How to create the darkest blacks with tannins and ferrous sulfate.

If you have been reading my blogs you have seen a thing or two about creating dark shades using the magical collusion of tannins with ferrous sulfate. (See this blog)

In this blog, we are going to explain why there is a particular order you should pay attention to.

First: how does it work?

The reaction between tannins and ferrous sulfate results in the formation of a dark-colored complex, commonly known as ferric tannate. This chemical reaction has been historically used in the production of ink for writing and drawing. So when we are using this combination on fabric, we are de facto creating black ink directly on our textile.

There are two components in this process:

1. Tannins. These are polyphenolic compounds found in various plant tissues, such as barks, hulls, and gall nuts. They contain phenolic hydroxyl groups that can react with metal ions.

2. Ferrous sulfate (iron(II) sulfate), which is a salt that contains iron(II) ions (Fe²⁺).

When tannins come into contact with ferrous sulfate, the iron(II) ions form a complex with the phenolic hydroxyl groups in the tannins.

This complex is black or dark blue-black in color, depending on the tannins used. In the dye bath and even outside of it, the iron(II) ions in the complex undergo oxidation to form iron(III) ions (Fe³⁺). This process may involve exposure to air or other oxidizing agents. The oxidation of iron(II) to iron(III) results in a change in color from a lighter shade to a darker color.

Usually, when making inks, the order of the materials does not matter. But when we are creating dark shades on fabric, it very much does. Why is this?

All mordants do not adhere the same way to protein (wool and silk) as they do to cellulose (cotton, hemp, linen, viscose, and rami). Therefore the mordant salt absorption levels are not the same.

This is because:

1. Mordants often contain metal ions that can form complexes with functional groups on fibers. Cellulose fibers primarily have hydroxyl (-OH) groups, which are less reactive with metal ions than the functional groups present in proteins, such as amino groups (-NH₂) and carboxyl groups (-COOH).

2. Mordants typically interact more readily with charged or polar functional groups. Protein fibers have amino and carboxyl groups that can readily form ionic interactions with metal ions, enhancing the affinity for mordants.

3. The structural differences between cellulose and proteins also affect the accessibility of reactive surfaces. The more complex structure of protein fibers allows for a greater variety of sites that can interact with mordants.

In practice, it means that when you are mordanting 100 grams of wool and 100 grams of cotton, each with 2 grams of ferrous sulfate, the wool will be more mordanted than the cotton.

On the other hand: Tannins generally have a higher affinity for cellulose than for proteins.

Here are some reasons for this:

1. Tannins, being polyphenolic compounds, have hydroxyl (-OH) groups that can form hydrogen bonds easily with the hydroxyl groups in cellulose.

2. The interaction between tannins and cellulose is often more favorable than the interaction between protein fibers and tannins due to the specific arrangement of hydroxyl groups in cellulose.

3. The hydroxyl groups in cellulose and tannins can engage in hydrogen bonding, which contributes to the strong affinity between them. Proteins have fewer available hydroxyl groups, and the interactions may be weaker compared to those with cellulose.

4. Tannins may also form ionic interactions with cellulose fibers. While proteins have amino and carboxyl groups that can engage in ionic interactions, the specific arrangement and availability of these groups may result in stronger interactions with cellulose.

5. Cellulose fibers have a linear and organized structure which allows for a more accessible surface area for interaction with tannins. The irregular and more complex structure of proteins limits the availability of reactive surfaces for tannin binding.

In practice, it means that when you are tanning 100 grams of wool and 100 grams of cotton, each with 20 grams of oak gall powder, the cotton will have higher levels of tannins than the wool.

For our fabrics, this complex explanation comes down to a simple 'rule' when you want to create the darkest grays and blacks on your fabrics:

For cellulose: Tannin first, Ferrous sulfate after.

For protein: Ferrous sulfate first, tannin after.

As always: go light on ferrous sulfate, a little goes a long way. I use a maximum of 2% WOF.

Exhibit A: cotton samples. 1 cutch, 2 cutch and then ferrous sulfate. 3 Ferrous sulfate and then cutch.

Exhibit B: Silk Samples. 1 dyed with oak galls. 2 oak galls and then ferrous sulfate. 3 Ferrous sulfate and then oak galls.

Exhibit C: Silk noil. 1 Silk noil dyed with pomegranate peels. 2 pomegranate peels with a post-mordant of ferrous sulfate. 3 Ferrous sulfate mordant and then dyed with pomegranate peels.

I am looking forward to seeing your results with this experiment!