Boric acid is sometimes used in shampoo bars as a mild acidifier, preservative booster, and scalp-soothing additive. However, it is heavily restricted in cosmetics in many regions, so the first rule is to keep the amount very low - around 0.5-1% - and to check your local regulations before making or selling these bars.

What boric acid does in a shampoo bar

Boric acid (H3BO3) can lower pH, boost preservation, and has mild antimicrobial/antifungal properties. Historically it was used in scalp tonics and dandruff rinses. In a rinse-off shampoo bar, it may help the formula feel slightly acidic and can support a healthy scalp environment. It is not a substitute for a broad-spectrum preservative or a medicated dandruff active.

Safety notes

  • Do not use boric acid on broken or irritated skin.
  • Do not use in products for children under 3 years old.
  • Pregnant and nursing women should avoid boric acid.
  • Never substitute borax for boric acid.
  • Wear gloves, eye protection, and a dust mask when handling the powder.
  • Check your country’s cosmetic regulations; boric acid is restricted or prohibited in some places.

Basic boric acid shampoo bar formula

This is a 100 g test batch. Scale up by multiplying every ingredient by the same factor.

  • 60 g SCI (sodium cocoyl isethionate) powder or noodles
  • 10 g SLSA (sodium lauryl sulfoacetate)
  • 10 g cocamidopropyl betaine
  • 5 g cetyl alcohol or BTMS-50
  • 5 g shea butter or mango butter
  • 5 g vegetable glycerin
  • 0.5-1 g boric acid
  • 2.5-3 g distilled water, warmed
  • 0.5 g broad-spectrum preservative (e.g., phenoxyethanol/ethylhexylglycerin)
  • 0.5-1 g fragrance or essential oil

Adjust the water slightly so the batch totals exactly 100 g. Because the formula contains water, include a preservative; do not rely on boric acid alone.

Step-by-step method

  1. Heat the distilled water to about 60-70°C. Sprinkle in the boric acid and stir until fully dissolved. Set aside.
  2. In a double boiler, gently melt the cetyl alcohol and shea butter with the SCI until the mixture becomes a soft, uniform paste. Keep the temperature around 70-75°C.
  3. Remove from heat. Stir in the cocamidopropyl betaine, glycerin, and the boric acid solution.
  4. Add the SLSA powder and preservative, folding gently to avoid dust. Work while the mixture is still warm and pliable.
  5. Add fragrance or essential oil and mix thoroughly.
  6. Press the dough into silicone soap molds or a lined bar mold. Tap firmly to remove air pockets.
  7. Leave the bars in the molds for 24-48 hours, then unmold.
  8. Place the bars on a rack and cure/dry for 1-2 weeks so they harden and become longer-lasting in the shower.

pH and troubleshooting

Shampoo bars should usually have a pH of 4.5-5.5 when diluted in water. Test a 1% solution with pH strips or a meter. Boric acid will help lower pH slightly, but if the pH is too high, use citric acid solution rather than adding more boric acid.

If the bars sweat or feel sticky, reduce the glycerin or water slightly next time. Store finished bars in a dry, well-ventilated area.