QUOTE(Dissever @ May 30 2012, 07:51 AM)
I am curious about the decision to use the Firm for the helmet, and the normal for the face. What prompted those choices?
Firm is stiffer than regular Sculpey, which was useful for the chin guards. Thin, and somewhat unsupported structures can be delicate in Super Sculpey. I don't use Firm that often, and am more familiar with Sculpey, so I used that as my go-to putty for the bulk of the head.
How would the figure have turned out differently if you used, say, the Firm for the face, and normal Sculpey for the helmet?
Not a huge difference, really. It's mostly a matter of how comfortable and familiar one feels with the putty.
What is the upper critical mass of Sculpey you could use on a transforming figure without worrying about pieces shearing off?
Cured Sculpey, especially when it's somewhat thin, is brittle. It won't warp under stress, it breaks. The actual neck joint on Ursa is made of Brown Stuff (a stiffer, harder version of Green Stuff, aka Kneadatite), and in vehicle mode the head hangs freely on a hinge, so the shearing stresses are avoided.
I wouldn't recommend Sculpey for any parts of a figure, that have to resist shearing forces. Either use a more elastic putty, or cast the parts in resin.