Mordanting with cow’s milk

When doing natural dyeing or eco-printing on plant-based fibres such as cotton, you need to prepare the fabric with something to help the plant dyes adhere. This could be mordants such as metals or tannins. Or you could use a protein-rich binder such as soy milk, cow’s milk, eggs or blood. These emulate the results that you will get dyeing protein fibres (wool, silk, leather).

Before I explain my process of preparing fabric with milk, I would like to clarify my use of language. Although milk and these other protein-rich materials are not technically mordants, the word mordant is often used informally to describe them. I don’t have a problem with this, because in its basic sense a mordant is something that helps plant dyes bind to fabric. So I apologise to dye puritans for conflating these words, but I find that more people understand what I mean if I speak about milk as a mordant rather than a binder.

For ethical reasons, I only use cow’s milk to mordant fabric when it would otherwise be going to waste. It doesn’t seem reasonable to buy it for this purpose when many people are starving and the dairy industry is horrific. But I have a friend who occasionally brings me bottles from their workplace that are close to their use-by date and were going to be thrown in the bin. Dumpster diving is also a good way to find wasted milk and put it to good use.

Step one: add water to your milk at a ratio of 1:1. Pre-wet your cotton fabric (so it will absorb evenly) and add it to the milk. There should be enough space for the fabric to move freely. Let it soak for about half a day, stirring occasionally. This is best done on a cold day or in a very shady spot or else the milk will quickly get smelly and this smell will remain in the fabric even after dyeing. I soak fabric for less time in cow’s milk than when using soy milk, because it does tend to go bad faster (especially when it is already close to the use-by date).

Mordanting cotton with milk

Step two: remove the fabric, squeeze out the excess milk and put it in your washing machine on a spin cycle. Then hang to dry. Putting it on a spin cycle means that there won’t be milk dripping down the side of the fabric as it dries, which would show up as streaks when you dye it.

Step three: if your milk is still good, add the fabric back in just until it is saturated, then remove and put through the spin cycle and dry again. Do this process twice. Adding extra layers strengthens the mordant, and it is done in quick dips so that the previously adhered milk won’t have time to come off. These extra steps can be omitted if your milk bucket is getting too smelly- you will still get good results from one round of mordanting.

Step four: once your fabric is dry, leave it to cure for at least a week before dyeing it. Again, this strengthens the mordant and helps it adhere strongly to the fabric. Then dye or eco-print with it as desired.

Mordanting with milk versus iron on cotton

This picture illustrates the difference between protein and metal mordants. The top piece of cotton was prepared with cow’s milk. The bottom piece was mordanted with iron. Both pieces were eco-printed with the same species of eucalyptus leaves. On the milk mordant, the leaves have printed a bright reddish-orange. This is similar to the results these leaves give on silk and wool, which is unsurprising because protein mordants make plant fibres dye like protein fibres. On the iron mordant, the leaves have printed olive-green and have some speckles and vein details. I find that iron mordants are great for bringing out details, while protein mordants give flatter results but sometimes more vivid colours.

I hope this has been useful for you and gives you a way to make use of milk that would otherwise go to waste. Feel free to pin the image below to return to this information later.

Instructions for mordanting cotton with milk, for eco-printing and natural dyeing

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