Meredith,
    What you've done is VERY interesting.  Am I correct in assuming that your latest test (1.5 lbs
oscillating 50 minutes) was with the aluminum rod hanging down and swinging from the stationary steel plate,
with the 1.5 lb weight at the bottom?  If so, how far below the axis was the weight.  The moment of inertia is an
important issue in the decay time, so I would really like to know this number to try and assess the
relative importance of air damping compared to the  internal friction associated with your axis.

  Randall