I make most of my pens out of Saguaro Cactus Skeletons, very old, very dry, very unstable. I have tried everything, jars, pressure cookers, 2 qt paint pots and didn't find anytrhing that could hold a high vacuum for a long time till I bought a vacuum chamber on ebay. $40., but it works like a champ. A little secret I discovered is to use vacuum grease on the gasket. You can order vacuum grease from any lab supply source. Maybe I'll take a picture of my set up and post it here, if I can figure out how. Good luck!