I am trying to create esters from natural resins (Copal, amber, dammar, etc.). This is based on chemistry from the first half of the 20th century. The esterified resins were used in making varnishes, before the advent of acrylic and other synthetic resins. To esterify the natural resins, I must first fuse the resin (called ‘running’) by melting at temperatures of 300° C. for one to two hours. I will run the resins in a round bottom flask with a reflux condenser. Then I will lower the temperature to 260° to 280° C. and add the esterifying agents (glycerin, phthalic anhydride, pentaerythritol, etc.) and react this mixture for another two to four hours. The esterifying agents will be added dropwise through an addition funnel. Then linseed oil or other drying oil will be added to the mixture at 200° C. to complete the varnish. I do not have a lot of experience with setting reactions of this type, just distillations at lower temperatures. I am concerned about the high temperatures of this reaction and how it may affect the glassware and the joints, especially since I was planning to use Glindemann PTFE sealing rings in the joints and a PTFE stopcock on the addition funnel. I know the upper temperature range of PFTE is supposed to be about 220° to 240° C. Does anyone see a problem with this setup? Would I be better off with silicone grease? I planned to heat the flask in either a sand bath or replace the sand with zinc granules (30 mesh) to get better heat transfer. Does anyone have experience with metal powders, especially zinc in sand baths? One more note: The glassware I have is made of heavy wall borosilicate glass. I get conflicting ideas about the upper temperature for this type of glassware.