Put 200 ml (7 fl oz) water in a bowl. Add the flour, charcoal, salt and oil and mix to form a dough (kneading is not necessary). Rest the dough for 30 minutes. Preheat the oven to 180°C (350°F) and line about 5 baking trays. Roll the dough as thinly as possible into rectangles approximately the size of your baking trays. Put the dough on the trays and bake for 11 minutes or until crunchy. Cool on the trays, smash into small pieces and store in an airtight tin for up to 1 month.