Why do fired bricks vary in colour from buff to red or black?

The colour of fired bricks is primarily determined by the composition of the clay used and the atmospheric conditions within the kiln during the firing process.12

Natural colours, ranging from buffs to reds, are largely influenced by the presence of iron within the clay. Under standard oxidizing conditions, this iron produces colours varying from buff to dark red, depending on the concentration of the mineral. Fireclays typically yield lighter shades like buffs and grays, while ordinary clays and shales are associated with the red spectrum.21

To achieve darker tones such as browns, purples, or blacks, manufacturers often use a process known as flashing. This involves regulating the kiln atmosphere to create a reducing environment—where a limited supply of air leads to the presence of carbon monoxide—or allowing smoke to act upon the surface of the brick. While these methods can produce deep grays and blacks, the colour change is generally limited to the surface, with the interior of the brick retaining its natural hue. Additionally, additives like manganese may be mixed into the clay to develop specific colours.12