Is it Stealth? (2015)

Reimar Horten claimed that he wanted to add charcoal to the adhesive layers of the plywood skin of the production model to render it invisible to radar, because the charcoal “should diffuse radar beams, and make the aircraft invisible on radar” (Horten and Selinger 1983). In 1983, Reimar Horten wrote in Nurflügel that he had planned to combine a mixture of sawdust, charcoal, and glue between the layers of wood that formed large areas of the exterior surface of the Ho 229 V3 production model. In a technical paper titled “Aviation Archeology of the Horten 229 v3 Aircraft” (AIAA 2010-9214), they summarized the goals and techniques used in their analysis:

“Modern RCS [radar cross-section, a measure of the detectability of an object with radar] tools were utilized to examine the one known Horten 229 V3 located at the Smithsonian facility in Washington, DC.

Source: airandspace.si.edu