Pick over and stalk the currants, then mash them with a wooden spoon, adding half a pint of water. Next put the fruit into a preserving-pan with the castor sugar, let it cook gently until it begins to simmer, stirring it all the time; then pass it through a hair sieve.
Put the lump sugar in a saucepan with three-quarters of a pint of boiling water; let the sugar dissolve, then boil it to a syrup. Pour this on to the fruit juice, adding one and a half pints more water. Leave it until cold, then serve.