Natural dyes for soap: a guide to colour stability

TL;DR: Natural dyes have an authentic, earthy look, but in the alkaline environment of soap, time changes. Stable dyes (turmeric, activated charcoal, clays) are safe choices; unstable ones (spirulina, matcha) turn olive/yellow-brown within the first few weeks. Always test in a four-week soap safe test.

Natural dyes are the first choice for manufacturers who want a soap without synthetic pigments. The results are organic, warm and earthy - not as richly unified as synthetics, but with an authentic character.

A crucial thing to know: most natural dyes change colour over time in the alkaline environment of soap. The spinach goes from green to olive. Matcha turns brown. Beetroot red fades quickly. That's not a mistake - it's a property of alkaline chemistry.

How do natural dyes work in alkaline soap?

What are the three main types of natural dyes?

Natural dyes are either:

• Lipophilic (fat-soluble): Carotenes, chlorophylls - dissolve in oils. You add as an infusion oil or powder dispersed in oil.
• Hydrophilic (water soluble): Anthocyanins (red/purple from red cabbage, blueberries) - these change colour or fade immediately in alkaline soap and are not suitable.
• Mineral (clays and oxides): Stable, reliable, available in many colours.

Key rule: Avoid hydrophilic vegetable dyes (red cabbage, spinach juice, beet juice). Oxidation and alkalinity will break them down.

Overview table of natural dyes

Detailed profiles of the most commonly used dyes

Turmeric (Curcuma longa)

Golden orange, very stable in soap. One of the few natural dyes that tolerate pH alkalinity well.

Dosage: 1 teaspoon (3-4 g) per 500 g of oils for golden colour. 2 tsp for deep orange.

Note: When using fresh soap for the first time, turmeric can slightly stain towels and skin (transient). After full maturation (6+ weeks) the effect is minimal.

Where to buy: Every supermarket (spice department). E-shops of health food.

Spirulina

Intense green, but oxidizes to olive or yellow-brown over time. In 6-8 weeks after production the colour is usually olive. This is normal and does not indicate defective soap.

How to slow down the color change: Add spirulina on late trace (as late as possible), store soap in a dark place, wrap in foil.

Dosage: 1 tsp/500 g for medium green. More = richer, but oxidizes faster.

Activated charcoal

The most stable dark dye. Deep black, retains colour for the life of the soap. See detailed guide Activated charcoal soap.

Clays (kaolin, rhassoul, French clays)

Mineral dyes - the most stable of all natural dyes. They do not undergo oxidation and do not change in an alkaline environment.

Kaolin (white): White to off-white. Adds a silky feel to the skin. Green French clay: Olive green, stable. Detoxifying effect for oily skin. Red clay (rose clay / red kaolin): Pink to reddish brown. Gentle, for sensitive skin. Rhassoul clay: Dark brown, from Morocco. Conditioning.

Clay dosage: 1-2 tablespoons per 500 g of oils.

Cocoa powder

Brown to dark brown colour, stable. Additional scent of chocolate (mild in finished soap). Combine with cocoa butter in recipe for a more pronounced chocolate effect.

Dosage: 1-2 tsp per 500 g of oils.

How to add natural dyes correctly?

What is the easiest method for even colouring?

Pre-soaking in oil (recommended method)

1. Weigh out 1-2 tablespoons of oil (sunflower or olive).
2. Stir the powder into a smooth paste without lumps.
3. Add the paste to the soap on the light trace.

Advantage: no lumps, even colouring.

What are the alternative methods?

Adding as an infusion oil

Maceration of herbs in oil (4-6 weeks or by heating) - the oil absorbs the dyes. Use in place of part of the recipe oil. See Herbal recipes.

Addition to the leaching solution (only for stable dyes)

Water with added cocoa or turmeric as part of the leaching solution. The dye undergoes an alkaline process - less suitable for fine dyes, excellent for stable dyes (cocoa powder, clays).

What not to do with natural dyes?

Do not combine turmeric with white (TiO₂): Turmeric reacts with titanium dioxide to form green spots in the cross section. Use them in separately layered sections.

Do not use colouring agents from food gels (food colouring): They are hydrophilic and fade or morph to unexpected colours in soap.

Do not expect the same colour as the powder: Alkaline environments and saponification will change virtually every natural dye at least a little.

Frequently asked questions

How long did the soap safe test take? 4 weeks. During these 4 weeks, the natural dyes emerge and show their true stability. After 4 weeks you can be sure what colour they will be in the final product.

Can I use multiple natural dyes together? Yes, but test the combinations in a small test soap. Some dyes react with each other - for example, matcha and spirulina together produce a duller colour than each individually.

Why did my natural dye turn blue/green in the soaping process? Natural dyes are sensitive to pH and alkalinity. What you see during solidification is a chemical transformation. During maturation, the colour stabilises - in many cases it will change once more.

Is it safe to buy natural dyes from e-stores? Yes, as long as you buy them from trusted retailers. Make sure it is for cosmetic use, not just for food preparation. Natural dyes from food departments sometimes contain binders or solvents that are not suitable in soap.

How long does natural soap with natural dyes keep its color? It depends on the stability of the dye. Stable dyes (turmeric, clays, activated carbon) practically do not change during the entire life of the soap. Unstable ones (spirulina, matcha) change during the first few weeks and then stabilize. Storage in a dark, cool place slows the changes.

Can I combine natural and synthetic dyes? Yes. Many designers use natural dyes for earthy tones and synthetic oxides for intense colors. Test the combination so they don't react unexpectedly (like turmeric with TiO₂).

See also:

• Synthetic dyes and micas - stable alternative to natural dyes
• Herbal recipes - herbs as colouring and functional ingredients
• Activated charcoal soap - the most stable dark dye in practice