: I agree--it's super light in air, but when you mix it with epoxy, it doesn't
: displace any epoxy to speak of. The resulting mix is as dense (unit
: weight) or even denser than solid epoxy alone. Wood flour and sawdust
: displace some epoxy (that which isn't absorbed by the wood fibers) and
: make the thickened epoxy less dense. Microballoons (glass and phenolic)
: are mostly hollow and really displace epoxy mass. While they feel heavier,
: they displace more epoxy molecules and actually make the mixture lighter.

: When you mix cabosil into epoxy, you don't change the volume of the original
: epoxy mixture much. You've added more mass, but kept the original volume,
: making the mixture denser and heavier. Notice when you're mixing it that
: you put a pretty substantial amount in your mixing cup, but it all gets
: mixed in, and the downy appearance is all gone. The thickened epoxy
: remaining is about the same volume as your unthickened epoxy, but it's got
: a bunch of cabosil particles packed into it.

: When you mix less dense fillers into epoxy, you increase the overall volume a
: great deal while only adding a little weight. The new mixture is less
: dense and lighter. (While it may not be as flowable, it is less
: dense--less weight per unit volume).

: A molecule of cabosil is as heavy as a molecule of glass fiber. The cabosil
: particle, though, is only a few molecules thick, and doesn't noticeably
: displace epoxy. Your glass fibers are just larger particles of silica
: (which is denser and heavier than epoxy) so they pack a lot more weight
: into the volume of epoxy that was displaced.

: That's why cabosil-thickened epoxy is harder to sand than
: microballoon-thickened epoxy. It's denser and harder.

: I've heard of folks using portland cement as a structural thickener. Cement
: is a fine dust and looks almost as fluffy as the cabosil (it is about as
: fluffy as microballoons) but it really makes a dense mixture.

: Shawn

When you mix up potting soil you usually use 1/3 soil, 1/3 silica and 1/3 pearlite (tiny foam balls). The job of the silica is to soak up water and hold it. So it would seem that silica would soak up the thin epoxy and in the end be a heavy mixture while the pearlites job is to seperate and loosen the soil. So maybe the pearlite could be used as a filler for lightness if strength is not what you are after. Also, I don't recall seeing any pearlite degrading from wet soil over a long period of time.

John