during a fractional distillation in the laboratory (or chemical plant), simple straight tubes can be packed with materials to increase surface area, and therefore the number of theoretical plates; in the same manner, the surface areas of simple laboratory glass condensers such as the liebig can be filled to improve performance. the same standard distillation packing materials can be used—glass beads, rings, or helices (such as fenske rings), "porcelain" raschig or lessing rings, or metal packings of aluminum, copper, nickel, and stainless steel of most of the preceding shapes (such as metal lessing and fenske types); glass packings have as a further benefit their chemical inertness relevant to distillations of reactive chemicals (like acid chlorides), while metal packings are easier to tamp to "ensure uniform packing".