Place fresh, fully ripe black mulberries, together with a few red ones, in a muslin bag.
Wearing rubber gloves, press the juice from the mulberries through the bag and into an earthenware or enamel container.
Measure the juice into an enamel or ceramic cooking pot and add 2 cups of sugar for every cup of juice.
Boil the mixture vigorously, stirring with a wooden spoon until the syrup thickens.
Remove from the heat, cool until tepid and pour into bottles.
Leave until completely cool, then seal with airtight lids and store.
When a bottle is unsealed for use, it should be kept in the refrigerator.
Use as a syrup on sweets or dilute 2 tablespoons in a glass of iced water to make a cordial drink.