Piezoelectricity is the charge that accumulates in solid materials in response to applied mechanical stress. Substances that exhibit piezoelectric properties to a greater or lesser extent are crystals (Tourmaline, Quartz, Topaz, Rochelle salt and sugar cane), certain ceramics and biological matter such as bone, DNA and various proteins. Quartz and Rochelle salt exhibit the greatest amount of piezoelectrical properties and since the ingredients and instructions for making Rochelle salt are relatively straightforward I decided to make some to play with.
I found some helpful guides here and here, but diverged from both guides several times without ill effect. Here are my instructions for making Rochelle salt (Potassium Sodium Tartrate), which yielded at least 5 tablespoons full of the salt and several large single crystals in a few days.
The recipe calls for cream of tartar (Potassium Bitartrate or Potassium Hydrogen Tartrate) and washing soda (soda ash or Sodium Carbonate). Cream of tartar is a byproduct of winemaking; as the grapes age this acidic salt forms on the barrels of wine and is collected and used in a variety of household purposes, from preventing sugars from crystallizing to an ingredient in baking powder to stabilizing egg whites in recipes. Washing soda is highly alkaline (pH of 11) and often used as a water softener. It can be extracted from the ashes of plants and is sometimes used to make German pretzels. Washing soda can be made by heating baking soda (Sodium Bicarbonate or Sodium Hydrogen Carbonate) in an oven to break and evaporate the bonds of hydrogen to leave the Sodium Carbonate.
I didn’t have access to washing soda at my local grocery store so I had to make some. There is a difference between store bought washing soda versus washing soda made at home from baking soda. Arm & Hammer washing soda or any similar brand is decahydrate (contains ten molecules of water of crystallization per molecule) and homemade washing soda is anhydrate (lacks the presence of physically attached water). This means that you will need less homemade washing soda to achieve the same or similar results of store-bought washing soda. In the picture above I started with 1 1/2 cups of baking soda on a baking sheet. This makes way more washing soda than you need for the Rochelle salt so I suggest saving it to clean with or make a homemade laundry detergent.
Spread the baking soda across the sheet in a thin even layer and bake at 300 degrees F for 2 hours, stirring and re-spreading into a thin layer every 30 minutes. Take out and allow to cool for 5 minutes and prepare in a small saucepan 10 tablespoons of cream of tartar in 1 cup of water.
Washing soda can be slightly irritating to the skin and mucous membranes so it might be best to ventilate the area and wear gloves. Heat the water and cream of tartar over medium heat until it is slightly boiling and add a teaspoon full of washing soda to the pot. You’ll notice a lot of bubbles and fizzing!
Keep adding teaspoons of washing soda to the solution until it reaches it’s saturation point. This will be suddenly apparent as the bubbling and fizzing will stop abruptly after adding the soda.
I added a total of 8 teaspoons of my homemade washing soda to reach the saturation point. Boil this solution uncovered for 5 more minutes to evaporate more of the water. Allow the pot to cool then place uncovered in the refrigerator or a cool place. After a few hours you should see crystals start to grow!
Keep in the refrigerator overnight (I kept mine in the coldest part of the refrigerator) and in the morning you should have a substantial growth of crystals.
Drain the excess fluid and place a few small crystals back in the dish as seeds and place in the refrigerator again. Separate and dry the other crystals on paper towels.
When tested a Rochelle salt crystal can generate a peak charge of 9.6 volts of electricity after being tapped. They can also be used as a laxative and in microphones. Here is my instructable on it. After they are dry you can store them in a glass jar for later use or crush them for use in my next experiment!
