I have been working on your problem for a while, because It sounds like a fun sort of thing to add into my mod, and I havee managed to make a bit of headway, a reaction to extract souls from skulls:
[REACTION:SOUL_GEM]
[NAME:extract souls into soul gems]
[SMELTER]
[REAGENT:1:BAR:NO_SUBTYPE:METAL:VOIDSTONE]
[REAGENT:1:SKULL:NO_SUBTYPE:BONE:CAT]
[REAGENT:1:ROUGH:NO_SUBTYPE:GLASS_GREEN:NO_MATGLOSS]
[PRODUCT:100:1:AMMO:ITEM_AMMO_SOULGEM_FULL:GLASS_GREEN:NO_MATGLOSS]
[PRODUCT:100:1:BAR:NO_SUBTYPE:ASH:NO_MATGLOSS]
[FUEL]
[ITEM_AMMO:ITEM_AMMO_SOULGEM_FULL]
[NAME:soul gem:soul gems]
[CLASS:SHOT]
[DAMAGE:900:GORE]
The reaction will produce a green galss ammo item called a soul gem, since it is not made of metal, you don't have to worry about people melting them down and making -soul essence- bucklers, and the gems in turn can be used in reactions:
[REACTION:SOUL_STEEL]
[NAME:make soul steel]
[SMELTER]
[REAGENT:1:BAR:NO_SUBTYPE:METAL:VOIDSTONE]
[REAGENT:1:BAR:NO_SUBTYPE:METAL:STEEL]
[REAGENT:1:AMMO:ITEM_AMMO_SOULGEM_FULL:GLASS_GREEN:NO_MATGLOSS]
[PRODUCT:100:BAR:NO_SUBTYPE:METAL:STEEL_SOUL]
[FUEL]
[MATGLOSS_METAL:STEEL_SOUL]
[NAME:soulsteel][ADJ:soulsteel][COLOR:0:7:1]
[VALUE:45]
[SPEC_HEAT:500]
[MELTING_POINT:12718]
[BOILING_POINT:14968]
[WEAPON][WEAPON_RANGED][AMMO][DIGGER][ARMOR][ANVIL][ANY_USE]
[DAMAGE_PERC:170]
[BLOCK_PERC:170]
[SOLID_DENSITY:7850]
However...there is still one little(big) problem: the reaction seems to want to use a completely different skull for the reaction then the one I used in my test reaction, cat skulls. I might be using the wrong syntax/whatever for the skulls, but I am at a loss at how to proceed, perhaps someone else has an appropriate fix?