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The following were selected from Karl's unpublished documents, unearthed posthumously.

1. A Few Facts About Myself (ca. 1940) notable for its unbridled enthusiasm.

2. 1940 Plan for Work evidently from a fellowship or grant application.

3. 1957 Account of Karl's visit to and work at Jugtown.

4. 1958 Poem/Statement on the website of Scott Ackerson, long-time friend of Karl and Becky, and ceramics teacher.

5. 1980-1993 formerly secret date codes in signatures.