When a bar of soap uses no synthetic colourant, its colour is the colour of what was mixed into it: the green of spirulina, the yellow of turmeric, the black of activated charcoal. These three sources of colour differ at the root. Two are organic molecules that can change and fade, one is almost pure carbon. This piece follows each pigment to see how it changes in an alkaline environment and under light, and why two batches of the same recipe are rarely identical.
Colour is a molecule that absorbs light
A substance has colour because it absorbs part of visible light and returns the rest. Which part is absorbed is decided by molecular structure, usually a long chain of alternating double bonds, or a large ring holding a metal ion at its centre. Change that structure slightly and the absorbed light changes with it, and so does the colour the eye sees.
In cold process soap, that structure is tested at three familiar stages. The six week piece records a pH of 9 to 10 in a freshly cut bar, a block centre that reaches 60 to 70°C during gel phase, then weeks on an open rack while the pH falls to 8 to 9. Alkali, heat, then light and air during curing: each stage can change a pigment.
The three colour sources in this piece respond to those stages in three ways. Spirulina carries two pigments, one that tolerates alkali fairly well and one that is far less stable. Turmeric carries curcumin, a molecule that changes colour with pH and breaks down faster in alkali. Activated charcoal has no pigment in that sense, so it barely changes.
Spirulina: the green of chlorophyll, the blue of phycocyanin
Spirulina is deep green because it carries two groups of pigments at once. The first is chlorophyll, the green pigment of every green plant: a porphyrin ring holding a magnesium ion at its centre, attached to a long tail called phytol. The second is phycocyanin, a protein carrying a blue pigment, characteristic of cyanobacteria. The colour of the powder is those two colours mixed.
Phycocyanin is the most vulnerable part. A 2022 review of phycocyanin stability records that it is most stable at pH 5.5 to 6.0, below 45°C and in the dark. A study on spirulina phycocyanin found it fairly stable between pH 4 and 9 but unstable above pH 10. A freshly cut bar sits at pH 9 to 10, close to that limit, and gel phase runs well above 45°C. So it is reasonable to infer that the blue part of the powder suffers first.
Chlorophyll tolerates alkali better. The first reaction of alkali with chlorophyll is cleaving the ester bond that holds the phytol tail, the same kind of saponification happening to the oils in the pot. The magnesium ion at the centre stays, so what remains is still green. Chlorophyll only turns olive brown when it loses its magnesium, which happens in acid, not in soap.
What fades chlorophyll is oxygen, light and time, and alkali speeds up its oxidation. Soap makers note that the green of spirulina can drift towards beige within a few weeks, even when the bars are kept in a box. Lê Mai's Bitter Melon Artisanal Soap No.094 uses spirulina with bitter melon powder and concentrated bitter melon juice, on a base of more than 30 percent sweet almond oil, cocoa butter and neem seed oil; the green of the bar is the colour of those ingredients after they have passed through the lye pot.
Turmeric: curcumin, a pH indicator
The yellow of turmeric comes from curcumin and the curcuminoids that accompany it. The library records that curcumin is absent from distilled turmeric oil and present only in the powder and in total CO2 extract. The piece on ginger, turmeric and galangal explains why: curcumin does not evaporate, so it never reaches the condenser. To bring turmeric colour into soap, the powder is needed, not the essential oil.
Curcumin is a natural pH indicator. In neutral or mildly acidic solution, most molecules are in their neutral form and appear yellow. A review of curcumin chemistry places the first change at a pH of about 7.5 to 8.5: the molecule begins to lose protons, the absorbed light shifts towards longer wavelengths, and the colour moves from yellow through orange to red. Different methods give quite different pKa values, so this is only a range.
In soap, turmeric sits on the alkaline side of that threshold. Soap makers note that turmeric powder gives bars from pale yellow to deep orange, depending on how much is used and how it goes in. After the curing weeks the bar is still at around pH 8 to 9, within or above that colour change range, so it is reasonable to infer that it will not quite return to the yellow of the dry powder.
Alkali does not only shift the colour of curcumin; it also speeds up its breakdown. Kinetic studies record that curcumin is more stable in acid and breaks down faster as the pH rises to neutral and alkaline: in a dilute buffer at pH 7.2 and 37°C, about 90 percent of it decomposed within 30 minutes. Breakdown is slower at high concentration and when curcumin is bound to lipids or surfactants, and a bar of soap is no dilute solution in a test tube, so the rate is not the same, but the direction of the reaction is.
Turmeric under light, and a bar made with beer
Light is curcumin's second test. The molecule absorbs blue light strongly, around 410 to 430 nm, and breaks down much faster when exposed to sunlight. Soap makers likewise note that the turmeric colour in a bar can fade over the curing weeks and after. From that mechanism one can infer that a turmeric bar kept near a window would fade first on the side facing the light, while the core keeps its colour longer; this is an inference, not a measured result.
Turmeric also leaves marks. The turmeric essential oil page records that the oil can leave yellow orange marks on skin and fabric, and the Turmeric CO2 extract page notes that the total extract contains curcuminoids that can stain skin, fabric and products. With turmeric powder in soap, soap makers likewise note that the lather can look slightly yellow orange when rinsed against a white sink.
Lê Mai's Turmeric and Beer Artisanal Soap No.093 takes its colour from turmeric powder, with beer in place of distilled water, on a base that includes tamanu oil and neem seed oil, with essential oil blend No.093. The product description records a fine lather and the pale yellow of turmeric. Blend No.093 contains turmeric oil, but as the library notes, that oil carries turmerones for scent, not curcumin for colour: colour is the powder's work.
Activated charcoal: a colour with nothing to fade
Activated charcoal is black for an entirely different reason. It is almost pure carbon, heated and activated to give it a very porous structure. Carbon in this form has no particular chain of double bonds or metal ring that absorbs one band of colour. It absorbs nearly the whole visible spectrum, so the eye sees black.
With no pigment to transform, charcoal barely changes in alkali, in gel phase heat or under light. Soap makers count charcoal among the few natural colours that stay true over time, along with cocoa powder and annatto. What changes is the amount: a little charcoal gives a pale grey bar, more gives a deep black one.
The porous surface of charcoal is why it is used elsewhere in fragrance work. The library's glossary records that an absolute can be decolourised, usually with activated carbon or molecular distillation, for products that need a pale colour, and that the odour may shift slightly from the original. There, charcoal takes colour away; in soap, charcoal is the colour.
Lê Mai's Lemongrass and Activated Charcoal Artisanal Soap No.061 uses activated charcoal for its black, on a base of more than 30 percent sweet almond oil, with lemongrass powder and lemongrass essential oil Cymbopogon flexuosus No.061. The lemongrass page in the library records citral at up to 90 percent of the oil. The scent of the bar passes through the lye pot in its own way, while the black stays almost unchanged from the day of cutting.
Why every batch is different
Two batches of the same recipe rarely give the same colour, and the first reason lies in the materials. Spirulina, turmeric and bitter melon powders are agricultural products. Their pigment content is not fixed; for turmeric, variety trials record that curcumin content varies with cultivar, growing region and time of harvest. A bag of turmeric deeper in colour than the last one can give a more orange bar even when weighed to the same gram.
The second reason is heat. How far a batch goes into gel phase depends on batch size, mould type, insulation and room temperature. During gel phase the block turns from opaque to slightly translucent, as the six week piece describes, and the visible colour shifts with it. For spirulina, the hotter and longer the gel phase runs above the 45°C threshold, the more readily phycocyanin breaks down.
The third reason is pH and time. The free alkali in each batch differs slightly, the speed at which pH falls during curing differs too, and curcumin changes colour with pH. Last comes the light on the curing rack and wherever the bar is kept. Two bars cut from the same block, one kept in a box and one near a window, can differ in colour after a few months.
These differences are not faults of the batch. They are a property of natural colour: an organic molecule can always change, while carbon almost stands still.
Spirulina fades over time, faster with heat and light.
Turmeric is yellow in the powder, orange in alkali, paler under light.
Activated charcoal does not change, because it has nothing to change.