# Reactive dyeing basics: why salt and soda ash (and how to stay safe) - URL: https://abdohoo.app/blog/reactive-dyeing-salt-soda-ash-basics - Product: Shadecard (https://sc.abdohoo.app) - Section: Textile & colour - Updated: 2026-09-30 > In reactive dyeing of cotton, salt pushes the dye onto the fibre (exhaustion) and soda ash raises the pH so the dye bonds to it (fixation). Unfixed dye must be washed off. Reactive dyes colour cotton, viscose and linen by forming a **chemical (covalent) bond** with the fibre. Getting there takes two helpers. **Salt** pushes the dye out of the water and onto the fibre – this is **exhaustion**. **Soda ash** (sodium carbonate) then makes the bath alkaline so the fibre and dye react – this is **fixation**. Some dye always reacts with water instead, and that loose dye must be washed off at the end or the colour will bleed. ## Stage 1: Exhaustion – what the salt does Reactive dyes are **anionic**: their molecules carry negatively charged groups that make them dissolve in water. Cotton, wet in the dyebath, also takes on a **negative surface charge**. Like charges repel, so on its own much of the dye would rather stay in the water. Adding salt changes that. The sodium ions crowd around the fibre and the dye and **screen the repulsion**, and the high salt concentration makes the dye less comfortable in the water. The dye migrates onto and into the fibre. Nothing is bonded yet; the dye is held loosely and can still move and even out. Practical points: - **Common salt** (sodium chloride) or **Glauber's salt** (sodium sulphate) are used. Follow what your dye supplier recommends. - If you use **Glauber's salt crystals** (the decahydrate), you need about 2.3 times the weight of the anhydrous form to get the same amount of sodium sulphate. - Add salt **in portions**, not all at once, so the dye goes on evenly. - **Deeper shades need more salt** than pale ones. Dye makers publish tables by depth. ## Stage 2: Fixation – what the soda ash does Once the dye has exhausted, the alkali goes in. Soda ash raises the pH of the bath to strongly alkaline. At that pH some of cellulose's hydroxyl groups lose a proton and become reactive, and they attack the dye's **reactive group**, forming a covalent bond. From then on, the dye is part of the fibre: that is why well-fixed reactive dyeings have excellent wash fastness. The temperature for fixation depends on the type of reactive dye – some fix in warm baths, others need hot baths – so the dye maker's method sets it. ## The side reaction: hydrolysis The same alkali also lets the dye react with **water**. A dye molecule that reacts with water can no longer bond to cotton. This **hydrolysed dye** still sits loosely in and on the fibre, and if it is not removed it comes out later in washing, staining other clothes and lowering fastness. That explains three rules of reactive dyeing: 1. **Add alkali after exhaustion**, so most of the dye is already on the fibre when it becomes reactive. 2. **Never put alkali in dye stock solutions**; dissolved dye plus alkali hydrolyses in the bottle. 3. **Wash off thoroughly**: rinse, neutralise, then soap at or near the boil and rinse again until the water runs clear. ## A typical exhaust sequence Exact times and temperatures come from the dye maker, but the shape is usually: StepWhat happensSet the bathWater at the dyeing temperature, fabric wetted outAdd dyePre-dissolved dye, run about 10 minutes to levelAdd saltIn two or three portions; run until exhaustionAdd soda ashDissolved, in portions; run for the fixation timeDrain and rinseRemove bath liquor and loose dyeNeutraliseMild acid rinse to bring the pH back downSoapNear-boil wash with a soaping agent to remove hydrolysed dyeFinal rinsesHot, then cold, until clear## Why liquor ratio matters Dye is dosed on the **weight of fabric** (% owf). Salt and soda ash are dosed per **litre of bath** (g/L). So if you change the liquor ratio – say from 1:10 in the lab to 1:6 in a machine – the dye stays the same but the absolute salt and soda change. This is a classic reason a lab dip and a bulk batch come out differently. Shadecard's recipe calculator follows the same logic: bath litres = fabric kg × liquor ratio, and salt and soda are that volume times their g/L. For 200 g of fabric at 1:10, the bath is 2.0 L, so 50 g/L of salt is 100 g and 15 g/L of soda ash is 30 g. The app notes that these figures are typical for a medium reactive shade and that every dye range publishes its own table. See [Use the recipe calculator](https://abdohoo.app/help/shadecard/recipe-calculator). ![Shadecard recipe calculator with Fabric weight, Dye depth and Water ratio fields, the Y R B ratio and the Salt (g/L) and Soda ash (g/L) fields for reactive dyes on cotton](https://abdohoo.app/media/library/shadecard/android-calc-method.webp)Salt and soda ash are entered per litre; the calculator converts them to grams for your bath## Handling salt, soda ash and dye powders safely Reactive dyeing uses everyday chemicals, but in quantity and hot. Treat them with respect. **Soda ash is an alkaline irritant.** It can seriously irritate eyes and skin. Wear safety glasses and chemical-resistant gloves when weighing and dissolving it. Add the powder slowly to water while stirring, and avoid breathing its dust. - **Dye powders:** reactive dye dust can cause allergic reactions, including breathing problems, in people exposed repeatedly. Weigh gently to avoid clouds, work in a ventilated area, and wear a dust mask rated for fine particles. Dissolving dyes into stock solutions once reduces how often you handle powder. - **Spills:** wipe powders up with a damp cloth instead of sweeping, which raises dust. - **Hot liquor:** soaping near the boil and hot drains can scald. Lift and pour carefully; let baths cool before handling. - **Acids for neutralising:** use dilute acid, and always add acid to water, never water to acid. - **Separation from food:** keep dyeing equipment, spoons and containers for dyeing only; do not eat or drink while working with dyes; store chemicals labelled and out of children's reach. - **Safety data sheets:** read the SDS for each dye, salt and alkali you buy; it lists the hazards and the protective equipment needed. - **Waste water:** spent reactive baths are salty, alkaline and coloured. Neutralise and dispose of them according to local rules; do not pour concentrated chemicals into drains. ## Starting points, not guarantees Salt and alkali amounts interact with dye strength, fabric preparation, water hardness and temperature. Whether your recipe came from experience, a supplier's pattern card or an app, it is a starting point: dye a lab dip first. See [The lab dip process explained](https://abdohoo.app/blog/lab-dip-process-explained) and [How to make and use a dye stock solution](https://abdohoo.app/blog/how-to-make-dye-stock-solution). ## Frequently asked questions ### Why is salt added in reactive dyeing? Cotton in water carries a negative surface charge and reactive dyes are negatively charged too. Salt screens that repulsion and makes the dye prefer the fibre, so it moves out of the bath and onto the cotton. ### What does soda ash do in reactive dyeing? It makes the bath alkaline. At high pH the cellulose becomes reactive and forms a covalent bond with the dye. This is fixation, which gives reactive dyes their good wash fastness. ### Can I add the soda ash at the start with the salt? Not in the usual exhaust method. Alkali also makes the dye react with water (hydrolysis). Adding it after the dye has exhausted onto the fibre means more dye fixes to the cotton instead of being wasted. ### How much salt and soda ash should I use? It depends on the dye range and depth of shade. Figures around 50 g/L salt and 15 g/L soda ash are typical for a medium shade, deeper shades need more and paler less; always follow your dye supplier's table. ### Is soda ash dangerous? It is an alkaline irritant, especially to the eyes. Wear safety glasses and gloves, avoid raising dust, and read the supplier's safety data sheet.