A method is described that allows large amounts of fluid to be sealed into platinum capsules without loss of fluid during welding, and using a minimum amount of metal. After loading the solution and other starting materials into the capsule, a preliminary seal is obtained by pressing the capsule walls around an inserted disc of platinum. This mechanical seal, which is produced in a two-piece die compressed in a vice, prevents loss of volatiles during subsequent mounting of the closure lid by arc welding. The result is a short, cylindrical capsule with a regular geometry. We have tested this capsule preparation technique in 36 consecutive piston cylinder experiments in which Pt95Rh05 capsules filled with aqueous solutions were subjected to 700–1100 °C and 1.0–3.5 GPa for 4–119 h. In 28 of the runs the amount of fluid lost during the experiment was less than 5 wt%. The new method is suitable for any kind of experiments in which volatile-rich charges need to be sealed into platinum capsules, and in which short capsules with a regular geometry need to be obtained.