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Saturated sodium chloride solutions are evaporated at elevated temperature. The aim of evaporation is to produce hollow pyramid salt crystals. Temperatures used range from 65°C to 85°C. With 80°C obtaining the best results yet. I was able to save a couple of large pyramid crystals before they were taken up in a crust.

All evaporation experiments do seem to produce some hollow pyramid crystals. However, most of the very small pyramid crystals encounter other very small crystals, clumping together. This happens faster than the growth of individual crystals. Resulting in a crust that covers the liquid-air interface, leaving no room for proper pyramid formation.

The salt is dissolved in both water and a mixture of ethanol and water in different ratios. Pure water solutions with NaCl gave the best results yet. But still, the crust formation is a large problem.

So, here comes the real question: Is there a method to counter crust formation, so that individual crystals can grow larger? Or am I looking for an answer in the wrong way, should I perhaps search for another crystallization method to produce NaCl pyramids?

Below a picture of the crust formation, and a picture of the best hollow pyramids I obtained yet.

Crust formation while evaporating a NaCl solution

Hollow pyramid salt crystals obtained from evaporation of a NaCl solution at 80°C

Thank you for reading this. I hope to find some valuable solutions to this problem.


Additional comments on preparation:

The general method used is to heat a temperature-controlled oil bath to the desired temperature, and add the beaker with salt solution to the oil bath. I use an oil bath to keep the relative humidity low around the beaker, for faster evaporation. I cover the beaker while heating to the desired temperature, then uncover it to start the evaporation.

Larger and shallower dishes can be used and are experimented with. The general trend I observe is that a layer of approximately 1 cm gives best results. A deeper filled dish gives too much crystallization at the same time on the interface. While a lower filled evaporation dish does not allow pyramid formation since the formation of hollow pyramids depends on slow sinkage of the crystals. The point is faced down, while the sides are grown on the liquid-air interface. So you could say, while crystallizing it is a reversed hollow pyramid.

A high evaporation rate is needed to create the pyramids, slower evaporation often leads to cubic salt crystals. It can also lead to pyramid-like crystals with a cuboid top, which is not desired.


Aditional comments on similarity with How to make pyramid-shaped NaCl salt crystals:

The problem is similar to the aforementioned topic, I read the topic before I opened this one. I try to build on the previous topic and further explore the knowledge on this very specific mechanism.

No solution was found in the previous topic. A valuable suggestion was made by @andselisk, using a water-ethanol mix for pyramidal crystals. However, the source mentioned for this statement does not seem to mention that. A similar statement is found on the Crystal-growing-pedia: https://en.crystalls.info/Sodium_chloride#Influence_of_impurities Details are lacking in these descriptions. I have not yet been able to produce pyramidal crystals using a water-ethanol solution.

Therefore, my main question remains. What method should be used to successfully and consistently produce pyramidal salt crystals? A derivative question from this is the search for a method to counter the crust formation. The solution for the latter will probably combine with my previous results to find a successful method to create pyramidal salt crystals.

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    $\begingroup$ The general answer for "how do I get larger crystals" questions is go slower. Not sure if that is useful for these very special crystals. $\endgroup$ – Karl Oct 10 at 19:14
  • $\begingroup$ Can you explain the method a little more in detail? I suppose you place a large dish on a hot plate. What variables can you alter? Can you use a larger (shallower) dish to spread the solution, or would this have the opposite of the desired effect? $\endgroup$ – Buck Thorn Oct 11 at 7:37
  • $\begingroup$ You could try to gently move the surface without disturbing the bulk. I agree with the comment by @Karl $\endgroup$ – Alchimista Oct 11 at 10:17
  • $\begingroup$ Thank you for your comments. The general method used is to heat a temperature controlled oil bath to the desired temperature, and add the beaker with salt solution to the oil bath. I use an oil bath in order to keep the relative humidity low around the beaker, for faster evaporation. I cover the beaker while heating to the desired temperature, then uncover it to start the evaporation. $\endgroup$ – Flex Oct 11 at 14:40
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    $\begingroup$ Possible duplicate of How to make pyramid-shaped NaCl salt crystals $\endgroup$ – Buck Thorn Oct 13 at 8:51

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