Homemade Blackberry Juice Liqueur (Jagendorf) Recipe

From Folk Wines, Cordials & Brandies, by M.A. Jagendorf:

For each gallon of juice, use 1 pound sugar. For each gallon, add 1 teaspoonful cloves and 1 teaspoonful allspice. You can also use cinnamon or any spice you favor. Bring to a boil and simmer for almost 10 minutes. Then let it cool off, and when it is quite cool, add a quart of neutral spirits (vodka, diluted grain) or even good rye or bourbon or brandy. Let the cordial rest for a month before using.