Tannins, Ferrous Sulphate, and using an Iron Blanket.

Updated: Oct 8, 2020

In this blog I will show the different colours achieved with tannins, and how to create a dark background using an iron Blanket in eco printing.

The classic way of creating blacks and other dark backgrounds in eco printing is the use of tannins in combination with ferrous sulphate (often called; iron). It's quick, easy and does not contain too many steps which makes it a popular choice, but it must be said that the ferrous sulphate will inevitably weaken your fibres over time. One extra reason to use it with good measuring and not just in the 'rusty nails bucket', where we have no real control over the amounts of ferrous sulphate.

In this test I worked with relatively low percentages (10%WOF) of tannins and only 1% WOF for the ferrous sulphate.

I used three types of textiles;

handwoven matka silk

a satin type silk and a handwoven

vintage linen.

I used a total of 80 grams of fabric per tannin type.

Dyeing with the tannins.

All the samples were dyed for one hour in water just below simmering point and then I let them cool down in the dye pot for another three hours.

The first; Acacia Mearnsii tannin, an extract created from the bark of an invasive, super fast growing tree originally from Australia. Also called black wattle. Because the tree is invasive, using it as a tannin is helping the environment.

It's a catechic tannin belonging to the same group as tea and cutch. The latest batch however turned out to be rather colourless when dyeing (see picture), other batches may be more reddish in colour. The extract dissolves easily in cold water.

The second; good old oak galls, easily available and not expensive at all. When used it gives a pleasant creamy colour. It needs slightly warm water to dissolve easily. This is the tannin most used to tan cellulose fibres, and the tanning agent used by the leather tanners of old.

Not all oak gall tannin powders are the same strength, the ones in the webshop have a very high tannin content.



This was the only tannin I feel would love a good bit of hot water to dissolve with ease. In cold water it stays clumpy. As a standalone dye it gives a buttery yellow. The fabric really needs to be rinsed well after dyeing because particles of the powder will stay on the fabric and give discolourations afterwards.

In India Myrobalan is also used as an Ayurvedic medicine for intestine problems and against parasites. For this they prepare a kind of tea from the whole nut. The smell of the myrobalan powder is pleasantly spicy and nutty.

From top to bottom;

vintage linen, matka silk, satin silk.

After the dyeing I let the samples all dry and took off small pieces to keep for reference. I remeasured the textile and per sample measured 1%WOF ferrous sulphate in three separate buckets with lukewarm tap water. For such small amounts of mordants or dyes I use a digital spoon for the best precision (see this article). I worked the samples really well in the iron water and then soaked for about 20 minutes. You could see the samples get darker. Then the samples got washed and dried again with a simple ph neutral detergent.

The results, it is easy to see the different shades created when adding the ferrous sulphate.

Myrobalan is giving the most 'black' on protein fabrics but took rather blotchy to the linen. Oak galls did best on the cellulose and was rather purple on the rest. Acacia tannin gives a more mauve purple, and I will re-test for higher %WOF to see what that does.

Side by side;

The Iron Blanket

Often in eco printing there is talk about the famous iron blanket. This is when we are using the reaction of tannins and ferrous sulphate like we saw above, and apply it in an eco printing piece.

How to create a dark background using tannins and an iron blanket;

1) Mordant your project piece (silk or wool) with the tannin of your choice at 10%WOF (I described above how I do this.)

2) Take a piece of cotton or linen that is the same size as your target fabric and weigh it, this will be your iron blanket.

3) Measure 1%WOF Ferrous sulphate and dilute it with plenty of warm water in a bucket. Soak the 'blanket' for about 10 minutes, working the fabric well in the water so to create an even distribution. Failing to do this will create dark spots in your final project!.

4) Take the silk or wool out of your tannin bath and rinse it so there will be no powder residue left. Wring it and spread it out on your work surface.

5) Take your leaves and lay them down side facing the target fabric.

6) Take the blanket out of the ferrous sulphate bucket and wring well.

7) Lay the iron blanket on top of your leaves. You now have a sandwich of target fabric, leaves and iron blanket. It is a good idea to now add a barrier of brown paper, or recycled plastic.

8) roll up the entire sandwich using a dowel and tie the 'sausage' well using rope or strips of cotton.

9) Steam or boil for 90 minutes.

If you want a step by step instruction with all the details and pictures you can find it here.

The iron will create a dark background where there are no leaves.

Tannins and Iron Blanket
Download PDF • 268KB

Did you make an eco print or natural dye project using this method? Don't forget to upload your pictures to our very active and supportive Facebook Group.

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Find all the tannins available on the website here.

The best blank fabrics for all your eco printing projects are here.

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