Leafcutter John has a great post about making Rochelle Salt, which is piezoelectric, and hence can be used as a contact mic: Real Sound Cookery.

My folks Peter and Judith are both crystallographers, so I thought I'd put them to work in the lab (kitchen) to make some nice crystals.

We followed a pretty similar procedure to Leafcutter John, along with some extra info from the crystal guide here

Simply put:

• Mixed 80g Cream of Tartar with 100g water, and brought to a gentle simmer
• Added Washing Soda (Sodium Carbonate) a teaspoon at a time
• It fizzed each time. We added the soda until it stopped fizzing and went clear
• Filtered immediately before it cools
• Left it in a bowl in the freezer till it made some crystals

We pulled out some of the good crystals. While we were doing this, we got some crystal showers - when you interfere with a saturated solution, lots of tiny crystals form. This is no good if you want single crystals!

In order to get better crystals, we decided to re-crystallise:

• wait till a lot of crystals have formed
• strain out the crystals using filter paper and a funnel
• wash very quickly with water
• put into water to redissolve. We used ~80g water for 60g crystals, and heated it in the microwave until it melted.

We wanted crystallisation to happen through evaporation, so it is slow, and allows large crystals to form. So we left the solution in clean glasses for a couple of days, and dangled seed crystals in to give a good nucleation point.

Here it is as a photo story...

We started with 80g of Cream of Tartar Added 100g of water Brought to a gentle simmer, stirring constantly Added Washing Soda a half teaspoon at a time It fizzed merrily It kept fizzing each time we added more Until at some point it stopped fizzing, and went clear. That means it's ready We filtered the resulting liquid quickly before it cooled The bowl went in the fridge, and we left it for a few hours. Concentration gradients around the crystals forming Crystal showers - when we fiddled around with the saturated solution, millions of tiny crystals precipitated out We got 8 or so squarish crystals, about 10mm a side, 1-3mm thick Wedged between a couple of bits of circuit board, clamped to the table, picking up some sounds. One nice clean single crystal Seed crystal, hanging on some fishing line in solution