All wood (excluding adulterations like plywood or resin impregnated materials) moves as a result of changes in heat and moisture content. It always moves in three directions based upon its orientation in the living tree/bush. Depending on the species, a stable wood moves the same amount in each of the three directions.
There are some things that you can do to wood to diminish the movement and the internal tensions which result. The purpose of boiling the briar blocks before being put out to dry is to break the cell walls, soften the lignum, and relieve the internal stresses. I have heard a lot about removing tannins and resins, but boiling, particularly for burls, has been a standard technique for a long, long time.
The primary problem of movement in a finished pipe is the disparity in movement of different matierials. A tenon might be tight in one season and looser in another. The incorporation of ivory, horn, etc. present an extra point of potential failure. Probably the main reason that screw joints were used so much on older pipes is because glue joints, between disparate materials, almost always fail over time. Our modern glues work better, but whether they are good enough we will discover over the next fifty years.
anthony
