you can probably buy something like this very easilly, or get Bentonite Clay as used in fireworx.
or you can make this dodgey imitation that probably works as well as anything else.
i get a tub of water putty powder [pic] which is cheap, chalky and superfine.
then i add some PVA craft glue flavouring to enough warm water to satisfy the application.
i spread the putty powder on a sheet of paper, and spray it with a fine mist of PVA/water, until it is damp and little darker. then i quickly gather up the paper so the dampened powder falls to the middle, and gently rattle it into the cardboard tube. the tube is standing on its end on a hard smooth surface, like a benchtop or thick metal plate or whatever, and the putty powder is prodded to the bottom until judged to be of sufficient depth, and then packed solid with some sort of rammer and a hammer, just like bentonite clay.
the resulting plug can be drilled straight away and shouldn't crumble or blow out. this method is fine for little [or maybe huge, i dare you] 1-use motors, but crap for bigger / shaped / multi-use nozzles.
the putty can also be mixed up like putty is intended to be, using the same ingredients as the packed stuff, but ground together into a thick homogenous paste with a mortar and pestle.
this appears to make a very hard, strong, slightly plastic substance. once it dries of course. i use it for forming nice nozzles with metal cores, as this PVA putty becomes chalky again when channeling gasses, and subsequently erodes quite fast. water is its elemental weakness, but damage can be repaired and surfaces smoothed with a little water+pva and perhaps a tiny bit of additional putty powder on one's finger.
recently i found that its ok to clean a nozzle made of PVA putty with water as long as its just a breif exposure and the nozzle is dried off straight away, since erosion repairs will probably be needed anyway. the best attribute of this substance is its strength and convenience, rather than its performance.
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