If you’ve ever tried natural dyeing and ended up with faded, dull, or washed-out colors, chances are you skipped a crucial step, mordants and modifiers. Mordants are the magic ingredient that takes plant-based dyes from soft and fleeting to bold, rich, and colorfast. They bind dye molecules to fabric, ensuring your beautiful shades don’t wash out after a few rinses. Without them, many natural dyes produce pale or uneven results that don’t last. With the right mordant, those beautiful hues you’ve worked so hard for can stay vibrant through washes and wear.
But mordants aren’t the only way to change how your colors turn out. Their close cousins, modifiers, can shift a single dye into an entire palette of shades. Modifiers can deepen, brighten, or completely transform the look. Together, mordants and modifiers are the real foundation of natural dyeing.
So before we dive into the different types you can use, let’s take a closer look at what mordants and modifiers actually are. How they work and why they’re so essential for bringing your colors to life?
What Are Mordants and Why Do They Matter in Natural Dyeing?
Mordants are substances that fix dyes to fabric, ensuring that the color bonds to the fiber. When a mordant, such as alum, iron, or copper, is introduced, it chemically bonds with both the dye and the fiber, locking the color in place. Mordants create what’s known as a dye lake: a stable, water-insoluble complex that enhances both color intensity and fastness.
Without a mordant, many natural dyes sit on the fabric’s surface, making them prone to fading or washing out. Think of mordants as little “bridges” that connect the dye to the fiber, making colors stronger and longer lasting. This is especially important for cellulose fibers like cotton and linen, because they can`t form a proper chemical bond with the dye. Cellulose fiber don`t have strong affinity to dye. Wool and silk on the other hand have a much stronger affinity to dye and don`t need as much coaxing as cellulose fiber.
The type of mordant used can dramatically shift the final color. For example, alum tends to brighten colors, iron deepens and darkens hues, and copper adds a greenish or muted tone. Dye lakes can also be formed outside fabric and become insoluble pigments, which can be used to make paints and inks.
What Are Modifiers in Natural Dyeing?
Modifiers don’t fix dye to fabric the way mordants do. Instead, they change the shade of a color that’s already there. With modifiers, one dye bath can give you a darker, warmer, cooler, or completely different colors.
The most common way to modify is by adjusting the pH. Adding vinegar (acid) can make colors brighter and warmer. Soda ash or baking soda (alkaline) often deepens or dulls them. Iron is another powerful modifier, turning yellows into olive greens or pinks into deep purples and grays.
Using modifiers after dyeing is where the magic happens. It’s a playful way to explore endless variations and create one-of-a-kind shades from the same dye source.
Common Mordants for Natural Dyeing
Let’s take a look at the mordants you’re most likely to come across. Each one has its own strengths, quirks, and color effects. It helps to know what to expect before you start experimenting.
Alum (Potassium Aluminum Sulfate)
Alum is the go-to mordant for most natural dyers, and for good reason. It’s affordable, easy to find, and works on both protein fibers (like wool and silk) and plant fibers (like cotton and linen). Alum tends to produce bright, clean, and true colors, making it perfect for achieving vibrant reds from madder or yellows from weld. It’s particularly effective with dyes that might otherwise produce weak or pale shades.
It is commonly used in combination with soda ash (washing soda) for cellulose fabrics and cream of tartar for wools. It comes in powdered crystal form and bigger crystals. The bigger crystals take longer time to dissolve and require more space for storage. You`ll usually need around 10%-20% WOF to mordant, in addition to 1-2% WOF soda ash if you are dyeing cellulose fibers.
Advantages:
- Widely available. It`s used in gardening and food so you can find it online, in supermarkets, in gardening stores and nurseries.
- It`s quite cheap and easy to buy in bulk
- Has good colorfastness
- Low toxicity. Alum is relatively safe to use. Still, it`s important to treat it with caution (wear gloves and mask when handling). Diluted alum (as in a spent mordant bath) can be poured out outside or down the drain.
Disadvantages:
- It`s not the best aluminium mordant you can get. Especially for cellulose fibers Aluminium Acetate and Aluminium Lactate tend to produce better results.
- Sometimes requires extra steps like tannin baths or additives.
- You need a higher amount compared to other mordants, so the cost advantage isn’t always as big as it looks.
Aluminium Acetate
If you’re working with cellulose fibers like cotton, linen, or hemp, aluminum acetate is one of the best mordants you can use. It helps plant-based fabrics bond with natural dyes more effectively, giving you better results. Unlike iron, it won’t darken or shift your shades—it keeps them true to the original dye.
While this mordant doesn’t need a tannin pre-treatment, it does need a calcium carbonate or wheat bran after bath to set the colors. You`ll need around 5-10% WOF to mordant, in addition to around 5% WOF calcium carbonate or wheat bran
Advantages
- Excellent for cellulose fibers—superior to alum on cotton and linen.
- Produces bright, intense, and true colors.
- Less risk of patchy or streaky results compared to tannin-based methods.
Disadvantages
- Needs an extra rinse step (chalk or wheat bran), which adds time.
- More expensive and harder to find than alum.
- The fine powder requires careful handling—wear a mask, gloves, and work in good ventilation.
Aluminium Lactate
Aluminum lactate (water-soluble aluminum salt derived from lactic acid) is a newer mordant made from byproducts of the sugar industry. It’s especially good for cellulose fibers like cotton, linen, and hemp. What makes it stand out is how simple it is to use—unlike alum or aluminum acetate, it doesn’t need any extra pre-treatments or after-baths. Just one step, and you’re ready to dye. You’ll usually use about 5–10% WOF (weight of fiber).
Advantages
- Works beautifully on cellulose fibers.
- Produces bright, clear colors with excellent fastness.
- No tannin bath required (unlike alum).
- No chalk or wheat bran bath needed (unlike acetate).
- Simple, straightforward mordanting process.
Disadvantages
- Harder to find than alum or aluminum acetate.
- Can be more expensive depending on the supplier.
Iron Mordant (Ferrous Sulfate, Iron Water)
Iron is a classic tool for natural dyers, known for its ability to darken and shift colors. Often called “saddening,” iron tends to deepen shades. Yellows turn olive green, reds and pinks shift toward purples or grays.
It is used on both cellulose and wool, but it’s best avoided on silk since it can be too harsh. It`s improves colorfastness of the dye, but should be used sparsely as it can be damaging for fabric.
Iron is usually used as a modifier after dyeing rather than as the main mordant. Typically, you dip your already mordanted and dyed fabric into a light iron solution for a few minutes. Even at very low concentrations (1–2% WOF), the effect is strong, so less is more. Always rinse well to reduce fabric damage.
One of the simplest and most sustainable ways to use iron is by making your own iron water. Pop some rusty nails, scraps, or steel wool into a jar with vinegar and water, let it sit for a week or two, and you’ll have a budget-friendly iron solution ready to use. The resulting liquid will have a rusty brown appearance and a faintly metallic smell.
⚠️ A note of caution: iron can stain everything it touches and may weaken fibers if left too long. It’s worth keeping a separate set of tools (jars, spoons, buckets) just for iron to avoid cross-contamination.
To modify your fabric, fill a bucket or big jar with enough water for you fabric to move around freely, add 1-2% WOF iron or a bit of the homemade iron solution, stir until it`s dissolved. Add dyed and wetted out fabric, move it around and agitate the fabric, you will see the change in color after a minute or so. Remove the fabric after 2-3 minutes. Don`t let fabric soak in the solution as it might damage the fiber.
Advantages:
- Adds depth and complexity. Iron is fantastic for creating complex shades and deeper, richer tones. It’s particularly great when you want to create an “antique” or vintage look.
- Modifies colors. Iron allows you to modify other dye colors, making it an excellent tool for experimenting with hues.
- Inexpensive and readily available. Iron is one of the most cost-effective mordants and can often be sourced from items like rusty nails.
- Effective even at low concentrations
Disadvantages:
- Iron residue can transfer to other fabrics or dye baths if equipment isn’t dedicated and labeled.
- Might be harsh and damaging on fabric. Especially silk.
- Will stain everything and everyone.
- Can irritate skin (always use gloves)
- Not suitable for vibrant, bright colors; tends toward muted tones
Copper
While alum and iron are the staples, copper brings something a little different to the dye pot. It’s known for giving fabrics a cool, greenish, or teal undertone, and it can also add depth and richness to many natural dyes. Copper can be used as a mordant on its own or as an after-bath modifier to shift colors. You only need a small amount—2–4% WOF is usually enough.
Copper mordant is available as a powder, but you can also make a gentler version yourself. Just place a few copper pieces (like old pennies or copper pipes) into a jar with vinegar and water. After a week or two, the liquid will turn light blue, creating a mild copper solution you can use.
Advantages
- Enhances greens and blues.
- Boosts colorfastness and longevity.
- Works on both protein and cellulose fibers.
- Useful as either a mordant or a post-dye modifier.
- Produces subtle but unique variations.
- Generally less harsh on fibers than iron or tin.
Disadvantages
- Toxic at higher concentrations, so handle and dispose of with care.
- Harder to find compared to alum or iron.
- More expensive than iron (though you need very little).
Tin (Stannous Chloride)
Tin produces brighter, clearer, and more vibrant colors in natural dye projects. It`s used with protein fibers (silk and wool) in small quantities. Used in very small amounts (1–2% WOF), it can act as both a mordant and a modifier. Because tin can make fibers brittle, it’s often paired with cream of tartar to soften its effect.
Tin is particularly striking with certain dyes. Cochineal becomes more intense and saturated, while madder shifts toward a vivid orange.
Advantages
- Creates brilliant, vivid colors.
- Enhances reds, oranges, and yellows.
- Improves lightfastness.
- Works as both a pre-mordant and a post-dye modifier.
Disadvantages
- One of the more expensive mordants (though only small amounts are needed).
- Can weaken fibers if used too much.
- Less available than alum or iron.
Chrome
Chrome was once widely used as a mordant. Today it’s largely avoided because it’s been proven to be highly toxic and carcinogenic. While all mordants require care, chrome carries serious health and environmental risks that make it unsafe for most dyers.
With so many safer and more effective options available, chrome simply isn’t worth considering. In fact, it’s usually only sold for professional or educational use, so hobby dyers are unlikely to have access to it anyway.
Symplocos (Plant-Based Mordant)
Symplocos is a plant native to Asia and the Americas and is used as a “bio mordant” (non metal). It`s an interesting mordant as it`s plant based, but its mordanting ability comes from the fact that it`s able to accumulate alum (leaves have the highest concentration), it`s a “hyperaccumulator” plant for alum.
It is considered to be more environmentally friendly as the production is less invasive and polluting. Alum and the metal mordants need to be mined or synthetically produced, putting strain on the natural environment. Symplocos on the other hand is harvested by picking up fallen leaves off the ground, no need to chop down trees.
Symplocos is sold as a powdered dry leaf and works on both protein and cellulose fibers. You’ll need quite a lot though—around 50% WOF, much higher than alum or acetate. The biggest drawback is the price: while alum is very affordable, symplocos can be many times more expensive for the same amount of fabric.
Advantages:
- Non-toxic and safer to handle than some metal-based mordants.
- Renewable, plant-based alternative to mined mordants.
- Works on a wide range of fibers (silk, wool, cotton, linen).
- Supports ethical, sustainable harvesting (often by local communities).
- Adds a subtle warmth or depth to colors, thanks to its natural tannin content.
Disadvantages:
- Expensive compared to alum or acetate.
- Requires much higher amounts (50% WOF).
- Not widely available—usually from specialty suppliers.
- Natural variations mean results can differ between batches.
- More complex process for cellulose fibers—less beginner-friendly.
Popular Modifiers in Natural Dyeing (pH Shifts and Afterbaths)
Modifiers are one of the most exciting aspects of natural dyeing, giving you the ability to tweak and transform your colors in unique ways. While mordants fix the dye to the fabric, modifiers alter the colors produced. By adding a modifier to your dye bath or applying it after the dyeing process, you can shift the final hue, intensify or dull the color.
They’re a wonderful way to explore and experiment, often turning a single dye source into a whole palette of unique tones you might not have expected.
Want to see how much a single dye can change? In Dyeing with Kitchen Scraps, I use mordants and modifiers with 11 accessible dye materials to create more than 130 photographed colours and hues on cellulose fabrics. Explore the ebook →
Tannin
Tannin is a natural compound found in plants like oak galls, sumac leaves, and even tea. It’s often used alongside alum to make colors deeper and more intense, especially on cellulose fabrics (cotton, linen, hemp), which don’t naturally bond well with dyes. By treating fabric with tannin first, alum has something to cling to, which in turn helps the dye stick. Without it, colors on cellulose tend to be pale and not very durable.
Tannins can also act as dyes themselves. When combined with iron, they create rich grays and charcoals. How much tannin you’ll need depends on the form:
- Tannic acid or powdered plant material → 20–25% WOF
- Extracts → about 10% WOF
- Whole plant pieces (like oak galls or pomegranate rind) → need a longer soak (1–3 days) before use
Advantages:
- Helps cellulose fibers hold onto mordants and dyes.
- Improves colorfastness, reducing fading and washout.
- Produces deeper, richer shades (especially with iron).
- Easy to source from common plants and household items.
- Works as both a modifier and a dye on its own.
Disadvantages:
- Adds an extra step to the process.
- Can shift colors unpredictably depending on the tannin source.
- Some tannins may leave fabric feeling a bit stiff.
Cream of Tartar
Cream of tartar is a gentle acidic salt that works as a pH modifier in dyeing. It’s mainly used with wool to lower the pH, which helps keep the fibers soft and protected during the dyeing process. It also brightens warm shades by neutralizing any iron in the water (since iron can dull or “sadden” colors). You’ll typically use about 5% WOF.
Advantages:
- Softens wool, keeping it smooth and flexible.
- Brightens reds, oranges, and yellows.
- Helps prevent wool from becoming brittle.
- Safe, non-toxic, and easy to find in grocery stores.
Disadvantages:
- Adds an extra step to the dyeing process.
- Only useful for protein fibers—no effect on cotton, linen, or silk.
Soda Ash (Sodium Carbonate)
Soda ash (also known as washing soda) is both a pH modifier and a scouring agent. It’s often added after dyeing to shift colors (for example, turning madder red into purple). It’s also used alongside alum when mordanting cellulose fibers and in making aluminum acetate.
To use it as a modifier, dissolve about 1–2% WOF soda ash in enough water for your fabric to move freely. Add the dyed, wet fabric and stir for a few minutes. You’ll see the color start to change—usually within 2–5 minutes. Remove once you’re happy with the result.
Advantages:
- Available and affordable – Found in grocery stores or online as a household cleaner.
- Essential for scouring plant-based fabrics (removes oils, waxes, residues).
- Shifts certain dyes toward cooler or deeper tones.
Disadvantages:
- Extra handling step – adds more time and ingredients to the dyeing process.
- Can weaken protein fibers like wool and silk
Soy milk
Soy milk works a little differently from traditional mordants. Instead of chemically bonding dye to fiber, it acts as a binder or fixative, helping pigments stick better to fabric. This makes it useful for cellulose fibers like cotton and linen, which are often harder to dye.
The proteins and fats in soy milk coat the fabric, creating a surface that mimics protein fibers such as silk or wool. Since natural dyes bond more easily to proteins, this treatment helps plant-based fabrics hold onto color more deeply and last longer.
Using soy milk does take some patience. Fabrics usually need to be dipped 2–3 times in a soy milk and water solution, with drying in between. I often use it as an extra step after tannin and alum, just before dyeing.
Advantages:
- Produces deeper, richer colors on cellulose fibers.
- Improves colorfastness and reduces fading.
- Makes plant-based fabrics behave more like silk or wool.
Disadvantages:
- Not a true mordant—still needs alum (or another mordant) for best results.
- Time-consuming—requires multiple dips and a curing period.
- Can spoil, so it must be used fresh and stored carefully.
What to do with Mordant Baths?
So, you’ve finished mordanting your fabric, and now you’re left with a bucket or pot full of used mordant solution. What’s next? Unlike spent dye baths, mordant baths need a bit more care when it comes to disposal. Some are harmless and easy to deal with. Others (especially those with metal salts) need to be handled more carefully to avoid harming the environment. Let’s look at the best ways to keep your practice safe and eco-friendly.
Can You Reuse a Mordant and modifier Bath?
Before you pour it out, see if you can reuse it! Many mordants can last through several rounds of fabric.
- Alum baths can usually be reused multiple times. Just top them up with a little fresh alum when they start to weaken.
- Tannin baths (like oak galls or sumac) often still have some life left for another round of mordanting or even as a color modifier.
If the bath looks too murky, weak, or just “spent,” then it’s time to dispose of it properly.
How to Dispose of a Spent Mordants and modifiers Bath Safely
Not every mordant bath can go straight down the drain. The right way to dispose of it depends on which mordant you’ve used. Here’s a quick guide:
1. Alum Mordant Bath (Generally Safe)
Alum is considered one of the safest mordants since it’s food-safe and not very harmful in small amounts. Still, it’s best to dilute it well before disposal.
- Best Disposal Method: Mix the bath with plenty of water and pour it down the drain with extra running water, or onto a non-plant area like gravel. Avoid dumping it directly into the garden, unless you are growing alum loving plants!
2. Iron and Copper Mordant Bath (Use Caution!)
Iron and copper can accumulate in soil and water sources, which may affect plants and wildlife. Handle these with care.
- Best Disposal Method: Dilute heavily and pour onto a dry, non-plant area like a driveway or sidewalk. If in doubt, check local guidelines to see if it’s safe to pour down the drain in your area.
3. Tin Mordant Baths (Hazardous – Do Not Drain!)
Tin mordant baths should never be poured down the drain or into the soil. They’re toxic to plants, wildlife, and water systems.
- Best Disposal Method: Store the liquid in a sealed container and take it to your local hazardous waste facility. Most cities have designated drop-off points for safe disposal.
Can You Compost a Mordants and modifiers?
It depends on the type of mordant and modifiers you’ve used:
- Plant-based mordants (like tannins, soy milk, or rhubarb leaves): Yes, these can go straight into the compost or garden, since they’re natural and biodegradable.
- Alum: In small amounts, diluted alum can be poured into soil, especially for acid-loving plants. But avoid adding too much in one spot, as aluminum can build up over time.
- Iron (ferrous sulfate) or Copper: Technically safe in very small, diluted amounts. But regular disposal this way can overload the soil, better to use sparingly.
- Tin or Chrome: No, these should never go into compost or soil. They’re toxic to plants and wildlife and must be disposed of properly.
Rule of thumb: If it’s plant-based, it’s compostable. If it’s a mined or synthetic metal salt, handle it with caution and dispose of it responsibly.
Wrapping Up
Mordants and modifiers are the hidden tools that can take your natural dyeing from pale and fleeting to rich, vibrant, and long-lasting. Once you get comfortable with them, you’ll find your dye pots become so much more versatile and exciting.
In the next part of this series, Natural Dyeing for Beginners Part 4: How to Choose Fabric for Natural Dyeing, we’ll look at another key ingredient for success: your fabric. Choosing the right fiber makes just as much difference as the dye itself, and we’ll explore which ones work best and why.
READY TO START DYEING?
Dyeing with Kitchen Scraps
Turn what you’ve learned into your first dye experiments. Dyeing with Kitchen Scraps is my 184-page beginner’s guide to natural dyeing on cellulose fabrics, using 11 accessible dye materials you can collect from your own kitchen.
184 PAGES · 11 DYE MATERIALS · 130+ COLOURS
