Crystal growth method for high clarity


Today I show you how to grow crystals with high clarity. In general, slow growth rate is needed to get high transparent crystals. There are two major ways to grow crystals slowly; slow evaporation of solution or slow cooling. These are easy to do but have some problems. For example, the former is sensitive to room temperature, especially if solubility changes drastically by temperature. And the latter is hard to control cooling speed. So I use the following two-layer diffusion method, as seen in hydrothermal crystallization of quartz.

・A fishing line (nylon, flexible and fine is better)
・2 fishing rubber stoppers
・A fishing basket
・A transparent bottle or testing tube
・A transparent box
・A acrylic rod (φ2mm)
・A filter bag
・A seed crystal
・Raw materials (powder is not recommended, rough crystals are better)
・Saturated solution

◆ Method
This time, I grow alum crystals.

1. Tie the seed crystal to the fishing line.
2. Cut off the shorter line near the knot.
3. Drill a hole in the center of the bottle lid through which the acrylic rod will pass.

4. Assemble these parts.

5. Put the raw materials into a filter bag and pack it into the basket.

6. Attach the seed crystal to the acrylic rod with a rubber stopper.
7. Adjust the basket so that it is positioned on the top half of the bottle.
8. Adjust the seed so that it is positioned on the bottom half of the bottle.

9. Pour the saturated solution into the bottle.
10. Set the bottle in the box.
11. Fill the box with water so that the liquid level is between the nutrients and the seed crystal.

11. Place the set in a place with little temperature change and grow the seed.

* Change the water in the box regularly.
* If many crystals precipitate at the bottom of the bottle, filter the saturated aqueous solution in the bottle to remove precipitated crystals.

◆ Comparison with the cooling method
This is an alum crystal grown by the two-layer diffusion method for a month.

And this shows difference between the general cooling method and the two-layer diffusion method.
Compared with the cooling method, the crystal size is smaller, but transparency is better.

Left: grown by the cooling method
Right: grown by the two-layer diffusion method

◆ Mechanism
In this method, crystal growth is driven by the temperature difference between cooler bottom layer of the bottle and hotter upper layer. First, water in the box is cooled by its evaporation. Therefore, the seed crystal grows. Second, the cooled bottom solution is carried up to the top of the bottle very slowly by convection. Third, upper layer of the bottle is heated by air because the bottle is sealed and can’t cool down by evaporation. So rough crystals in the basket dissolve and make solution saturated again. And then, saturated solution is carried down again.
By repeating these steps, the seed grows slowly and be good transparency!

This method can be applied not only for alum, but also for almost all substances whose solubility decreases when cooled. This is also a very easy method because once it is prepared, crystals grow by leaving them alone. Furthermore, this method is somewhat insensitive to changes in room temperature.
I hope you will try making various crystals!



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