Dissolve the salt in the cold water in a large bowl, add the lemon halves and place a clean plate on top to keep them submerged in the salted water. Cover the bowl and leave to stand for six days, stirring daily.
On the seventh day, rinse and drain the lemons. Put them in a saucepan, add enough boiling water to cover, and boil for 15 minutes. Pour into a colander, rinse under cold water, and drain well.
Meanwhile, pour the vinegar into a saucepan, add the spices, bring to the boil and boil for 3 minutes. Remove the pan from the heat and strain the vinegar through a sieve lined with muslin or a clean tea towel, into a heatproof jug.
Pack the lemons in clean, warm jars and pour the hot vinegar over them. Cover the jars immediately with vinegarproof lids. When the pickle is cold, label and date the jars and store in a cool, dry, airy cupboard for about three months before using.