Bricks are made from a greenish blue clay in which iron exists in the ferrous state.
"An Introduction to Chemical Science"
R.P. Williams
On being heated, ferrous salts are oxidized to ferric, and their color is changed to red.
"An Introduction to Chemical Science"
R.P. Williams
It is thus possible to work with much larger quantities of material when ferrous sulfate is employed.
"Organic-Syntheses"
Conant, James Bryant