefully without disturbing what is left at the bottom of first bottle.
Repeat the same with second bottle and move the 90% content of the second bottle in the third bottle without disturbing what is left at the bottom of the second bottle.
Use mix in the third bottle for polishing and figuring. To remove excess water just let the cerium oxide settle in the bottom in 15-30 min. Once settled, you can remove the excess water from top.

Cold Pressing

Before we start polishing, we need to make sure good contact between mirror and pitch lap. This is achieved using cold pressing. It is done by spreading some polishing mix over the lap and placing mirror over it. Now put a heavy weight over the mirror for around 15-30 minutes. It will press the mirror surface over the lap and create a very good contact for polishing. I used a 5 liter water can as weight as shown in the images below. I also used a damp cloth to cover mirror and tool to prevent the water evaporation.

Start Polishing

Note: I initially made a sub diameter lap which I won’t recommend after getting experts feedback. Keep that in mind since most of the images/video following will show working using sub diameter lap.

Set the lap on the work post.
Make sure work area is clean!
Squirt some polishing mix over the lap. Don’t use too much. A thinner mix work well.
Spread the polishing mix evenly over the lap using your finders. It is also to feel and remove any stray particles from the lap.
Place the mirror carefully over the lap
Rotate it over the lap if you like (to spread CeO₂ evenly)
Gently start 1/3 CoC stroke and speed it up if you feel all is right!
I see I was working on around 2 strokes per second which is 120 strokes per minute which is not a recommended speed 😀
I roughly count from 1-500 and rotate the Mirror anti-clockwise around 15-30 degree on every count of 8, 16, 24, 32, 40, 48 like..
- one..two..three..four..five..six..seven..eight.. turn the mirror
- nine, ten, eleven, twelve, thirteen, fourteen, fifteen, sixteen.. turn the mirror and so on. Number of count before turning the mirror could be more or less for you.
After completing the rough count of 500, rotate the Lap clockwise around 30 degree.
Repeat the polishing by counting from 1-500 in the same manner mentioned in point 9
Complete a full cycle (Twelve, 30 degree rotate of Lap) and inspect the mirror.
Considering 2-3 minute per rotate, you will finish the one session in 30 minutes around without stopping.
I suggest to do at-least 2-3 session per day (around 1.5 hours each day). At this speed you would be able to complete the polishing in a weeks time for 6″ mirror.
It took me more than 20 hours to polish my mirror because I made a sub-diameter lap which impacted the polishing at edge and I had to spend so much additional time to polish the mirror! I wish somebody would have told me earlier to make a full size lap 😐
You may not realize and need to look carefully that I am turning the mirror after 8 or so strokes. It happens when I open my fingers slightly and use my left ring finger to turn the mirror. Look at the moving cerium spots at the mirror’s edge to see the rotation.

Polishing using Mirror on Top (MoT) Position

In the following video you can see me working in Tool on Top position in the similar way. I had to do several sessions of ToT to polish the mirror edge for the reason I mentioned about the sub-diameter lap.

Polishing using Tool on Top (ToT) Position

Polishing Log

Its a good idea to maintain a polishing log to keep the track and feel motivated. It took me 22.5 hrs to polish my 6 inch mirror. Download the attached file to see the actual polishing log.

6-inch-f8-polishing-log Download

Lap Maintenance

After few sessions, channels on the lap will become narrow and you need to re-channel it for the optimal working of lap. Use a sharp blade or knife to cut the channels. Its a good idea to submerge the lap in shallow water bowl to prevent the chipping.

In the first pic below, you can see how badly channels were closed. You can also see clearly that the lap is a sub-diameter. It is also an ugly looking lap as someone said :D.

I also used a fine netting to create micro facets on the lap. This is done to increase the polishing action. Keep the net (Kinda one used to make mosquito nets) between mirror and lap and cold press for around 15-30 minutes. If you don’t have net, you can use use the razor blade to create the micro channel however I now no longer suggest it over the netting method.

Following are the pics of full sized lap which I made after experts feedback on Cloudy Nights.

Result

In following pics you can see the mirror face after stages of polishing sessions. You can see edge was not quite polished in first 5 pics. Last 3 images are complete face polish edge to edge.

When to call the polishing done? It is when all the pits are polished out and you don’t see any uneven reflection from the mirror surface. A better way to use a handheld laser pointer to assess the polishing quality. Please see this Stellafane link to understand how to use the laser pointer method. Since I didn’t have a laser pointer, I relied on analyzing reflection from various light sources to assess the polishing quality