Quick answer: Mimosa tenuiflora root bark powder is used in soap as a natural colourant. Disperse about one teaspoon of powder per pound (454 g) of oils in a little oil and stir it in at light trace. Expect shades from dusty rose to deep brown, with a light natural speckle. Make a small test batch first, because the colour shifts as the soap cures. The project planner works out the amount for your batch size.
What the powder does in soap
The inner root bark of Mimosa tenuiflora (also sold as Mimosa hostilis or tepezcohuite) is rich in tannins and red-purple pigments. In soap it works as a plant-based colourant and adds very fine texture. In cold-process soap the high pH of the lye pushes most tannin-based colours toward brown, so the purple tone of the raw powder usually ends up as a warm rose-brown in the finished bar.
How much to use, by batch size
Soap suppliers such as Bramble Berry suggest about one dispersed teaspoon of a natural colourant per pound of soap as a starting rate. The table scales that up. A level teaspoon of root bark powder weighs roughly 3 g, but powders vary, so weigh it if you want repeatable results.
| Oils in recipe | Light | Medium | Deep |
|---|---|---|---|
| 1 lb (454 g) | 1 tsp | 2 tsp | 3 tsp (1 tbsp) |
| 2 lb (907 g) | 2 tsp | 4 tsp | 2 tbsp |
| 5 lb (2.27 kg) | 5 tsp | 10 tsp | 5 tbsp |
For any other batch size, the project planner works out the amount and prints the steps.
Three ways to add it
| Method | How | Result |
|---|---|---|
| Powder at trace | Mix the powder into a tablespoon of light oil, then stir into the batter at light trace | Strongest colour, visible speckle |
| Oil infusion | Steep the powder or shredded bark in warm olive oil for a few hours, strain, and use the oil as part of your recipe | Smooth, even colour without specks |
| Water infusion | Simmer the bark in distilled water, strain, cool completely and use it as your lye water | Even colour, usually browner |
For an oil infusion, Bramble Berry suggests starting at about one tablespoon of powder or botanical per pound of oil, in an oil with a neutral colour and a long shelf life.
Why dispersing matters
Dry powder dropped straight into soap batter clumps, and the clumps show up as dark spots in the cut bar. The standard method is to mix one teaspoon of powder into one tablespoon of a light oil until smooth, then add that mixture a teaspoon at a time at trace. For melt-and-pour, use glycerin or 99% isopropyl alcohol in place of the oil, because extra oil can make the base sweat.
Cold-process soap, step by step
- Run your recipe through a lye calculator and weigh every ingredient.
- Disperse the root bark powder in a tablespoon of oil taken from the recipe. One teaspoon of powder per pound of oils is a common starting point for botanical powders.
- Make the lye solution and combine it with the oils as usual.
- At light trace, stir in the dispersed powder until the colour is even.
- Pour, insulate if your recipe calls for it, and unmould after 24 to 48 hours.
- Cure for four to six weeks. Note the colour at unmoulding and again after curing, so you can adjust the next batch.
Gel phase and colour
Cold-process soap that is kept warm after pouring goes through gel phase, a stage where the soap heats up and turns translucent before it sets. Natural colourants generally look stronger in soap that has gelled. To encourage it, cover the mould and wrap it in a towel for the first 24 hours. If you prefer a paler, more matte bar, keep the mould cool instead.
Melt-and-pour soap
Use less powder, about half a teaspoon per pound of base, and mix it with a little glycerin first so it does not clump. Stir as the base cools and pour at a slightly lower temperature so the particles stay suspended instead of sinking.
Getting the colour you want
- pH: the lye in cold-process soap pushes tannin colours toward brown. Melt-and-pour base is milder, so the shade stays closer to the pink-purple of the powder.
- Amount: more powder gives a deeper brown but also more speckle and a scratchier bar. Heavy use can tint the lather.
- Oils: dark or green oils such as unrefined olive or hemp add their own tint. Pale oils give a cleaner colour.
- Method: powder at trace gives the strongest colour with speckle; infused oil gives a smooth, lighter colour.
- Sieving: pass the powder through a fine mesh if you want a smoother bar.
- Cure time: the colour keeps shifting for the first few weeks. Judge it after the cure, not at unmoulding.
- Notes: record the amount, the method and the cured colour for every batch.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Dark spots in the bar | Powder not dispersed | Mix it into oil first and stir until smooth |
| Bar feels scratchy | Too much powder, or coarse particles | Use less, and sieve the powder through a fine mesh |
| Lather is tinted | Colourant level too high | Stay near one teaspoon per pound |
| Brown where you wanted rose | High pH and dark oils | Use pale oils, use less powder, or switch to melt-and-pour |
| Powder sank in melt-and-pour | Base poured too hot and thin | Let it cool and thicken slightly, stirring, before pouring |
| Colour faded on the shelf | Light exposure | Store bars cool, dark and dry |
Safety and labelling
- Lye (sodium hydroxide) is caustic. Wear goggles and gloves, add lye to water and never the reverse, and work in a ventilated space.
- Wear a dust mask when measuring fine powder.
- Patch test a finished bar before regular use, and do not make health claims for your soap.
- On ingredient labels the bark is listed under its botanical name, Mimosa tenuiflora bark.
- If you plan to sell your soap, read the section on selling below.
Selling your soap in the United States
Whether a bar is legally “soap” or a cosmetic depends on what it is made of and how you describe it. The FDA sets three conditions, covering composition, the cleaning ingredients and intended use. A bar that meets all three is regulated by the Consumer Product Safety Commission. If you market it as moisturising, deodorising or beautifying, it becomes a cosmetic, and cosmetics have their own rules, including on colour additives. Read the FDA guidance and the CPSC soap page before you sell, and keep your label claims to cleaning.
Frequently asked questions
What colour will my soap be?
Usually between dusty rose and deep brown. The result depends on how much powder you use, the method, your oils and how long the bar has cured.
Powder or shredded bark for soap?
Powder for adding directly to the batter. Shredded bark is better for infusing oil or water, because it strains out cleanly. See whole vs shredded vs powdered root bark.
Will the colour fade?
Natural colourants soften over time, especially in strong light. Store finished bars in a cool, dark, dry place.
Sources
Mimosa Tenuiflora Root sells root bark as a raw material for dyeing, soap making and cosmetic crafting only. It is not sold for human consumption.