If you talk about renovation of KBr or NaCl crystals used in IR-spectroscopy (because after some time of usage they become a bit turbid) - then recover them by polishing on a flat/smooth surface of velvet. Put on velvet a bit of dry ethanol and move the crystal - maning trajectrory like "8". It's in theory.  But in fact you will reach a quite OK result if as "polishing liquid " you will use - simply water.

I guess this simple method will work good - for any type of hydroscopic single crystal.

Leonid

Dear Leonid V Vladimirov

                                                Thanks for your reply. I face a problem when polishing crystal on fine velvet cloth. The crystal have an tendency that it will readily observed water molecule from atmosphere. It will spoil the crystal nature.

                                               I have an doubt that by using the readily available polishing liquid, is it possible to avoid the water absorption. Need clarification, Kindly suggest if any specific polishing liquid for these type of crystals.   

In that case you have to change the environment. Usually people are using the glove box with argon atmosphere where you can polish your crystal.

In theory - glove box and argon atmosphere - is only the way which is recommended in theory. But if you ever used this theoretical way - you should know that it is extremely inconvenient. The way which I described is used by in practice. I saw many times how the professional optics are polishing-renovating the KBr and NaCl and I did it myself.

I told you that you can add/place a bit of water on velvet - and it will perfectly work - because the-water-on-velvet - is the best solvent for the outer layer of the optical crystal - thus it removes only the upper layer which is opaque because of it sorbed water during a very long time. When the upper layer of the crystal is removed by dis-solution in water-on-velvet - the crystal becomes quite transparent. Funny enough -

if as na solvent-on-velvet you will use ethanol - then only the water which ethanol contains in concentration of very few %%% - will be the driving force of crystal renovation.

All the above described procedure takes only a few minutes. Have patience and try it again.

Good Luck.        Leonid