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Simple experiments on the internet show how to grow salt (NaCl) crystals, but the crystals are small and flawed. Several companies sell flawless optics, like windows, made of NaCl, which are presumably cut and polished from large crystals. Rectangular windows of size 50mm x 25mm x 2mm are available at fairly low cost. Where would I find the procedure for growing such large, flawless NaCl crystals?

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    $\begingroup$ I would guess that salt is first dried, then pressed into a disc under a vacuum. I highly doubt that the commercial windows are made from a single crystal. $\endgroup$
    – MaxW
    Commented Nov 16, 2023 at 15:24
  • $\begingroup$ I remember having read once somewhere that adding urea to the salt solution helps growing nice big crystals of NaCl. $\endgroup$
    – Maurice
    Commented Nov 16, 2023 at 15:57

1 Answer 1

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First, you need a good source of comparatively pure $\ce{NaCl}$; table salt won't do. Not only are there accidental impurities, such as salts of other alkali metals, but anticaking agents and iodides often are added intentionally, and the anticaking agents are intended to reduce crystallization.

Second, if you're growing crystals from water solution,

  • It must be done slowly, over weeks or months,
  • Water must be allowed to get out without dust getting in, and
  • The container must be undisturbed, free of vibration and temperature change or gradients.

However, for (near) perfect crystals, don't grow them from solution, but rather from a melt under controlled conditions, such as is done in the Czochralski process and in the similar Bridgman–Stockbarger technique.

While those techniques might be beyond the average home chemist's resources for crystallizing $\ce{NaCl}$, it's not hard to get good $\ce{Bi}$ crystals by the similar process of withdrawing the seed from the melt by hand, as in this video. Of course, they have hopper crystal defects, as could $\ce{NaCl}$.

See Crystals and Crystal Growing by Morrison and Holden for far more information.

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    $\begingroup$ To complement the answer (+1), about NaCl windows for Lasers, there is the conference proceeding «Polytran NaCl Windows For LASL Antares $\ce{CO2}$ Laser System» by V. E. Straughan at Los Alamos Conference on Optics '79 (doi 10.1117/12.957732) which outlines the state of the art already available then. This basically is: melting (ca $\pu{800 C}$) and slow cooling of NaCl (though not the household commercial one) to obtain ingots of 18" ($\approx \pu{45 cm}$) diameter by Stockbarger technique, cutting, polishing, selling. And by now, we're 4 decades later. $\endgroup$
    – Buttonwood
    Commented Nov 16, 2023 at 18:20

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