Making Rochelle Salt

02 Apr 2015

An experiment

I found this blog post by Alice Rzezonka about rochelle salt crystals and immediately thought, I gotta do this.

Rochelle salt are piezoelectric. Piezoelectricity means electricity resulting from pressure. So if you smack them, they will emit an electrical charge. It's not a very big one. I think the ones I created generated about ~1V. That's not very impressive, but since they emit ANY voltage, you can use them as a sensor.

Just like in the blog post, I used the arduino Knock sketch and was able to change the intensity of an LED while smacking 2 rochelle salt crystals together.

Pretty cool right?

To make the crystals you need the following:

  • Washing Powder (OR baking soda that you have cooked in the oven at 300 degrees for 2 hrs)
  • Cream of Tartar (NOTE: some of the grocery story cream of tartars are impure. Try to buy one that mentions it being a byproduct of winemaking)
  • distilled water

First you need to create a saturated solution with the cream of tartar. That's about 10 tablespoons per 1 cup of water. Mix throughly.

Then boil the mixture, until it reaches about 160 degrees. At this point, turn the heat down to simmer and then slowly mix half teaspoons of the washing powder, until the solution stops fizzing and the water becomes clear. Boil this a little more (1-2 mins) and then transfer to a baking tin and let cool.

If it worked you will start to see little square crystals after an hour:

mini crystals

TROUBLESHOOTING: If you see sludge similar to ice shavings, then one of 2 things could have happened:
Your cream of tartar wasn't pure or your mixture didn't reach it's saturation point when mixing in the washing powder.

You can try reboiling the solution and adding a tiny bit of washing powder. If it still fizzes, then add a little more until the fizzing stops and it becomes clear. Transfer to a pan and let cool.

If you let it rest overnight you should see a lot of crystal growth:


I picked out the biggest ones for this experiment. You can grow larger ones than these, but I had issues with the purity of my cream of tartar.


Check out Alice Rzezonka's blog post for the fritzing diagram and how to test your crystals after you create them.

Published on 02 Apr 2015 Find me on Twitter!