Alizarin: a mordant dye
Alizarin is the synthetically produced varient of madder, which can be used to dye fabrics, especially cotton, red. A metal ion known as a mordant reacts with the dye making a structure held together by relatively weak chemical bonds and the colour of the dye is dependent on this mordant. Common mordants are aluminium (Al3+), iron(II) (Fe2+) and barium (Ba2+).
In order to dye with alizarin, the cotton must first be soaked in an alkaline solution so that the mordant can bind with -OH groups in the fabric.
The fabric must then be placed in a solution containing the mordant.
Finally, the fabric must be transfered to a solution of alizarin.
The structure of Alizarin
OH OH
\ /
O C == C
\\ / \
H C -- C C -- H
\ / \\ //
C -- C C -- C
// \\ / \
H -- C C -- C H
\ / \\
C == C O
/ \
H H