Salts, Soda Ash & Lime in Natural Dyeing

Your studio reference chart

These substances are grouped together because their names often cause confusion. Chemically, they are not all salts.

An assistant is not automatically a mordant

Many natural dyes need a mordant to help colour bond durably with the fibre. Alum is a familiar example. An assistant may instead clean the cloth, change bath conditions or support a specific process.

Adding a white powder does not necessarily “fix” a colour. Table salt cannot stand in for an aluminium mordant, and soda ash cannot replace the preparation required by your chosen dye and fibre. Plant fibres and animal fibres also need different treatment. Read more about mordants and fibre differences.

Table salt, Glauber’s salt & Epsom salt

Table salt

Sodium chloride · NaCl

Salt is familiar from the kitchen, but in chemistry the word describes a broad family of compounds. Sodium chloride is only one of them.

Table salt appears in some direct and fibre-reactive dyeing methods. That does not make it a useful addition to every natural dye bath. Follow a complete recipe instead of adding salt as insurance against fading.

Glauber’s salt

Sodium sulfate decahydrate · Na₂SO₄·10H₂O

Glauber’s salt is a dyeing assistant used in particular processes. Its effect depends on the dye system; it is not a guarantee of stronger or more even natural colour.

Check whether a recipe calls for hydrated or anhydrous sodium sulfate. The hydrated form contains water within its crystal structure, so equal weights do not deliver equal amounts of sodium sulfate. Historical reference on these forms.

Epsom salt

Commonly magnesium sulfate heptahydrate · MgSO₄·7H₂O

Epsom salt is not Glauber’s salt: one contains magnesium, the other sodium. Treat it as a recipe-specific ingredient. Do not substitute between them because the crystals look alike. Choose a clearly labelled product without perfume or other bath-product additives.

Baking soda & soda ash: know the difference

Baking soda: NaHCO₃ · Soda ash: Na₂CO₃

Baking soda is sodium bicarbonate. Soda ash is sodium carbonate. Baking soda is the milder alkali; soda ash makes a more strongly alkaline solution. They are not equivalent spoon-for-spoon substitutes.

A material also does not have one fixed working pH regardless of how you use it. Concentration, water and the other ingredients in the bath matter. Where pH is important, measure it rather than guessing from the ingredient name.

Where soda ash is useful

For cotton, linen and hemp, soda ash is often used with a suitable detergent during scouring. This thorough cleaning removes substances that can interfere with wetting and dye penetration. Fabric can look clean and still need preparation.

Wool and silk need a different approach. Do not transfer a hot, alkaline cotton-scouring method to protein fibres. Use a process designed for your material. Compare cellulose and protein scouring methods.

Is washing soda the same thing?

Both names refer to sodium carbonate, but the form matters. Soda ash generally means the anhydrous form; washing soda may be hydrated crystals. Water of crystallisation changes how much active sodium carbonate is present in a given weight.

Read the product label before substituting. Laundry washing powder is a formulated mixture and is not a reliable replacement for pure soda ash. More on washing soda and soda ash.

Chalk, hydrated lime & quicklime

These names deserve a careful look before you start an indigo vat. Their chemistry and their behaviour are different.

Chalk: calcium carbonate

CaCO₃

Chalk provides calcium in certain dyeing methods. Madder is a useful example: natural dyer Jenny Dean describes adding chalk to brighten her madder colours. That is a specific use, rather than a reason to add it to every pot. Jenny Dean’s madder notes.

Chalk also appears in particular printing and fixing procedures. Follow the complete method, including its quantities and sequence. Choose a product with a known composition rather than relying on a general gardening label.

Find chalk (calcium carbonate) in the store →

Hydrated lime: calcium hydroxide

Ca(OH)₂ · Also called slaked lime or calx

Hydrated lime supplies alkalinity in certain indigo vats. It is different from chalk, and different again from quicklime. If your method specifies it, use hydrated lime with a clearly stated chemical identity.

Quicklime: calcium oxide

CaO

Quicklime is highly reactive and is not a substitute for the hydrated lime named in a vat recipe. Do not choose between them on the basis of the word “lime” alone.

Caustic soda: sodium hydroxide

NaOH · Also called lye

Lye is a strong alkali used in certain indigo methods. It can cause severe burns. It is not soda ash, and a recipe using one cannot be converted to the other by exchanging equal spoonfuls.

Small tests make useful discoveries

A side-by-side sample is more informative than an extra scoop in the pot. Keep one untreated comparison and change only one variable at a time. This is especially helpful when exploring a calcium addition or a pH adjustment.

Record the fibre and dry weight, exact product, amount added, preparation and mordant, water source, temperature, timing and any pH readings. Compare samples after rinsing and drying; a wet swatch can give a misleading impression of the finished colour.

Keep dye utensils separate from food equipment. Label containers with chemical names, avoid creating airborne powder, follow each product’s safety data sheet and store materials away from children and pets. Strong alkalis require suitable gloves and eye protection. Dispose of baths according to their contents and your local requirements.

Choose supplies for your method

A few useful studio essentials

Build your shelf around the processes you actually use. Start with the complete recipe, then select the ingredients it calls for.

Soda ash

For cellulose scouring and methods requiring sodium carbonate.

View soda ash

Chalk

Calcium carbonate for recipes that call for a calcium addition.

View chalk

Questions from the dye studio