About alum: I've consulted several books, a tanner and a doctor who
specializes in toxic effects on workers. Alum makes injuries like acids
if handled a)by a sensitive person or b) in rather high concentration,
one doesn't exclude the other. Try to buy it not powdered but
crystallized, wear a mask if handling it dry and gloves when handling
it in solution, barrier creams may not be sufficient.
I don't use a mask but a thin cotton scarf folded several times and
knotted around my face like John Wayne. For me as a paste paper maker
that's sufficient, but then I need alum only in thin solution and once
in a blue moon.
This problem is nothing compared to what the tanners and dyers of old
had to deal with - tannins in high concentration with nor protection at
all and dirt, dust, germs, animal decay into the bargain. With
About base paper: try Hahnemühle Natural Line. It's cheaper than the
Ingres, smoother and more obliging, feels thinner. Ilona Hesse of
Leipzig, who is a third generation marbler, uses it as one of her
stock-in-trades and says it works fine. It's conservation grade, and
that's important to me as a restorer!