PART II RED MERCURY DIRTY BOMB (forget the plutonium and the uranium !)

The lenses were cast in special molds made from cellulose acetate. Each lens consisted of two types of explosive, one fast-detonating Composition-B [60% RDX, 39% TNT, 1% wax] and the other slow-detonating Baratol. Each lens had three pieces: two made of high velocity explosive, and one of low velocity explosive. The outermost piece of high velocity explosive had a conical cavity in its inner surface into which fitted an appropriately shaped piece of slow explosive. When the lenses were in place, the fast-detonating part of each lens touched the layer of explosive

http://www.globalsecurity.org/wmd/intro/nuke-he.htm