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Joined: Sat Sep 26, 2009 10:39 am
Mon Apr 12, 2010 7:48 pm
Mirror Making - Part 5
MIRROR MAKING – FINE GRINDING
After hogging out the initial curve in the surface of the mirror, all subsequent grinding stages are for one purpose, to grind out all the pits until the glass has a smooth enough surface to commence polishing. If the surface is too foggy (grainy), all polishing will do is give you a shiny fog and when it is coated, it will look terrible and won’t reflect light efficiently. Also, it will be difficult to test and figure the mirror without a highly polished surface.
Grit is just fancy gravel. When you rub it between two hard pieces of glass, it eventually breaks down. As this “gravel” breaks down, it chips and scratches the surface of each piece of glass. This chipping and scratching is how the curve is dug out. The problem is that after rough grinding, the surface looks more like non-slip pavement (or sandpaper) than a mirror.
Fine grinding breaks down those large pits into smaller ones that correspond to each grit size you are using. Starting with 120 grit, the idea is to grind the glass and remove the larger pits from the 60 or 80 grit you started with. It will be easy to tell the difference after about an hour of grinding. The pits scarring the glass will become smaller. There will likely be a few large pits here and there also. You must keep going until all the pits are uniform. Any large pits you leave will only complicate things later down the road.
In the rough grinding, you might want to hurry things up and use a long stroke that overhangs too far. Or, you might press down too hard on the mirror as you grind. These bad habits will be the most likely cause of a hole in the center. If you take your time and use a moderate stroke as I described in rough grinding, this hole in the middle of the mirror should not be a problem.
One thing to keep in mind is that you are now starting with a finer abrasive, which means you will get mud faster. If there is a large hole in the middle, when combined with the 120 or finer grits, it can create a suction and you’ll end up with the mirror and tool stuck together. I already mentioned how to break them apart so I won’t go into that again. It is easy to see through the glass (if you are using a clear glass material). When the tool is wetted and charged and the mirror is centered over the tool, you will see a bubble in the middle while contact gets better and better toward the edge. This defect becomes more visible as the grit starts to break down. In that case, you should shorten your stroke a bit and try to flatten out the curve. Use this short stroke until the glass makes reasonably even contact. It may not be perfect, but that is okay at these early grinding stages. What you don’t want is a huge bubble in the middle. Another red flag is after grinding a while, you start to notice the pits are disappearing from around the perimeter of the mirror but are not in the middle.
Even with no deep hole in the center of the mirror, the finer abrasives are going to create mud faster and faster, and the whole feel of the grinding is going to change. The noise will be less, and the grinding less aggressive. This is all part of preparing the glass surface to be turned into a highly polished one.
My advice is to fine grind 120 until all the large pits are gone, then go for another 15 to 20 minutes just to make sure. Clean off the mirror, hold it up to the light (or use a magnifying glass) and look for any pits you might have missed. Also look for smoothness of the fog. If it looks odd or somehow uneven, clean that up with more fine grinding before you go to the next stage.
In my old mirror making books, it says to examine the surface with a magnifying glass under a bright light to check the size of the pits. I’ve done this and found it only partially effective. A quick scan is okay but to try and examine every square inch of the glass has given me a headache. Instead, I do a quick scan and only concentrate on any “suspect” spots. When I made my 16”, one suspected spot was simply a stain in the glass from something foreign that got into the grit. Whatever it was, it wasn’t abrasive or I might have had a lot more of that stage to grind.
Once you are satisfied that there are no large pits left, clean your work area including the mirror and tool, check the bevel around each of them, then go to 220 grit and so on. Once you get into the 300 grit stages, it’s going to feel like you are starting with mud instead of grit. This is normal. Also, the finer you go, the harder it is going to be to see the difference in pit sizes. I remember in the 320 grit range, I just ground until the fog changed evenly over the whole mirror. I must also add that if you missed a pit or two in an earlier stage, they are really going to stand out!
It is extremely important that you watch for cross-contamination. There is nothing more frustrating than to have a nice smooth 440 grit fog and hear a screech as a piece of larger grit migrates between the two pieces of glass. You may have to go back a few stages to get rid of that sleek. It’s happened to me. I’ve also had the bevel chip and scratch the glass. I’ve had to go back several grit stages for a few of those reasons. It’s not the end of the world, but it certainly makes your job that much harder to do.
By the time you get to the 400 grits, it’s almost going to feel like lubricating mud versus grit. These final stages prepare the glass surface for the polishing. At this point, you will have a surface reflective enough to do a sun focal length test. You can verify focal length by taking the mirror outside and aiming it at a wall or fence. Move it back and forth until the bright spot is focused, or as sharp as you can get it. Now measure from the wall to the mirror face, and that is what your present focal length is. I used the sun method after each stage, starting by wetting the mirror in a bucket until I didn’t need to wet it at all. In any case, I just let the mirror fall where it might in focal length and worked with it. If you are a super perfectionist, you’ll need to back up and either deepen or flatten the curve to get your ballpark focal length. Keep in mind that with each stage of grinding, the focal length will still change slightly, but it will change less and less with each stage.
To deepen the mirror and shorten the focal length, increase the length of the stroke. To shallow out the mirror and lengthen the focal length, shorten the stroke.
I must mention here that there is another way to check the focal length, especially if you are working at night. Put the mirror on the mirror stand (which I haven’t described yet). Spray the mirror with clean water. Now plug in a lamp or shop light with a clear light bulb in it. While standing a few feet away from the mirror, wave the light in front of you until you can see it’s reflection in the mirror. Now back up until the filament within the light bulb focuses on the mirror face. That, my friends, is your rough focal length. During my mirror making days, I used both, but favored the sun method until I was into the polishing stages.
When you get to the final grit stage, which is almost a polish anyway, you should be able to see clear through the glass without it being wet. That is when you know it is time to start polishing. There should be no or just a hint of fogging. If not, keep grinding until you do. Any significant fog will likely NOT polish out no matter how long you keep at it.
In the next part, we will get into making a mirror stand and Focault tester.
After hogging out the initial curve in the surface of the mirror, all subsequent grinding stages are for one purpose, to grind out all the pits until the glass has a smooth enough surface to commence polishing. If the surface is too foggy (grainy), all polishing will do is give you a shiny fog and when it is coated, it will look terrible and won’t reflect light efficiently. Also, it will be difficult to test and figure the mirror without a highly polished surface.
Grit is just fancy gravel. When you rub it between two hard pieces of glass, it eventually breaks down. As this “gravel” breaks down, it chips and scratches the surface of each piece of glass. This chipping and scratching is how the curve is dug out. The problem is that after rough grinding, the surface looks more like non-slip pavement (or sandpaper) than a mirror.
Fine grinding breaks down those large pits into smaller ones that correspond to each grit size you are using. Starting with 120 grit, the idea is to grind the glass and remove the larger pits from the 60 or 80 grit you started with. It will be easy to tell the difference after about an hour of grinding. The pits scarring the glass will become smaller. There will likely be a few large pits here and there also. You must keep going until all the pits are uniform. Any large pits you leave will only complicate things later down the road.
In the rough grinding, you might want to hurry things up and use a long stroke that overhangs too far. Or, you might press down too hard on the mirror as you grind. These bad habits will be the most likely cause of a hole in the center. If you take your time and use a moderate stroke as I described in rough grinding, this hole in the middle of the mirror should not be a problem.
One thing to keep in mind is that you are now starting with a finer abrasive, which means you will get mud faster. If there is a large hole in the middle, when combined with the 120 or finer grits, it can create a suction and you’ll end up with the mirror and tool stuck together. I already mentioned how to break them apart so I won’t go into that again. It is easy to see through the glass (if you are using a clear glass material). When the tool is wetted and charged and the mirror is centered over the tool, you will see a bubble in the middle while contact gets better and better toward the edge. This defect becomes more visible as the grit starts to break down. In that case, you should shorten your stroke a bit and try to flatten out the curve. Use this short stroke until the glass makes reasonably even contact. It may not be perfect, but that is okay at these early grinding stages. What you don’t want is a huge bubble in the middle. Another red flag is after grinding a while, you start to notice the pits are disappearing from around the perimeter of the mirror but are not in the middle.
Even with no deep hole in the center of the mirror, the finer abrasives are going to create mud faster and faster, and the whole feel of the grinding is going to change. The noise will be less, and the grinding less aggressive. This is all part of preparing the glass surface to be turned into a highly polished one.
My advice is to fine grind 120 until all the large pits are gone, then go for another 15 to 20 minutes just to make sure. Clean off the mirror, hold it up to the light (or use a magnifying glass) and look for any pits you might have missed. Also look for smoothness of the fog. If it looks odd or somehow uneven, clean that up with more fine grinding before you go to the next stage.
In my old mirror making books, it says to examine the surface with a magnifying glass under a bright light to check the size of the pits. I’ve done this and found it only partially effective. A quick scan is okay but to try and examine every square inch of the glass has given me a headache. Instead, I do a quick scan and only concentrate on any “suspect” spots. When I made my 16”, one suspected spot was simply a stain in the glass from something foreign that got into the grit. Whatever it was, it wasn’t abrasive or I might have had a lot more of that stage to grind.
Once you are satisfied that there are no large pits left, clean your work area including the mirror and tool, check the bevel around each of them, then go to 220 grit and so on. Once you get into the 300 grit stages, it’s going to feel like you are starting with mud instead of grit. This is normal. Also, the finer you go, the harder it is going to be to see the difference in pit sizes. I remember in the 320 grit range, I just ground until the fog changed evenly over the whole mirror. I must also add that if you missed a pit or two in an earlier stage, they are really going to stand out!
It is extremely important that you watch for cross-contamination. There is nothing more frustrating than to have a nice smooth 440 grit fog and hear a screech as a piece of larger grit migrates between the two pieces of glass. You may have to go back a few stages to get rid of that sleek. It’s happened to me. I’ve also had the bevel chip and scratch the glass. I’ve had to go back several grit stages for a few of those reasons. It’s not the end of the world, but it certainly makes your job that much harder to do.
By the time you get to the 400 grits, it’s almost going to feel like lubricating mud versus grit. These final stages prepare the glass surface for the polishing. At this point, you will have a surface reflective enough to do a sun focal length test. You can verify focal length by taking the mirror outside and aiming it at a wall or fence. Move it back and forth until the bright spot is focused, or as sharp as you can get it. Now measure from the wall to the mirror face, and that is what your present focal length is. I used the sun method after each stage, starting by wetting the mirror in a bucket until I didn’t need to wet it at all. In any case, I just let the mirror fall where it might in focal length and worked with it. If you are a super perfectionist, you’ll need to back up and either deepen or flatten the curve to get your ballpark focal length. Keep in mind that with each stage of grinding, the focal length will still change slightly, but it will change less and less with each stage.
To deepen the mirror and shorten the focal length, increase the length of the stroke. To shallow out the mirror and lengthen the focal length, shorten the stroke.
I must mention here that there is another way to check the focal length, especially if you are working at night. Put the mirror on the mirror stand (which I haven’t described yet). Spray the mirror with clean water. Now plug in a lamp or shop light with a clear light bulb in it. While standing a few feet away from the mirror, wave the light in front of you until you can see it’s reflection in the mirror. Now back up until the filament within the light bulb focuses on the mirror face. That, my friends, is your rough focal length. During my mirror making days, I used both, but favored the sun method until I was into the polishing stages.
When you get to the final grit stage, which is almost a polish anyway, you should be able to see clear through the glass without it being wet. That is when you know it is time to start polishing. There should be no or just a hint of fogging. If not, keep grinding until you do. Any significant fog will likely NOT polish out no matter how long you keep at it.
In the next part, we will get into making a mirror stand and Focault tester.