Grate the soap with a cheese grater.
Add the grated soap, washing soda and essential oil (if using) to your food processor.
Blend until you have a fine powder. You may want to lay a dish towel over the top of your food processor to prevent a fine mist of powder from floating into the air. Also, let it settle a bit before opening the container or the powder will float onto your kitchen counter.
Pour into a clean, airtight container.