Put 4tablespoons of water in a bowl. Wash the oranges thoroughly and rub the lumps of sugar over the skins so that they imbibe the perfumed oil in the zest. Put the sugar lumps to melt in the water and add the caster or icing sugar. Cut the oranges in half and squeeze out the juice, straining it into the bowl through a sieve or strainer. Mix well with a whisk to dissolve the sugar. Transfer to the ice-cream maker and freeze.
To serve the sorbet scoop it into balls with a tablespoon dipped in hot water and pile into well-chilled ice-cream dishes.