Put pears in bowl with cold water to cover. Add lemon juice to prevent discoloration.
In large nonreactive saucepan, combine sugar, vinegar, lemon slices, cloves, allspice, ginger, cinnamon and 3 cups of water. Bring to boil over high heat, stirring to dissolve sugar.
Drain pears and add to spiced syrup. Bring to boil, reduce heat to moderate and simmer until pears are tender but still firm, about 3 minutes.
Remove from heat and let pears macerate in syrup overnight. The next day, using slotted spoon, transfer pears to 2 clean pint canning jars. Boil syrup oven high heat until it registers 200° on candy thermometer and is almost at the thread stage, about 35 minutes.
Pour syrup over pears. Let cool; 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 refrigerator for up to a month.) Makes about 2 pints.