c for between two and eight minutes and preferably about five minutes has been found to provide adequate fillet formation within a brazed wick a vacuum of at least 10 u2212 5 torr or lower has been found to be sufficient and if hydrogen furnaces are to be used the hydrogen furnace should use wet hydrogen in one embodiment the assembly is vacuum fired at 1020 u00b0 c for 5 minutes in a vacuum of is 5 u00d7 10 u2212 5 torr or lower referring to fig7 14 and 16 17 grooved brazed wick structure 38 may also be advantageously formed from metal particles 27 combined with brazing compound 30 more particularly a mandrel 40 fig1 is used to create grooved wick structure 38 that comprises a plurality of parallel lands 45 that are spaced apart by parallel grooves 47 lands 45 of mandrel 40 form grooves 50 of finished brazed grooved wick structure 38 and grooves 47 of mandrel 40 form lands 52 finished brazed grooved wick structure 38 each land 52 is formed as an inverted substantially u201c v u201d shaped or pyramidal protrusion having sloped side walls 54 a 54 b and is spaced apart from adjacent lands grooves 50 separate lands 52 and are arranged in substantially parallel longitudinally or transversely oriented rows that extend at least through evaporator section 5 the terminal portions of grooves 50 adjacent to e g a peripheral edge wall 20 may be unbounded by further porous structures in one embodiment a relatively thin layer of brazed metal particles is deposited upon inner surface 18 of bottom wall 12 so as.