Lifting cut Tamil Nadu indigo cakes on a palette knife
Historic indigo cakes in Dresden University Museum
Blue indigo pigment was traditionally supplied as indigo cakes (also known as indigo blocks or indigo rocks).
Indigo pigment is extracted from leaves of indigo in a large pool or concrete tank. The pool is drained and indigo paste is left on the bottom. The paste is put in a large wooden mould to dry, but it is cut into square blocks before it dries completely. The indigo blocks are cut ‘by eye’ and are not exactly the same size. Indigo blocks cut in the West Indies in the 1600s were 1½” square while those from India were larger.
Our blocks are from Tamil Nadu indigo and approximately 63 mm by 54 mm and 30 mm high (2½” by 2” by 1¼”). The amount of paste that is put in the mould to dry is also not always the same, so some blocks may be higher than others. This is the result of a small scale operation run by several generations of the same family. Many blocks (but not all) are wrapped in cotton string to provide some protection during transit. The blocks are quite brittle and soft and are not difficult to break.
Indigo was traded in the form of blocks for centuries. In the past, some blocks were individually pressed in beautiful moulds carved with the logo of the company and the weight. Indigo cakes are very compact and didn’t use much space in traders’ luggage. They were the ideal traders' commodity: high value, low weight, low volume, and water-resistant. Well-dried indigo does not go off and can last for centuries. In fact, Jenny Balfour-Paul successfully used indigo cakes recovered from a three-centuries-old Caribbean ship wreck to dye with.
Uses for indigo cakes:
Indigo blocks are great for displays in museums and educational institutions.
If you are making a fermentation or urine vat, Liles suggests using indigo cakes. He recommends making a little bag of fine cotton and putting a piece of indigo cake inside. You should then tie the bag shut and suspend it in the vat. Every day, morning and evening, rub the bag to release a little indigo into the vat.
Some artists use the cakes to draw with, in a similar way to drawing with a charcoal stick. This can be quite messy but you can use disposable plastic gloves to hold the indigo block.
You can also grind the cakes and use them in indigo vat recipes that use spectralite. If you need only a small amount you can use a mortar and pestle with a little water. A coffee grinder used just for dyes would work for larger amounts but be careful when you do this as it can generate a large amount of dust.