I agree that the porosity is likely to be from over-heating, or too
harsh/oxidizing a flame. In my personal opinion, the speediness or
slowness of the melting is irrelevant. DO use a reducing flame, and
keep it on the metal while it is melting (this is a situation where
AS, like SS, does not do well with oxygen introduced to the melt),
and stop heating as soon as it is fully melted.
I hope you figure this out— I looked at your work, and it is
really lovely! So, I am rooting for you. Please do keep us informed
about what turns out to solve the problem.