Brick Colour

The colour of clay bricks and tiles was determined by the naturally occurring minerals that are kiln fired to burn in their colour and strength.  The composition of the raw materials as well as the firing process would cause each batch to differ.  The resultant colour variation was inherent in the process and part of theivisual appeal of the bricks.  Usually those bricks contained the following ingredients:

1. Silica (sand) between 50% to 60% by weight
2. Alumina (clay) between 20% to 30% by weight
3. Lime between 2 to 5% by weight
4. Iron oxide up to 7% by weight
5. Magnesia – less than 1% by weight

Colour is not only determined by the colour of the clay and its chemical composition, but also:

1. The colour of the sand used in the moulding;
2. the moisture content before firing;
3. the fuel used to fire the kiln;
4. the volume of air during firing; and
5. the temperature in the kiln during firing.

Colour	Natural Colourants in Clay	Prepared Chemical Colourant
Black	Manganese and a large percentage of iron	Manganese umber (produces yellow, brown, purple, and black)
Blue/Green	Alkalies (ie Feldspar) Sometimes cobalt oxide	Iron chromite, black cupric oxide (Chrome flour)
Bright Red	High %  of iron oxide	Red iron oxide (Haematite)
Brown	Large amount of lime	Brown iron oxide
Cream	Very low  iron	High lime content
Dark Blue/Purple	High iron oxide burnt at high temperature	Manganese umber (produces yellow, brown, purple, and black)
Red	Iron oxide	Red iron oxide (Haematite)
White	Kaolin clay, free from iron	High lime content
Yellow/Orange	Iron oxide and magnesia Sometimes lead antimonate	Manganese umber (produces yellow, brown, purple, and black)