Brown Stuff is the stiffer counterpart to the soft green sculpting putties. Where the green type stretches and suits cloth, the brown type holds a sharp edge and resists slumping, which is what you want for armour plate, weapon parts, buckles and any detail that should look machined rather than draped. Sculptors often blend the two to land between flexibility and edge retention. Blending it with the soft green type is ordinary practice and moves the mix towards whichever behaviour a part calls for, so the two are usually bought together out of two-part epoxy putties. Armatures, wire frames, holders and the shapers that cut plate and buckles are indexed in sculpting tools and materials.
