Materials

In the conservation field, technical studies to determine production specifications of macro technological artifacts, such as air and spacecraft, are unusual because typically these specifications are readily available. This is not the case with the Horten because it was constructed towards the end of the war when resources were scarce, supply lines were compromised, and record keeping was perfunctory. There is no documentation from the Gothaer Waggonfabrik workshop about material specifications. Our technical study aimed to compare physical evidence on the jet with historical wartime accounts of materials used to contextualize our findings.

Literature Review

A literature review of plywood technology was the starting point for developing a list of likely materials that we could expect to find on the Horten as well as the techniques by which they were applied. The research emphasized natural and synthetic plywood adhesives and protective coatings that were in use in Germany before and during WWII. In addition to published literature, many sources were found in the Combined Intelligence Objectives Sub-Committee Reports found in the Library of Congress and German Captured Air Technical Documents which are on microfilm in the Museum's Archives. The Combined Intelligence Reports date to 1946 and are summaries and full manuscripts of interviews and interrogations conducted by British and American intelligence officers. Detailed information about German industry and individuals of interest were gathered, including the Horten brothers. The German Captured Air Technical Documents are scientific reports from 1943 to 1946 acquired by the Allies from factories and labs. These reports, largely in German, describe material property tests of plywood, wood veneers from differing species, adhesive formulations, and protective coatings.

Reference Materials

Reference samples of cured thermosetting urea and phenol formaldehyde resins were obtained from Georgia Pacific Chemicals. Matthias Schleinzer and Josef Griener of Bitz Co., Hans Vornlocher of Eichelsdörfer (both aircraft restoration companies), and Peter Selinger (co-author of the book Nurflügel) provided plywood samples made of adhesives mentioned in the literature review. Wooden elements on the Horten were compared to wood reference samples and independently confirmed by wood identification experts Terry Conners, from the University of Kentucky, and Larry Osborn, from the Appalachian Hardwood Center in West Virginia.

Sampling Methods

A protocol was established for removing small samples from representative and non-deteriorated areas of the Horten. Samples of plywood in cross section, individual veneers, adhesives, inclusions within the adhesives, and original protective coatings were carefully recorded, numbered, and photographed with high resolution images taken with a Hirox KH-7700 digital microscope. Sampling the plywood was initially complicated due to the extensive deterioration, which made obtaining intact samples difficult. However, once we removed the steel fairings we discovered areas of sound wood that had been protected from the elements and chose to take our samples from there.

In our search for charcoal in the plywood adhesives, two methods were devised to extract black particles for analysis. Method One: The adhesive was crushed with a mortar and pestle, followed by soaking in sodium hydroxide (pH 14) for several hours to further break apart the adhesive matrix and release any inclusions. Particles were pipetted onto a glass slide, allowed to dry, and the black particles were separated out with tweezers. Method Two: Intact samples of adhesive were prepared into thin sections (30-50 microns) by embedding the sample in clear polyester resin, followed by cutting the cured resin sample with a diamond blade saw, and polishing with Micro-Mesh sheets.