There are a few ways to thicken preserves naturally without using pectin. First, let it set and cool completely in the fridge. The longer it sets, the thicker and more gelled it will become. Sometimes you need to let it cool overnight. If it is still not as thick as you'd like after setting overnight, return the preserves to the crockpot and cook again on low for 4 hours. You can repeat this until the preserves are reduced to your liking. Another option is to add chia seeds. Chia seeds have a natural gelling ability and can help to thicken preserves and jams. Add 1 tbsp of chia seeds to every 1 cup of pear preserves and stir. You can puree the chia and pear mixture if you do not want the seeds to be visible.