TY  - RPRT
TI  - Letter Circular 643: structural clay products, stone, and masonry (supersedes Letter Circular 496)
PY  - 1941
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 643r1941
DO  - 10.6028/NBS.LCIRC.643r1941
ER  -