There are a few benefits to the mason jar method over decarboxylating on a baking sheet in the oven. For starters, decarbing in a mason jar significantly minimizes strong cannabis odors. Another benefit concerns the terpenes that evaporate in the process; they will stick to the glass of the jar and if you use the same jar for your infusion, you can preserve them. Finally, there is no direct contact with the air in the oven, and it is less likely that you will burn any of your cannabis during the heating process.
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