depending on the material and the rate of smoking. For this reason, clay pipes in particular are often held by the stem. Meerschaum pipes are held in a square of chamois leather, with gloves, or else by the stem in order to prevent uneven coloring of the material. Cleaning The ash and the last bits of unburned tobacco, known as dottle, should be cleaned out with a suitable pipe tool. A soft or bristle pipe cleaner, which may be moistened with strong spirits, is then run through the airways of the stem and shank to remove any moisture, ash, and other residue before the pipe is allowed to dry. A pipe should be allowed to cool before removing the stem to avoid the possibility of warping it. A cake of ash eventually develops inside the bowl. This is generally considered desirable for controlling overall heat. However, if it becomes too thick, it may expand faster than the bowl of the pipe itself when heated, cracking the bowl. Before reaching this point, it needs to be scraped down with a reamer. It is generally recommended to keep the cake at approximately the thickness of a U.S. dime (about 1/20 of an inch or 1.5 mm), though sometimes the cake is removed entirely as part of efforts to eliminate flavors or aromas. Cake is considered undesirable in meerschaum pipes because it can easily crack the bowl or interfere with the mineral's natural porosity. Meerschaum also softens when heated so it is recommended to allow meerschaum pipes to cool before cleaning as people have been known to push pipe cleaners through the walls of heated pipes. Regardless whether a pipe is cleaned after every smoke, over time there is a buildup of cake in the bowl and tars in the internals of a smoking pipe. The cake can be controlled by gentle reaming, but a buildup of tars in the shank and airway of a pipe is more difficult to deal with. This may require the services of a professional pipe restorer to properly clean and sanitize the pipe. Sweetening When tobacco is burned, oils from adjoining not yet ignited particles vaporize and condense into the existing cake on the walls of the bowl and shank. Over time, these oils can oxidize and turn rancid, causing the pipe to give a sour or bitter smoke. A purported countermeasure involves filling the bowl with