In a large saucepan, cover the chicken wings with the cold water and bring to a simmer over moderate heat, skimming as necessary. Add the parsley, scallions and salt and simmer for 2 hours, skimming occasionally. Line a colander with 4 layers of dampened cheesecloth. Set the colander over a heatproof bowl and pour in the stock. Let cool to room temperature, then refrigerate for up to 3 days or freeze for up to 1 month. Skim off the fat before using.