I have 40 pounds of Drystall as well as white pumice bought from a nursery.
When I put it into a bowl of water it sinks. If you look at Drystall and
pumice under microscope, they are identical.

Your data point on Crater Lake is interesting, and I don't know why your
pumice behaves differently than mine. If I read the pumice raft idea
correctly, this just means that sometimes a large piece of pumice traps a
large air pocket internally.

Perlite, however, is a significant floater.

Sorry for top posting but I could not get my reader to quote your post
correctly.

--
W




"Chris" wrote in message
...
On Feb 24, 4:21 pm, "W" wrote:
"songbird" wrote in message

...

W wrote:
What is the most economical source for a large bag of small red lava
rock,
to be used as a replacement for pumice in a succulent soil mixture? I
also
want to use the red lava rock as a mulch.

the stuff floats away easily, so not the
best mulch if you get heavy rains, winds or
floods.

Are you confusing lava rock with perlite? Perlite definitely floats.
Volcanic aggregates (i.e., either white pumice or red lava rock) sink in
water.

I did a quick test with 10 smallish red lava rocks in a bucket of water,
and
they all 10 sank.

--
W

Lava rocks are not pumice. Pumice floats. Having tossed a few hundred
pumice pieces into Crater Lake, I can attest to this. You might also
look at

http://en.wikipedia.org/wiki/Pumice_raft