Carving hard plaster

I have a series of small plaster figures, each more than 5 years old, which I would like to modify and hopefully improve before recasting in bronze or other metal. The plaster was an alpha-hemihydrate type, hence very hard. I found it carves rather nicely, but slowly, with an HSS burr in a pendent drill. This leaves chatter and other tool marks on the surface of the plaster, but frequently I found these marks rather appealing. It also ‘carves’ well with a small coarse alumina-composition grinding tool in a pendent drill. Various shapes are available, unfortunately I find that the smaller and more precise shapes are a less coarse compound and take longer to cut.
Having some of the old rubber moulds available, which I made around the same time, I tried pouring copies in a much softer (’potters’) plaster, but the incidence of air bubbles was so high as to make the casts nearly useless. I expect that if the unset mix, and subsequently the moulds containing the freshly-poured mix, were subject to reduced pressure with a vacuum pump, they may become just what I need. Unfortunately I mislaid my aspirator some time ago, and this kind of cheap and effective pump now seems difficult to find.