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Comment by symmetricsaurus | Hacker News Reader

If the scratches are on the glass it could work even better to use a liquid with the same refractive index as the glass. That should make the scratches go away completely.

One option is glycerin as can be seen in this video: https://www.youtube.com/watch?v=pNWCB_GoQA4 (maybe it only works with pyrex glass ?)

Another option is to mix to fluids together, one with a higher index of refraction and one with lower. That way you can tune it to your particular scanner (although I would think most are the same).

How you see if your mix has the same refractive index as the glass I haven't figured out yet.

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> How you see if your mix has the same refractive index as the glass I haven't figured out yet.

You can do this by direct observation but it is not going to be 100% accurate:

You'd need some kind of etched grating for reference.

Drop the glass+grating into the mix, check for changes to the grid lines where you know the glass resides. If you can spot it then you still have a difference.

You could also simply measure the refractive indices of both. (For most microscopy glass you can find out the exact index of refraction from the manufacturer so you really only need to measure one.)

If I could put the glass into the liquid it would be fairly easy. But, isn't the glass stuck to the top of a flatbed scanner in this case? Makes it a bit hard to put it into a liquid without breaking the scanner.

In that case you might want to try to do some absolute measurements on the dimensions of the grid.

So you get (scannerglass + fluid + gridsample), then measure the grid point distance (by taking as large a distance as you can across the grid) and compare to what you know it should be. The closer the grid measurements are to the actual dimensions the closer the index of refraction of the fluid will match the glass.

I think the scratches are in exactly the same place each time, which suggests they're on the chip, unless the chip was repositioned very carefully.

The vertical portions of the scratches in the dry pic are accentuated, while the horizontal portions of the same scrates are highlighted in the wet pic. The only thing I can think that would cause that is the chip being rotated 90 degrees in relation to the scanner light.

Similarly, you can scrub some scratched up glasses with toothpaste to fill in the air gaps. Good as new (at least until it rains).

Toothpaste has some abrasives in it, I wouldn't use it on the glasses.

Toothpaste is mostly soap, so try that instead. Or mineral oil.

That doesn't work the same way, that polishes the edges of the scratches so they're not as distinct.

Is this related to the old lifehack about smearing toothpaste on your watch's sapphire crystal screen?

You can use toothpase to polish the bottom of a soda / beer can, which you can then use to focus sunlight to start a fire. I've done it.

There are better ways to start a fire.

That's using the abrasive silica in the toothpaste (aka "sand") as a very mild polishing agent.

Silica is not even close to hard enough to polish sapphire, and most toothpaste, instead of containing silica, contains chalk, which is not even close to as hard as silica. The problem with brushing your teeth with silica is that silica is not a very mild polishing agent; it's a pretty aggressive one, and it will wear away your tooth enamel pretty quickly. If those are your permanent teeth, that enamel ain’t coming back.

Silica and maybe even chalk will work fine to polish glass, though.

I have a correction. I went out and bought a tube of Colgate ("Triple Acción! Extra Blancura!") and it does indeed contain silica as an abrasive. (And Colgate is hardly some weird off-brand toothpaste.). The difference is that it’s hydrated silica, which is amorphous rather than crystalline, and is somewhat softer than normal silica. (And, as an extra special bonus, not carcinogenic like normal silica!) This is the same stuff known as "silica gel" that comes in little pouches with your electronics labeled "DESICCANT — DO NOT EAT".

Amorphous hydrated silica is still hard enough to polish glass and abrade your tooth enamel. Chalk is not, although I'm not sure it's hard enough to abrade away your tartar deposits.

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