When scientists are measuring the uranium and lead in a rock—specifically in the mineral zircon, found in many igneous rocks—to determine its age (U/Pb geochronology), they need to dissolve the zircon. Zircon is a very stable mineral, so to dissolve zircon, the mineral grains are subjected to acids at high temperatures (~200 °C) and pressures. Thick steel pressure vessels are needed to contain an inner teflon vessel when it heats up and the liquid inside boils.
In the picture above, there are two pressure vessels. On the right of the red marker, a smaller vessel is used when the zircons from one rock sample are being partially dissolved to remove the exterior surface (chemical abrasion). To the left of the red marker, the large vessel is used for the final dissolution, when zircon grains are on teflon racks with individual teflon capsules.