Epsom salt (magnesium sulfate)
A bath-salt and bath-bomb ingredient; its soap sources advise against it in bar soap. Fisher, The Complete Photo Guide to Soap Making primary source
Opens the calculator’s starter recipe with this additive in it. No source gives a dose to fill in, so enter your own. The batch opens as hot process, because it usually goes in after the cook.
What it is
Epsom salt is magnesium sulfate heptahydrate: crystals in which water of crystallisation is about half the weight, 50.5 % in one maker’s data. Its US drug labels measure it by the 5 g teaspoon.
JECFA; PubChem primary source, K+S primary source, DailyMed primary source
How much
No source gives a dose for soap: enter your own.
When and how
- Usually: after the cook.
- Also used: at light trace.
- Stir it in as it is.
- Not before the cook: there it reacts with the lye and uses it up.
JECFA; PubChem primary source
What it does to the recipe
- Every 100 g takes 32.46 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.
JECFA; PubChem primary source, Thomssen, Soap-Making Manual primary source
Worth knowing
- Caution: In a salt bar it draws water from the air and turns the soap mushy. Fisher, The Complete Photo Guide to Soap Making primary source
- Caution: Magnesium makes insoluble magnesium soap, the scum of hard water, which cuts lather. Old soapmaking manuals keep magnesium salts out of the water. Thomssen, Soap-Making Manual primary source, Simmons & Appleton primary source
Spoons and drops
- 1 tsp ≈ 5 g (4.34–5 g)
- 1 tbsp ≈ 15 g (13–15 g)
- 1 cup ≈ 240 g (208–240 g)
INCI name
Magnesium Sulfate
- CAS number
- 7487-88-9, 10034-99-8, 18939-43-0
- EC number
- 231-298-2
- In the finished bar
- with NaOH, Magnesium Hydroxide, Sodium Sulfate; with KOH, Magnesium Hydroxide, Potassium Sulfate
It changes only while there is free lye: stirred in after the cook, it stays as it went in.
Guides that discuss it
Sources
David Fisher, The Complete Photo Guide to Soap Making (Creative Publishing International, 2018), p. 115: for salt bars, "stick with sea salt or regular table salt. Don’t use Epsom or Dead Sea salts as they contain too much magnesium and other minerals that will attract water from the air and turn your soap into a mushy mess". Pp. 132–135: Epsom salts in three bath salt recipes and in the dry phase of bath bombs.
Magnesium sulfate heptahydrate, 246.47 g/mol (JECFA monograph 4, 2007); sodium hydroxide precipitates magnesium hydroxide from a magnesium salt solution (PubChem). 2 × 39.997 ÷ 246.47 = 0.3246 g NaOH per gram.
E. G. Thomssen, Soap-Making Manual (1922): hard water makes "insoluble soaps of lime and magnesia"; transparent soap with "10-20 per cent. sugar of the weight of the soap" (p. 83).
W. H. Simmons and H. A. Appleton, The Handbook of Soap Manufacture (1908): bicarbonate "converts the caustic alkali into carbonate", and excess carbonate effloresces; the water must be "free from calcium and magnesium salts"; sugar "may caramelize if the temperature gets too high".
US National Library of Medicine, DailyMed, Epsom salt (magnesium sulfate heptahydrate) labels: "each teaspoon (5 grams)".
K+S, Epsom Salt pure, technical data sheet (2019): water 50.5 %, bulk density about 980 kg/m³. Giles technical grade (2016): 55–58 lb/ft³ (0.88–0.93 g/mL).