So this morning I fired up the gas burner to test casting aluminium into a green sand mould. The small crucible wouldn't fit into my gas forge, so the metal didn't melt as well as I hoped, but I managed to pour enough to test the concept. And it worked!
I made a green sand mould for a tusk, which involved making a damp mixture of fine beach sand and ground up kitty litter (bentonite clay), then tamping it down around a previously turned tapered former. When the former was withdrawn, the packed sand kept the shape, ready to receive the aluminium. The idea is that the sand is porous enough to allow any steam to vent through the sand instead of exploding out the top.
There was not enough molted aluminium to fill the mould so my finished tusk is only about 2/3 as long as it should be. When it was demoulded, I filed off the sand and fitted it into the lathe for freehand turning. You can see some rough areas from the casting which I decided not to remove by turning, because I quite like the weathered look.
Next step is to excavate the coal forge from behind the shed, ready for a big pour next weekend. The coal forge is much hotter and copes with a large volume of metal than my gas burner, so should get some large castings hopefully.