I'v been using this IRL inspired solution.
If you take a magnet to any sandy beach you'll find a black sand sticks to it.
I don't suggest using a magnet you like as it is very hard to get it all off!
[REACTION:SANDIRON]
[NAME:Process Sand For Iron]
[BUILDING:SMELTER:NONE]
[REAGENT:B:10:BOX:NONE:NONE:NONE][BAG][CONTAINS_SAND]
[PRODUCT:100:2:BAR:NO_SUBTYPE:METAL:IRON][PRODUCT_DIMENSION:150]
[PRODUCT:100:3:BOULDER:NO_SUBTYPE:OBSIDIAN:NONE]
[FUEL]
[SKILL:SMELT]
Slag is a custom material copied from obsidian.
http://en.wikipedia.org/wiki/IronsandAs a rule I dislike cheating in a serious embark. It makes me feel better to have a 'real world' solution.
Similarly an earlier mod:
[REACTION:BONEBURN]
[NAME:Burn Bones for Calcite flux]
[BUILDING:TRASHBURNER:NONE]
[REAGENT:bones:1:NONE:NONE:NONE:NONE][USE_BODY_COMPONENT][ANY_BONE_MATERIAL]
[PRODUCT:100:1:BOULDER:NO_SUBTYPE:INORGANIC:CALBONE]
[SKILL:SMELT]
[REACTION:TRASH_VERMIN]
[NAME:Burn Dead Vermin for flux and potash]
[BUILDING:TRASHBURNER:NONE]
[REAGENT:A:10:REMAINS:NONE:NONE:NONE]
[PRODUCT:100:3:BAR:NONE:POTASH:NONE][PRODUCT_DIMENSION:150]
[PRODUCT:100:1:BOULDER:NO_SUBTYPE:INORGANIC:CALBONE]
[SKILL:POTASH_MAKING]
http://www.appropedia.org/Iron_and_SteelCalcined bone has been used as a flux.
more on the building and materials
calcined bones are a usable flux
[INORGANIC:CALBONE]
[USE_MATERIAL_TEMPLATE:STONE_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:calcined bones][DISPLAY_COLOR:7:7:1][TILE:'177']
[REACTION_CLASS:FLUX]
[MATERIAL_VALUE:3]
[IS_STONE]
[MELTING_POINT:12902]
[INORGANIC:SLAG]
[USE_MATERIAL_TEMPLATE:STONE_TEMPLATE]
[MELTING_POINT:13600]
[BOILING_POINT:16000]
[STATE_NAME_ADJ:ALL_SOLID:slag][DISPLAY_COLOR:0:7:1][TILE:177][LAVA]
[MAX_EDGE:20000]
[MATERIAL_VALUE:1]
[IS_STONE]
and the trashburning building
[OBJECT:BUILDING]
[BUILDING_WORKSHOP:TRASHBURNER]
[NAME:Burn Barrel]
[NAME_COLOR:7:0:1]
[WORK_LOCATION:1:1]
[BUILD_LABOR:SMELT]
[BUILD_KEY:NONE]
[DIM:2:1]
[TILE:0:1:32:32]
[COLOR:0:1:0:0:0:0:0:0]
[TILE:1:1:32:182]
[COLOR:1:1:0:0:0:0:0:1]
[TILE:3:1:32:182]
[COLOR:2:1:0:0:0:0:0:1]
[TILE:3:1:32:182]
[COLOR:3:1:0:0:0:4:0:0]
[BUILD_ITEM:1:BARREL:NONE:METAL:NONE][CAN_USE_ARTIFACT][EMPTY]
[BUILD_ITEM:1:BUCKET:NONE:NONE:NONE][EMPTY][CAN_USE_ARTIFACT]