A Quick Guide to Mordanting Fabric for Botanical Printing and Natural Dyeing
When possible, take advantage of sunlight and the outdoors for drying and working with mordanted fabrics…
Purpose:
Mordanting prepares silk fibers to bond with natural dyes, ensuring deeper, longer-lasting color and better definition in botanical prints.
The Classic Alum Mordant: Potassium aluminum sulfate
Potassium aluminum sulfate, commonly called alum, is one of the most traditional and versatile mordants used in natural dyeing. It works beautifully on protein fibers like silk and wool, and can also be used on plant-based fabrics when paired with tannin. Alum helps dyes bond securely to fibers, giving you clear, lasting color without harsh chemicals.
A Bit of History
Alum has been used for thousands of years—from ancient Egypt and Greece to Renaissance textile workshops. Its gentle yet effective action made it the go-to mordant for silk, wool, and fine fabrics. Even today, it remains a favorite among natural dyers for its safety and reliability.
What It Is
A double salt made of aluminum and potassium sulfate, found naturally as the mineral alunite and refined for dyeing use.
Why It’s Great
Ideal for protein fibers (silk, wool, mohair, alpaca)
Safe, non-toxic, and easy to handle
Produces bright, even colors with good wash and light fastness
Works in combination with tannin for cellulose fibers
Readily available and affordable for studio or home use
Safety
Wear gloves when handling powders
Avoid inhaling—use a mask if working with dry alum
Store in a dry place, away from children and pets
Sourcing
Sold as a fine crystalline powder for dyeing and textile work; look for “alum mordant” or “potassium aluminum sulfate” from trusted dye suppliers.
How to Use
Pre-wash (SCOUR) fabric or fiber to remove any oils or finishes.
Prepare the bath: dissolve alum at 10–15% WOF (weight of fiber) in hot water.
Fill your pot with enough warm water to cover the fibers.
Add the dissolved alum and stir well.
Add fibers: place pre-wetted materials into the mordant bath.
Heat gently: keep temperature around 160–180°F (70–80°C) for about an hour.
Allow to cool in the bath or let sit overnight for deeper penetration.
Rinse in cool water and air dry.
Fibers are now ready for dyeing.
Alum offers the perfect balance of tradition, safety, and brilliance—making it a staple in every natural dyer’s toolkit.
Silk charmeuse pillowcases mordanted in PAS and overdyed with natural dyes— then botanically printed with plants. Learn this technqiue in my new atelier offerings.
ALUMINUM ACETATE MORDANT
Aluminum acetate is one of the best mordants for natural fibers like cotton, linen, and hemp and you can use it on silk. It bonds tightly to plant fibers, helping dyes stick better and last longer through washing and sunlight. You can use it on its own or combine it with a tannin mordant for extra-bright, durable colors on cellulose fabrics.
What It Is
A crystalline salt made from aluminum and acetic acid.
Why It’s Great
Works beautifully on cellulose fibers (cotton, linen, hemp, viscose, Tencel)
Produces clear, luminous colors—especially yellows and reds
Creates a stronger bond than alum alone
While you can make it at home by mixing alum, washing soda, and vinegar, the pre-made crystalline version is much more reliable and easy to use
Safety
Always wear gloves and protective clothing
Avoid breathing in the powder—use a dust mask if needed
Can irritate skin and eyes—handle with care
How to Use
Scour your fabric: Wash in warm water with a small amount of neutral soap (like Orvus or a gentle castile). Rinse thoroughly to remove oils and sericin residues.
Dissolve 10–15% WOF (weight of fiber) in warm water.
Add to a pot of hot water to create your mordant bath.
Submerge pre-wetted fibers.
Maintain the temperature at 160–180°F (70–80°C) for about one hour.
(Optional) Let soak overnight for richer color uptake.
Rinse well in cool water.
Your fibers are now ready to dye and/or print with.
My eco-printed designs on organic cotton for Pottery Barn— Cellulose fibers require a bit more finesse to readily absorb natural dyes. Check out my cellulose course to create stunning imprints from plants with these innovative techniques.
Iron Dip After Mordanting
Ferrous Sulfate: A Powerful Color Modifier
Ferrous sulfate, commonly called iron, is one of the most powerful modifiers used in natural dyeing. Rather than acting primarily as a mordant, iron interacts with the dyes already present in the fiber, often deepening, muting, or shifting the color.
Iron is known for “saddening” a color—a traditional dyeing term meaning that it takes a bright, clear shade and moves it toward a deeper, earthier, more subdued tone. Oranges can become burnt orange or rust, yellows can shift toward olive, moss, or khaki, and reds may become plum, brown, or charcoal depending on the dye and mordant.
Because iron is so strong, a little goes a long way. Too much iron or prolonged exposure can make colors muddy and can weaken or damage protein fibers such as silk and wool over time. For this reason, use the lowest concentration necessary, watch the fiber closely, and rinse thoroughly after the iron dip.
After mordanting your fiber with PAS (potassium aluminum sulfate), aluminum triformate, or aluminum acetate, you can use an iron dip to deepen or modify the color.
Calculate 2–3% ferrous sulfate based on the dry Weight of Fiber (WOF). Dissolve the calculated amount of ferrous sulfate in 1 liter of water to create the iron bath.
For example, for 100 g of fiber, use 2–3 g of ferrous sulfate dissolved in 1 liter of water.
Prepare the bath immediately before use and use it fresh to minimize oxidation. Immerse the mordanted fiber and move it gently through the bath for even coverage. Iron can act quickly, so monitor the color closely and remove the fiber when you reach the desired shade.
Rinse the fiber thoroughly after the iron dip.
Remember: The 2–3% measurement is based on the weight of your fiber; 1 liter is the volume of water used to make the bath.
Ferrous sulfate will deepen the plant colors and it critical when printing with leaves.
Iron + Tannins: Deeper, More Realistic Prints
When printing with leaves, iron can play an important role in developing richer, more recognizable botanical impressions. The iron reacts with tannins and other naturally occurring compounds in the leaves, creating darker, more complex pigments that reveal the leaf’s veins, edges, and subtle details.
With alum alone, tannin-rich leaves often produce pale, acidic yellow or yellow-green impressions. An iron treatment shifts these colors toward deeper olive, green, gray, brown, and charcoal tones, giving the print greater definition, depth, and a more natural, realistic appearance.
When printing with leaves, iron is often essential for obtaining realistic impressions and richer pigmentation—alum alone may produce bright, acidic yellow imprints.
When printing with leaves, iron is essential to obtain more realistic impressions and pigmentation— alum alone will result in acid yellow imprints.
Iron is known for “saddening” a color—a traditional dyeing term meaning that it takes a bright, clear shade and moves it toward a deeper, earthier, more subdued tone.
Order a signed copy of Printing from the Garden, my comprehensive botanical printing book and natural dyeing guide.
Books ordered now will ship June 8th— thank you for your patience. This beautifully illustrated eco printing book teaches how to create one-of-a-kind prints using flowers and leaves, featuring step-by-step instruction and a compendium of 30 plants documenting their stunning effects on fabric and paper.
A beautiful blank canvas for your botanical practice. This luxurious 20 momme silk charmeuse pillowcase has been professionally pre-mordanted with my personal mordanting method, so it is ready for eco printing, botanical printing, bundle dyeing, and natural dyeing.
Skip the scouring and mordanting and go straight to the creative part: gathering flowers, leaves, and botanicals and transforming them into a one-of-a-kind silk textile.
DIVE IN DEEPER WITH ME by taking an atelier archive workshop, online, at your own pace, from home.
Join me on a monthly journey in botanical printing and beginner sewing technqiues in the Flora Obscura Atelier— learn more here.
There is a moment—when you unroll the bundle, when the leaves fall away—where cloth becomes something else entirely.
Botanical printing is not just a technique. It is a way of working closely with plants, allowing their color, form, and structure to leave a lasting imprint.
In this course, you’ll learn how to begin.