1. Put the pears in a bowl with cold water to cover. Add the lemon juice to prevent discoloration.
2. In a large nonreactive saucepan, combine the sugar, vinegar, lemon slices, cloves, allspice, ginger, cinnamon and 3 cups of water. Bring to a boil over high heat, stirring to dissolve the sugar.
3. Drain the pears and add them to the spiced syrup. Bring to a boil, reduce the heat to moderate and simmer until the pears are tender but still firm, about 3 minutes.
4. Remove from the heat and let the pears macerate at room temperature in the syrup overnight. The next day, using a slotted spoon, transfer the pears to clean pint canning jars or a glass bowl. Boil the syrup over high heat until it registers 200F on a candy thermometer and is almost at the thread stage, about 35 minutes.
5. Pour the syrup over the pears. Let cool, then cover and refrigerate until ready to serve. (The pears will become a more beautiful color after 2 or 3 days in the syrup. They keep well in the refrigerator for up to a month.)