Let’s say you have a mirror– not some cheesey $2 makeup mirror, but a research-grade aluminum mirror– and it has some crud on it, say a film of junk deposited during your Summer Institute for Hot MEtal Chemistry. Like, say, the mirror on the right in this picture:
How do you get that mirror clean?
Well, first, you assemble your tools:
You’ll need the mirror, some lens tissue (special soft, lint-free paper), a hemostat (I prefer the curved-tip kind shown here, but you can use the straight ones, too), and some solvent (the bottle here is methanol, but you sometimes need to use acetone).
The goal here is to get the dirt off the mirror without scratching it or leaving new kinds of dirt on the mirror surface. Thus, it’s crucial to avoid touching the mirror with your hands, or anything that’s been in contact with your hands, lest you leave finger grease all over the optical surface. (You can, in principle, wear latex gloves to do this, assuming your hands are of a size that one size fits, but I don’t generally bother.) Thus, take a sheet of lens tissue, and fold it in half along the long direction, holding it only by the ends. Then fold it in half again, making a long narrow strip.
Still holding the strip only by the ends, fold it in half along the short axis, then in half again. The easiest way to do this is to hold both ends and pull it taut, then sort of bump the middle part up against the edge of some (clean) object, like the table, to get the fold started. The key thing here is to make sure that the outside surface never comes in contact with anything dirty. Once you have it all folded up, clamp it in the hemostat, as shown in the (slightly blurry) picture at right.
Then, take your solvent, and put a drop or two onto the lens tissue. You don’t want to overdo it– at extreme levels, the paper will disintegrate, but long before that, you’ll be leaving puddles of solvent on the mirror. I usually go with 2-3 drops from a dropper bottle like the one shown in the picture above, and then shake the hemostat for a few seconds to get rid of any excess.
Then take the hemostat, and bring the clean, wet surface of the lens tissue into contact with one edge of the mirror. Pressing the tissue firmly into the mirror, swipe the tissue across the mirror from one edge to another, once and only once. Do not swipe it back and forth– then you’re just moving the dirt around. Swipe it across, then discard the tissue.
(If the mirror is exceptionally dirty, you might need to make several passes, and in that case, you might as well make a couple of passes with the same tissue. But on the final cleaning pass, you use one piece of lens tissue for one swipe in one direction, to eliminate any traces of dirt or solvent left by previous passes.
(The choice and quality of solvents is also important here: you need to use the expensive spectroscopic grade methanol, not the reagent grade stuff, and if you need to use acetone, it should be the highest-purity acetone you can get from your chemistry department. If you end up using acetone (as I did here), you need to follow it up with a lighter cleaning with methanol– acetone tends to leave more of a residue, that can cloud the mirror slightly.)
If all goes well, you will have cut a big swath through the dirt on the mirror, as seen in the picture at left. Good work.
Now, you just keep repeating those steps until you’ve cleaned the whole thing.
There aren’t too many pitfalls to avoid, here. The biggest one is that you want to make sure you don’t scratch the mirror surface. This means that you need to be extremely careful not to drag the metal tip of the hemostat across the surface of the mirror (duh), but also, you want to avoid trapping any dust or grit between the lens tissue and the mirror. If you do, and the trapped material is harder than the coating, you’ll ruin the mirror (I did this to the output coupler on a Ti:Sapph laser once, which was not pretty). If you’re not sure whether there’s stuff on the mirror, blast it with canned air, which usually removes anything dangerous.
At the end of this process, you should have a nice, clean mirror– maybe not exactly as good as new, especially if you baked soot onto it over the summer, but suitable for use in scientific experiments. And you’ll also have a big pile of used lens tissue:
but that’s just the price we pay for clean optics.
And that’s how you clean a mirror. Any questions?