Tobacco pipes are made from a number of different materials, though briar wood is the most common one. Pipes consist of three major parts: the mouthpiece, stem, and bowl. The mouthpiece and stem are typically one piece that is crafted out of vulcanite or Lucite. The bowl is a separate piece that is carved out wood, clay, corn cob, stone, and even metal. Each of these materials offers its own unique smoking experience, but some are better than others. Additionally, some bowls are better when treated and some are better left untreated.
The primary issue in the question of treated vs. untreated bowls is preventing damage to the bowl when smoking. Not only does the bowl have to handle the heat from the lit tobacco, it also has to deal with moisture and ash. One wrong move and any one (or a combination of factors) can cause irreparable damage to the pipe or cause it to sour.
The most popular material pipes are made from is briar wood and for a specific reason. This wood is heat resistant and absorbs moisture very well. It is also amendable to being carved in a variety of shapes. Briar wood pipes can typically be used without being treated, though it is still important to manage the cake and clean them on a regular basis.
Other types of woods like cherry, maple, plum, and ash are not so flexible. For pipes made with these woods, it is a good idea to treat the bowl. Some manufacturers pre-treat their pipes to help them resist heat and handle moisture better. However, pipe smokers can treat their bowls at home by slowly building up a protective layer of ash (also called cake). The layer should be about the same width as a U.S. dime. Any bigger than that and there is a risk of the cake expanding too much and breaking the bowl. Rubbing the inside of the bowl with a mixture of honey and water can also mitigate the effects of heat and moisture.
Materials like clay, meerschaum, and calabash also typically require a “break in” period. Though porous, pipes made from these materials don’t always handle heat well. It is best to use them cautiously the first few times to get a feel for how the material handles heat and moisture before using with them on a regular basis.
Lastly, corn cob pipes are like briar pipes in that they can be used untreated. Though considered less elegant than pipes made from other materials, they are very functional pieces. They are inexpensive, provide a very cool smoke, and are perfect for beginners.