Sorry, the 1890 U.S. census was NOT distroyed in the 1921 fire.
The U.S. censuses were housed on the 5th floor of the Department of Commerce building. Except the 1890 U.S. census was in the basement. In 1921, a fire in the Department of Commerce building was contained to the 1st floor. When the fire was extinguished, everyone went home. The water used to extinguish the fire drained down into the basement. The 1890 U.S. census was water damaged. In 1932 a list of document to be destroyed to make room for more document was submitted to Congress. The 1890 U.S. census was number 26 on that list. In 1933, Congress authorized it's destruction. And in 1935, it was destroyed by the Department of Commerce.
In 1942, some schedules were found to have survived destruction. And in 1950, more schedules were found. Over 6100 schedules of the 1890 U.S. census still exist.
You can read about it here: http://www.archives.gov/publications/prologue/1996/spring/18...