This may work: (has for me) 1. Set all spheres to positive using the attributes panel PRIOR to grouping them. 2. Still PRIOR to grouping, select individual spheres and pull up material editor. Use the rendering options triangle and set the object recieve shadows to false. You could also set object cast shadows to false if the spud doesn't need to cast shadow on supporting surface or neighboring spud. 3. NOW group them and render.... This works for me when I use intersecting shapes to create swirling or "controlled" smoke and mist. Check my gallery images with fire and or smoke. They all had the same problem you speak of until I applied above... good luck! (Why a spud?)