Soda Ash for Natural Dyeing, Plus Chalk and Gum Arabic

Mordants get most of the attention, but small helpers called dye assists often decide whether your color is clear, deep and even. This guide covers three of them: soda ash, calcium carbonate (chalk) and gum arabic. You will learn how to use soda ash for natural dyeing, when to add chalk to madder and weld, and how to thicken mordant pastes for printing.

Cream of tartar is another common assist. It has its own article: see our cream of tartar guide.

Quick overview

Assist	What it is	Main uses	Typical rate
Soda ash	Sodium carbonate, an alkali	Scouring cellulose, neutralizing alum, alkaline color shifts	2–5% WOF to scour; 1.5–2% WOF with alum
Calcium carbonate	Chalk, a mild alkali and calcium source	Madder and weld baths, dunging after mordant printing	1–6% WOF in madder; 4–8% WOF in weld; 10 g/L as a post-bath
Gum arabic	Acacia tree gum, water soluble	Thickening mordant pastes and dye paints	About 1 part gum to 4 parts liquid for printing; less for painting

Use the WOF calculator to turn percentages into grams.

Soda ash (sodium carbonate)

Soda ash is a strong but safe-to-handle alkali. It does three jobs in a natural dye studio.

1. Scouring cotton and linen

New cotton and linen carry natural waxes, pectins and factory finishes. These block mordants and dyes and cause patchy color. Hot alkaline scouring removes most of them. Research on cotton shows that some calcium-bound pectin can remain even after strong alkaline scouring (Gamble, 2010). That is one reason a thorough scour matters.

1. Weigh the dry fabric.
2. Use soda ash at 2–5% WOF. Botanical Colors suggests about 1% for prepared-for-dyeing fabric, 1–2% for light fabric and 2–4% for heavy fabric. Start at 2% for most fabrics.
3. Add a few drops of pH-neutral soap.
4. Dissolve the soda ash in hot water, then add it to a pot with enough water for the fabric to move freely.
5. Add the wet fabric. Hold at 70–90 °C (160–195 °F) for 30 minutes for light fabric, up to 2 hours for heavy or unbleached cloth. Stir often.
6. Rinse very well. Leftover alkali can block the mordant.

More detail is in our guide on how to prepare fiber and fabric for dyeing.

Tip: Never use soda ash to scour wool or silk. Alkali damages protein fibers.

2. Neutralizing alum on cellulose

Alum is acidic in water. On cotton and linen, dyers add a small amount of soda ash to the alum bath. This partly neutralizes the alum and makes it bind more readily to tannin-treated cellulose.

• Alum: 12–15% WOF.
• Soda ash: 1.5–2% WOF. Botanical Colors uses 12% aluminum sulfate with 1.5% soda ash.

Dissolve the alum and the soda ash in separate containers of hot water. Pour the soda ash slowly into the alum. It will fizz a lot, so use a big container. Full steps are in our guide to mordanting cotton and linen and the alum mordant guide.

3. Alkaline color shifts

Many plant colors change with pH. A short alkaline rinse can push a color in a new direction:

• Madder shifts from brick and orange-red toward raspberry and pinker reds. In one wool study, a sodium carbonate treatment gave the most intense raspberry red of all the treatments tested (Schmidt-Przewoźna and Kicińska-Jakubowska, 2023).
• Weld and other yellows become deeper and more golden.
• Safflower red is extracted with an alkaline soak, then neutralized with acid (see our safflower recipe).

To test a shift, dissolve about 1 g soda ash per litre of warm water. Dip a dyed sample for a few minutes, watch the color, then rinse well. For wool and silk, keep dips short and cool.

Tip: Alkaline shifts are not always permanent. Acidic sweat, rain or washing can move the color back. Use them for effect, and test before dyeing a large batch.

Soda ash safety

• Wear gloves and avoid breathing the dust.
• Always add soda ash to water, not water to soda ash.
• Keep dedicated dye tools. Never use them for food.

Calcium carbonate (chalk)

Calcium carbonate is pure chalk. It raises pH gently and adds calcium to the bath. Some dyes, especially madder and weld, give much richer color when calcium is present.

Chalk in a madder bath

Madder "loves calcium." Hard, chalky water gives bright reds. Soft or slightly acidic water often gives brown or orange-brown. In a Botanical Colors test, adding chalk turned a weak brown madder bath into deep reds on every mordant.

• Rate: 1–6% WOF. Botanical Colors uses about 1%. Wild Colours uses 6 g per 100 g fiber (6%). Start at 2–3%, and use more in very soft water.
• Mix the chalk with a little hot water and stir it into the bath before you add the fiber.
• Keep the bath below 80 °C (176 °F) for clear reds.

Use it with madder root powder (50–100% WOF) or madder extract (3–10% WOF). See our guide on how to dye with madder and our madder troubleshooting guide.

Chalk in a weld bath

Calcium deepens and brightens weld yellows. Wild Colours adds 2–4 g chalk to a pot for 50 g of fiber, which is 4–8% WOF. Botanical Colors gets results with less, about 1%. Start at 4% in soft water and less in hard water.

Simmer the chalk with the weld rather than adding it at the end. If a yellow looks weak, lift the fiber out, stir in a little dissolved chalk, then return the fiber. Use with weld powder or weld extract (0.25–3% WOF). Recipes are in our weld dye recipes.

Chalk as a post-bath (dunging)

After mordanting cotton with aluminum acetate, or after printing a mordant paste, dyers use a chalk bath to "fix" the mordant. This step is called dunging. It removes loose mordant and thickener so it does not bleed into the dye bath.

1. Dissolve or suspend 10 g calcium carbonate per litre of warm water.
2. Optional: add a muslin bag of wheat bran, about half a cup per litre.
3. Move the dry, mordanted fabric through the bath for 10–30 minutes.
4. Rinse well, then dye.

Which chalk to buy

Use pure calcium carbonate. Do not use blackboard chalk (often calcium sulfate), garden lime (calcium hydroxide) or climbing chalk (magnesium carbonate). They behave differently.

Gum arabic for mordant printing and painting

Gum arabic is a natural gum from acacia trees. It dissolves in warm water and makes a smooth, sticky paste. In natural dyeing it is a thickener, not a mordant. It holds a mordant or dye in place so you can stamp, brush or screen print a pattern.

How much gum arabic to use

Gum arabic is a weak thickener compared with guar gum. One printing tutorial used about 20 g gum arabic where 1 g guar would do the same job. Supplier guidance for thin paints starts at 30–60 g per litre.

• Thin paste for brushing: about 5–10% (50–100 g per litre).
• Thick paste for stamping or screen printing: about 1 part gum to 4 parts liquid (20–25%).
• Add more gum until the paste holds a sharp line.

Sprinkle the powder slowly into warm water while stirring, then let it hydrate for about 30 minutes.

Basic mordant paste recipes (100 mL each)

Alum paste (prints become red with madder, yellow with weld):

• 100 mL hot water
• 10 g alum, fully dissolved
• 1 g soda ash, added slowly
• 10–25 g gum arabic

Iron paste (prints become grey to black on tannin-treated cloth):

• 100 mL warm water
• 2 g ferrous sulfate
• 10–25 g gum arabic

Printing steps

1. Scour cotton or linen. For best alum prints, give it a tannin bath first, for example with white oak gall powder (see our tannin guide).
2. Print or paint the paste onto dry fabric.
3. Let it dry for 1–2 days.
4. Dunge in chalk, 10 g/L, for 10–15 minutes. This also washes out the gum.
5. Rinse and dye. The printed areas take up more color than the background.

Tip: Add a pinch of dye to clear alum paste so you can see where you printed. Gum arabic also works as the binder in oak gall ink.

Find all three assists in our dye assists and modifiers collection.

Troubleshooting

• Patchy color on cotton: scour again with soda ash and rinse until the water runs clear.
• Cloudy alum bath: too much soda ash, or it was added too fast. Stay at 1.5–2% WOF and pour slowly.
• Madder still brown with chalk: the bath was too hot. Keep it under 80 °C (176 °F).
• White residue on fabric: undissolved chalk. Rinse well and mix chalk with hot water before adding it.
• Prints bleed or have fuzzy edges: the paste was too thin. Add gum arabic, and print on dry fabric.
• Background stained after printing: loose mordant was not removed. Dunge longer and rinse well before dyeing.

Frequently asked questions

Can I use soda ash to scour wool or silk?

No. Strong alkali damages protein fibers. Scour wool and silk with a pH-neutral soap in warm water instead. Keep soda ash for cotton, linen and other cellulose fibers.

Is washing soda the same as soda ash?

Both are sodium carbonate. Washing soda often contains water of crystallization, so it is weaker by weight. Plain, fragrance-free washing soda works for scouring, but you may need a little more. Check that it has no added detergents or perfume.

Can I use blackboard chalk instead of calcium carbonate?

Usually not. Many blackboard chalks are calcium sulfate. Garden lime is calcium hydroxide and climbing chalk is magnesium carbonate. Use pure calcium carbonate for dyeing.

Why does my madder turn brown instead of red?

Soft or acidic water and high heat are the usual causes. Add calcium carbonate at 1–6% WOF and keep the bath below 80 °C (176 °F).

Is gum arabic a mordant?

No. Gum arabic is a thickener. It holds a mordant such as alum or iron in place while you print or paint. The mordant does the binding. The gum is washed out afterwards.

Sources and further reading

• Schmidt-Przewoźna, K. and Kicińska-Jakubowska, A. (2023). Rubia cordifolia roots as a raw material for dyeing wool in red colours. Herba Polonica 69(4), 54–60.
• Gamble, G. R. (2010). Examination of Factors Affecting Post-Scoured Fiber Metal Content. Beltwide Cotton Conferences.
• Botanical Colors. Chalk and madder experiment.
• Wild Colours. Dyeing with weld.
• Kaliko. Printing with plant dyes.

Related: Cream of tartar guide · Mordants for natural dyeing · How to prepare fabric for dyeing