Borax (sodium tetraborate decahydrate)
In liquid soap: takes up spare lye and thickens the soap.
Opens the calculator’s starter recipe, 1000 g of oils, with this additive at the dose it fills in, ready to change. The batch opens as hot process, because it usually goes in after the cook.
What it is
Borax is sodium tetraborate decahydrate, a salt whose water of crystallisation is 47 % of its weight. It dissolves poorly, 4.7 % at 20 °C, into a mildly alkaline solution of about pH 9.2.
U.S. Borax primary source, PubChem (HSDB) primary source
How much
- 1.18 % of the oils filled in (from a published recipe) Makela, Making Natural Milk Soap primary source
- 1.03 % of the batch in hot process (from a published recipe) Fisher, The Complete Photo Guide to Soap Making primary source
The calculator fills in 1.18 % of the oils: 11.8 g for the 1000 g of oils in its starter recipe.
When and how
- Usually: after the cook.
- Also used: into the oils.
- Stir it in as it is.
Fisher, The Complete Photo Guide to Soap Making primary source, Makela, Making Natural Milk Soap primary source
What it does to the recipe
- Every 100 g takes 20.98 g of sodium hydroxide while there is free lye, and the calculator adds that to the lye. After the cook none is left, so it takes none.
PubChem (HSDB) primary source, U.S. Borax primary source, Stoichiometry primary source
Worth knowing
- Caution: In the EU and Great Britain, where soap is a cosmetic, borates have been banned since May 2020. California bans them in cosmetics from 2027, and Canada allows up to 5 % with a warning on the label. Regulation (EU) 2019/1966 primary source, California Health and Safety Code §108980 primary source, Health Canada, Cosmetic Ingredient Hotlist primary source
- Soapmakers use it mainly in liquid soap made with no superfat: a borax solution stirred into the paste takes up the excess lye and thickens the soap. Fisher, The Complete Photo Guide to Soap Making primary source, U.S. Borax primary source
Spoons and drops
- 1 tsp ≈ 3.79 g
- 1 tbsp ≈ 11.4 g
- 1 cup ≈ 182 g
U.S. Borax primary source
INCI name
Sodium Borate
- CAS number
- 1330-43-4, 1303-96-4
- EC number
- 215-540-4
Guides that discuss it
Sources
Hazardous Substances Data Bank, via PubChem: sodium metaborate is made "By action of sodium hydroxide on disodium tetraborate" (HSDB 5045); boric acid "Ka = 5.80X10-10 at 25 °C (pKa = 9.24)" (HSDB 1432); borax "pH at 20 °C: 9.3 (0.1% solution); 9.2 (1.0% solution)" (HSDB 328). Na2B4O7·10H2O + 2 NaOH → 4 NaB(OH)4 + 3 H2O: 2 × 39.997 ÷ 381.37 = 0.2098 g NaOH per gram.
U.S. Borax (Rio Tinto), Borax Decahydrate product data sheet (9/2024) and product page: "Molecular weight 381.37"; "Bulk density: 769 kg/m3 (48 lb/ft3)"; "Water of crystallization, H2O: 47.24%"; "Dissolved in water, it hydrolyzes to give a mildly alkaline solution, and thus capable of neutralizing acids. It also combines with strong alkalis to form compounds of lower pH."; solubility 4.70 % at 20 °C.
Neutralisation stoichiometry with IUPAC 2013 molar masses: grams of NaOH per gram = protons × 39.997 ÷ molar mass. Disodium EDTA dihydrate (372.24 g/mol) takes 2; sodium bicarbonate (84.007 g/mol) 1; magnesium sulfate heptahydrate (246.47 g/mol) 2; borax decahydrate (381.37 g/mol) 2, to sodium metaborate; malic acid (134.087 g/mol) 2; citric acid (192.124 g/mol) 3. An acidity given in milliequivalents per litre takes that many millimoles of NaOH per litre.
David Fisher, The Complete Photo Guide to Soap Making (Creative Publishing International, 2018), pp. 161–165: liquid soap is made at 0 % superfat and, "To buffer any excess lye, the soap is neutralized using a borax solution": 1 oz (28 g) borax in 4 oz (113 g) very hot water, "2 tablespoons (15 ml)" of it per pound of paste (two tablespoons are 30 mL: the book disagrees with itself), more to thicken; cream soap, "0.5 ounce (14 g)" of borax in 4 oz of hot water for a 3 lb batch, whipped in after the cook.
Casey Makela, Making Natural Milk Soap (Storey, 1999; from Milk-Based Soaps, 1997): "Herbs will not keep their original color; most fade in reaction to the lye"; "½ cup (118 ml)" of cornmeal, "½ cup" of raw almonds blended "to a medium-coarse powder", ¾ cup of dried lavender flowers or rose petals, and "½ cup (118 ml) rolled oats" blended "to a medium-coarse powder", per batch of about 85 oz (2410 g) of oils, added "to the liquid when it’s blended for the first time"; "1 ounce (28.4 g) borax" in every recipe, added to the oils with the sugar and glycerin as the lye-milk goes in; soap "light brown if you use cinnamon and cloves".
Commission Regulation (EU) 2019/1966, Annex I, replacing entry 1396 of Annex II (prohibited substances) to Regulation (EC) No 1223/2009: "Borates, tetraborates, octaborates and boric acid salts and esters, including: ... Disodium tetraborate decahydrate; borax decahydrate [11]", CAS 1303-96-4; Article 3: "Article 1 shall apply from 1 May 2020." Great Britain kept the entry in its copy of Annex II.
California Health and Safety Code §108980(b)(19)(C), as amended by AB 60 (2025): "Borates, tetraborates, octaborates, and boric acid salts and esters", borax decahydrate among them, may not be in a cosmetic sold in California from 1 January 2027.
Health Canada, Cosmetic Ingredient Hotlist, "Boric acid and its salts" (borax, sodium borate): as a pH adjuster at most 0.1 %; in other cosmetics at most 5 %, with the warning "Do not use on broken or abraded skin, not to be used by children under three years of age."