For us fibre crafters, there’s nothing quite like the artistic creations that indie dyers produce through the process of hand-dyeing fibres. But what exactly is it that makes this process possible? This inquisitive knitter is here to investigate.
I will be looking into the chemical reactions that allow dyes to bind successfully to wool fibres and how pH plays a role in the dyeing process. I will be looking at sheep wool specifically, but keep in mind this process will be vastly different for different fibre types due to the difference in chemical composition.
Before we get too excited, let’s talk about the chemical composition of sheep wool. Wool is classified as a keratin fibre, specifically a “hard keratin” due to the higher composition of sulphur-containing cystine amino acids than can be found in soft keratin (such as skin). Chemical composition of wool fibres can vary even within the same animal. It has been noted that diet plays an important role in the percentage of cystine groups present in the fibre. Cystine is highly important when looking at wool composition because it plays a vital role in the structural integrity of the fibre and contains the chemical group that forms disulphide bridges (a major stabilising force). Disulphide bonds are covalent bonds that stabilise the wool fibre and are one of several “crosslinking” bonds in wool. You will also get noncovalent bonding/interactions between groups such as hydrogen bonding and ionic interactions (salt bridges) and these noncovalent interactions are most disturbed in the presence of water. All of these interactions play an important role in how wool looks and behaves on a larger scale.
Wool is what is considered an “amphoteric” fibre which means that it can function as either an acid or a base and therefore react with acids and bases with equal efficiency. Wool contains an equal number of amino (basic) and carboxyl (acidic) groups and can achieve an isoelectric state from a range of pH 4-8. This means that soaking our fibre in a bath with a pH within this range will cause both amino and carboxyl groups to ionise while still keeping the maximum number of salt bridges to stabilise the fibre. This is important because it means that wool is extremely receptive to the use of mordants within this pH range while still maintaining fibre integrity and minimising damage. So there are a lot of terms being thrown around here—let’s recap.
Amphoteric – a chemical compound that contains acidic and basic groups which makes it equally receptive to chemical reactions involving acids and chemical reactions involving bases.
Cystine – an amino acid group that contains sulphur and helps form disulphide bridges, giving structure and support to our wool.
Ion – charged chemical element/compound that is ready to react with the opposite charge and form a bond (such as those involved in salt bridges).
Isoelectric – when both the amino and carboxyl groups exist as ions and the overall compound has a neutral charge (one positively charged group and one negatively charged group equaling out to a neutral charge overall).
Mordant – a substance that helps dye bind to a fibre. In other words, a fixative.
pH – a range of numbers from 1-14 that indicate how acidic or basic a solution is.
Wool – Ha just kidding. You got this 😉
Ok so we know that wool is amphoteric now but why does that help us? Let’s talk about acid dyes first. A very large portion of indie dyers will use acid dyes as their dye of choice. Here is a really general idea of what goes into using acid dyes:
- Soak wool in acid solution
- Apply dyes and heat
Obviously there is a bit more that goes into it than that, but you get the idea. We’ve been throwing the word “acid” around quite a bit here and that can sometimes be a scary word. Don’t worry. The acid we are using here is very weak—usually citric acid or acetic acid (vinegar). Very chill stuff. Wool is most workable in the presence of acid dyes at a pH of 4.5-5. Going back to our chemistry lessons, we know that water (a neutral molecule) has a pH of 7. On a scale of 1-14 (7 being neutral), these solutions are very weakly acidic. If the solution were too acidic, it would break the peptide bonds (bonds between proteins) in the wool and damage the fibre.
Treating our fibre with a weakly acidic solution has helped us prepare the fibre for dyeing. Wool, as a mostly proteinaceous fibre, is made up of strings of amino acids. Each amino acid will have different groups attached to it that we can manipulate to help us dye our fibre. Remember how we said earlier that wool contains both basic amino groups and acidic carboxyl groups? At the moment, we are focusing on those amino groups. In the presence of our weak acid, the NH2 (amino) group will pick up one of those extra hydrogen ions floating around that were so kindly donated by the acid in our solution. We now have an NH3+ group – a charged ion that is ready for bonding. The slightly basic amino groups are ready to bind with the slightly acidic sulfonyl (or carboxyl) group designed to be a part of the structure of most acid dyes. Unfortunately, just pouring the dye on our prepared fibre isn’t quite enough to make those bonds stick which is why we need to heat our wool in the presence of the dyes. The heat increases the activity of the ions in solution and helps make sure we get as much bonding of the dye as possible. And there you have it!
So that’s the story of how acid dyes work, but what about natural dyes? That’s a great question. This is where mordants become really essential. There are two types of natural dyes: adjective and substantive. Substantive dyes (ex: indigo) do not require mordanting, but since most natural dyes fall into the adjective category, I am just going to focus on those (P.S. As someone who mostly works with acid dyes, I do not pretend to be an expert in natural dyeing, however I found an awesome resource for anyone wanting to get into natural dyeing and have included it at the end of this post).
Natural dyeing is a bit more finicky (but also awesome) than dyeing with acid dyes. It is usually pretty clear what colour you are going to get with an acid dye. You purchased sky blue from the supplier? Probably going to be sky blue. But natural dyes are a whole different story. They can completely change colour depending on whether you dye them at an acidic pH or basic pH and the type of mordant that you use can completely change the game as well. Makes it a bit fun, right? 😉 Tin and chrome mordants cause colours to brighten, while iron mordants darken colours. Alum (or aluminium sulphate) is the most common mordant for natural dyeing. Many of the other compounds that were used in the past have been abandoned due to their highly toxic nature.
Natural dyes will not remain in the fibre without the presence of a mordant. The mordant is the essential glue that attaches the dye to the fibre and makes it stay there. This is different from our acetic/citric acid because those compounds, while essential to the success of acid dyeing, simply help change the chemical composition of the wool rather than become a permanent part of the finished product.
The pH of our dye bath is more important here to determine colour rather than to prepare the fibre for dye uptake. A neutral pH of 7 is still perfectly fine for dyeing naturally. Many dyers will use something called “modifiers” to change the pH of their dye bath, resulting in the same dyeing compound producing a different colour.
Unfortunately, it is not entirely understood why mordants work so well. They just do.
“We shall never know by what chances primitive man discovered that salt, vinegar from fermenting fruit, natural alum, and stale urine helped to fix and enhance the colours of his yarns, but for many centuries these four substances were used as mordants.”
Jill Goodwin, A Dyer’s Manual
For the aspiring natural dyer:
Shout out to Samantha Jane—your website is amazing!
For more information: