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.