Great question — answering as the UphillTech team:
You want a wood with high porosity to take up as much of the mineral laden water as possible. Perhaps balsa if you can find a block of it, poplar or willow would be good too. Cedar might work. Hardwoods will definitely not work well .. nor will any pine variety as their natural pitch will resist water uptake.
Full answer by vulcan_: https://crafts.stackexchange.com/a/4269 (CC BY-SA 4.0).
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