CREATE TABLE table_6449 ("Birth" CLOB, "Marriage" CLOB, "Became Duchess" CLOB, "Ceased to be Duchess" CLOB, "Death" CLOB, "Spouse" CLOB)

What is the Birth of the lady that has a Became Duchess on 18 august 1593 husband's accession?
SELECT "Birth" FROM table_6449 WHERE "Became Duchess" = '18 august 1593 husband''s accession'
SELECT "Birth" FROM "table_6449" WHERE "Became Duchess" = '18 august 1593 husband''s accession'