What was the pilum made of?

[Hibernicus]

I too prefer ASH

hardwood / soft wood etc.... A bit of basic research on wood will reveal a variety of characteristics: density, flexibility, compressibility etc..

With all factors considered ash is the best choice.

The major advantages coppiced wood has over timber for use in pilum shafts:
1) its far easier to harvest (smaller diameter!)
2) grows to a usable size sooner
3) a whole pole is stronger than a quartered split, rived or sawn board that's been shaped round
4) far far less labor to make the shaft

A coppiced pole can be harvested and shaped in 15-20 minutes.
A finished metal shaft can be attached in about the same amount of time.