Why not? Is this dangerous to have the jar covered? I understand it will lower the boiling point, but will you not also evaporate a portion of the ethanol?Heating the alcohol mixture can be done very safely using a hot water bath. You will need an accurate candy or quick read thermometer. Place about 1 inch of water in a wide, vertical-edged pan (9” wide x 3” high). Bring the water to a low simmer. The rum/cannabis mixture should be in a small (1 pint) mason jar. Do NOT cover the jar.
I'm using 95% ethanol, btw. good stuff.