Rinse the grains in a sieve under running water, then dump into the slow cooker. Add 6 cups (1.4 L) water. Cover and cook on low for 3 to 4 hours, until tender, checking them occasionally to make sure they don’t overcook. Stir in the salt, if using, in the last 30 minutes or so. Drain in a sieve. Store covered in the refrigerator for up to 5 days or in the freezer for up to 6 months.