Needing some piezoelectric crystals I decided to make some rochelle salt crystals. The steps for doing this are well detailed starting here. The steps I followed were to first make some rochelle salt crystals so that I'd have a seed crystal and then to use that seed crystal and the rest of the rochelle salt to make a larger crystal. As of this writing I've finished the first step only.

Making the Rochelle Salt

To make the rochelle salt, follow the instructions here.

Some tips that I learned by doing:

• The instructions say in step 6, to "filter the hot solution by using filter paper of a coffee filter." The key word here is "hot". One time, my filtering was taking a while because I spilled some unfiltered solution into the destination container and had to start over again. All this time, the solution was cooling so I actually got crystals forming inside the filter, slowing the filtering down further. If this happens, heat the solution again. You could also try changing the filter if you are almost finished but if you have a lot left, you'll only end up wasting solution by having it crystalize in the new filter.
• The instructions say in step 8, to "store in a cool place for several days". Mine took 20 days to fully crystalize. I had it sitting on a window sill and it crystalized faster on days when it was cool (5 - 15 degrees C).

The resulting rochelle salt crystals after removing from the measuring cup that it had crystalized in over 20 days. The bigger chunks are on the left.

Testing the Rochelle Salt

Before going on to the next step I wanted to check the piezoelectric properties of what I'd done so far.

I selected what looked like my largest single crystal and wedged it lightly between two electrical contact surfaces. The surfaces were made by packing layers of paper towel behind aluminum foil. This gave me an aluminum foil cushion. This allowed me to press both sides of the crystal with these cushions, resulting in good electrical contact without putting too much pressure on the crystal.	To do the test, I placed the flat end of a plactic bodied marker on the crystal surface and tapped lightly on the other end of the marker with the flat of a pair of pliers.
The result was this 9.6 volt peak-to-peak alternating voltage. The initial sharp peak was due to the initial tap. The downward shorter peak after that was probably the relaxation after the tap.	The total time for both peaks was 4.2 milliseconds.

I was curious if the peaks would reverse polarity if I reversed the scope leads (done here).

The result was the expected reversal in polarities.

Here is the result of a really hard tap... 26 volts.

Making One Large Rochelle Salt Crystal

The next step would be to make one large rochelle salt crystal using the next step here. Hopefully it would end up as nice as this one.