Replacement for clay/balsa

Does anybody remember the substitute material for balsa wood? It was in FSM about a year or two ago where one of the featured modeler used this lightweight material that had similar characteristics of Balsa wood. I can't remember what he was scratchbuilding or even what issue of FSM it was. I was wondering if anybody has used it or similar material and what their opinions on it was. I also would not mind know where I can order some and how much it is. That boths me that I cannot remember. I usually have good recolection on that stuff.