Place sugar, salt, cinnamon, cloves, all spice, lemon zest, lemon juice and bourbon all into a heavy bottom saucepan. Stir to combine. Over medium heat, stir the mixture to prevent burning. Reduce the mixture by about half - or until the mixture is thick and no longer runny. Make sure to stir constantly to ensure as much moisture is evaporating as possible. The thickening time will take anywhere from 1 to 2 hours depending on the liquid content of the apples.