So, by heating alcohol in a sealed jar, you are basically making a bomb. Since alcohol boils at 170f…..if will be vaporizing and continually increasing the internal pressure of the jar. Potentially, the jar could explode, the lid could fail, etc…..causing a rapid expulsion of alcohol vapor that will most definitely be in the form of some type of fire.Just remember if u do it with lid off your gonna have flammable alcohol vapors venting off into your house!
By leaving the jar open, the vapors mix with the steam from the water bath, and it all gets diluted into the house….which should have open windows etc. It won’t be enough volume of vapor to reach the explosive limit of the house. In my opinion, open jar is safer….doing it outside is even safer than that….covering the weed in booze and storing in the refrigerator for a week is safest of all.