After his retirement in 1977 (see timeline), Karl was approached several times by art historians interested in writing a biographical account of his career. He always refused these offers of collaboration. Karl was a very private and modest person. Also, throughout his life, he suffered from depression, some lack of self confidence, and difficulty believing in the magnitude of his accomplishments. These factors probably contributed to his refusals.
The following were selected from Karl's unpublished documents, unearthed posthumously.