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Real Sound Cookery – Make a contact mic with baking soda and cream of tartar.

I’ve been into contact mics for a very long time, and have used them to record sounds for my records since 1999. There is something magical about how an innocuous little brass disc covered in ceramic piezo crystals can act as a sonic microscope, revealing tiny sound-worlds easily and cheaply. For a bit more info on contact pickups have a look at some of my previous posts: Shit I’m a Geek / The joy of Piezoelectricity and Leafcutter’s DIY Steel Can Hydrophone & Preamp. Step-by-step guide *schematic link fixed!*

I recently stumbled on a great video tutorial by Collin’s Lab entitled  ‘Homebrew Piezo’ in it he shows how to make Potassium sodium tartrate  crystals also known as “Rochelle Salt”. These crystals are fairly amazing in that they are made from ingredients available cheaply from your local grocery store or supermarket. I suggest you watch it as it shows the process really clearly. Later in this post I’ll show my experience of following Collin’s method and do something Collin didn’t – Made some sound with it!

If you want to make your own crystals you’ll need:

• 200 g / 7 oz Cream of Tartar (aka Potassium Bitartrate, aka potassium hydrogen tartrate) from the grocer/supermarket
• about 120 g / 4 oz Sodium Carbonate  (aka soda ash) OR same amount of baking soda (Sodium Bicarbonate) if you cant find the Sodium Carbonate.
• Pyrex jug or bowl
• Pan big enough to fit the bowl inside
• Filter paper or Kitchen towel
• Thermometer useful but not essential
• Clean Baking tray and Oven if you are using Baking Soda instead of Sodium Carbonate

The Method is pretty simple, follow Collin’s Video to the letter! If you cant see the video for some reason here are the basic steps:

Collin asks for Sodium Carbonate  (soda ash) in the video, I could not find this but luckily you can make it very easily. Get hold of some Sodium Bicarbonate from the chemist or supermarket and spread it thinly on a clean baking tray. Now bake this for an hour or so at about 150 deg C / 300 deg F stirring the powder every 15 mins or so. Doing this will give of water vapor and Carbon Dioxide (open a window maybe) and after an hour your Sodium Bicarbonate will have turned into Sodium Carbonate!

1. Using your Pyrex jug Dissolve 200 gr / 7oz Cream of Tartar (Potassium Bitartrate) into 1 cup cold water. Stir.
2. Pop the jug into a pan of water, turn on the heat until the water bath reaches about 82 C / 180 F. If you don’t have a thermometer the water will start simmering at this point so keep an eye out for little bubbles in the pan.
3. When the water bath is at 82 C / 180 F put half a teaspoon of Sodium Carbonate into the Pyrex jug and stir, it’ll bubble up and then subside.
4. When the bubbling subsides repeat step 3. until the milky mixture goes clear (slight yellow tint).
5. Your mixture is now ready to filter, while still hot pour into another heat proof container (a clean jam jar is good) through filter paper or kitchen towel to get rid of any solids. Don’t mess about here as crystals will form as soon as the mixture cools and you don’t want that happening in the filter.
6. Place the container in a coolish spot cover loosely and wait for the crystals to form!

This is my solution just after pouring:

 

 

So I seem to have generated thousands of tiny crystals as opposed to a few big ones like Collin managed – not sure why this happened but maybe linked to a fairly short cooling time. I wanted to see if the crystals were in fact piezo electric so I arranged the following rig to hold the tiny crystal against two contacts which were connected to a little battery powered guitar amp.

Holding a music box against the clamp I was able to record this sound

Not quite as good as the stuff in my mic drawer but it’s amazing considering it’s made from cookery ingredients!

I’m going to let the crystals in the jars carry on growing and I’ll add another post when I’ve managed to grow a bigger crystal.

Give it a go and let me know how you get on!

J x.

UPDATE – Check out my larger crystals.

Tags: contact mic, crystal, Homebrew Piezo, homemade contact mic, piezo, Rochelle Salt

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