How much salt goes in a salt bar?
A salt bar is ordinary soap with fine salt stirred in at trace, so much salt that it's counted against the oils: 50% of the oil weight is the light end, 100% is the classic "spa" bar that's hard enough to knock on. The salt doesn't react with anything, so it never changes the lye. What it changes is the size of the batch, and that is easy to get wrong, because salt weighs a lot and takes up surprisingly little room.
| Salt, % of oils | Per 1,000 g oils | Per pound of oils | Table-salt cups | Batch weight | Batch volume |
|---|---|---|---|---|---|
| 25% | 250 g | 4.0 oz | 0.9 | +18% | +8% |
| 50% | 500 g | 8.0 oz | 1.7 | +35% | +15% |
| 75% | 750 g | 12.0 oz | 2.6 | +53% | +23% |
| 100% | 1,000 g | 16.0 oz | 3.4 | +70% | +31% |
An 80% coconut / 20% olive salt bar at 20% superfat and 33% lye concentration (139.0 g NaOH, 282 g water per kilo of oils). Salt crystal density 2.17 g/cm³ (CRC Handbook); batter 0.95 g/mL; cups from USDA's 292 g per cup of table salt (coarser salt weighs less per cup, so weigh it). Batch volume adds the salt's crystal volume to the plain batter's.
1,000 g of oils (800 g coconut, 200 g olive) at 20% superfat need 139.0 g of NaOH in 282 g of water at 33%, exactly as they would without salt. Add 750 g of salt at trace. The batch now weighs 2,171 g, 53% more than the soap alone. But the salt is 2.17 times denser than water, so it occupies only 346 mL, and the batter grows by just 23%.
Why salt bars are mostly coconut oil
Salt kills lather, and the reason is salting out: soap is a salt that dissolves in water, and adding a lot of another salt (sodium chloride) pushes it back out of solution, which is literally how soap was separated from its lye for centuries. Long-chain soaps (from olive, palm, tallow) salt out easily and stop lathering. Short-chain lauric and myristic soaps, from coconut, palm kernel and babassu, stay dissolved in salty water. As classicbells puts it, a soap with a high cleansing number "has the ability to dissolve easily in… salt water", and for 100% coconut soap "adding salt does not reduce the solubility enough to make the soap solidify and separate". Coconut soap was sold as marine soap for exactly this reason: it lathers in seawater.
| Recipe (20% superfat) | Lauric + myristic | Cleansing | Bubbly | Hardness | Conditioning |
|---|---|---|---|---|---|
| 100% coconut | 67% | 67 | 67 | 79 | 10 |
| 80% coconut, 20% olive | 53% | 54 | 54 | 67 | 24 |
| 70% coconut, 20% olive, 10% castor | 47% | 47 | 56 | 59 | 33 |
| 50% coconut, 50% olive | 34% | 34 | 34 | 48 | 46 |
| Classic bar (30% coconut) | 20% | 20 | 25 | 45 | 51 |
SoapCalc-style quality indices, worked out from the combined fatty acid profile of each blend's oils. They are relative numbers for comparing recipes, not claims about skin.
That's also why salt bars are made at 15–20% superfat. A soap that's mostly coconut has a very high cleansing number (67 at 100%, against a customary 12–22), and the extra free oil is there to balance it. The high superfat is safe here in a way it wouldn't be for a linoleic-heavy bar: coconut is saturated fat, slow to go rancid. Published salt bar recipes almost all sit at 70–100% coconut; the table shows what goes as the share drops. The 50/50 blend has 34% lather-forming acids against 53% at 80%, and with salt in it the difference is felt in the hand.
Will a salt bar fit my mold? Volume, not weight
Because salt is so dense, the batter doesn't grow in proportion to the salt you add. Two common ways of fitting a salt bar to a mold are to scale the oils so the total weight (oils, lye, water and salt) matches what the mold holds as plain soap, or to count the salt "as you would your oils". Neither accounts for density. Here is a 1,000 mL mold (a 12-cavity 3 oz tray is about that) filled each way:
| Salt, % of oils | Oils that fill it (by volume) | Oils if you fit by weight | Mold filled by the weight method |
|---|---|---|---|
| 0% | 669 g | 669 g | 100% |
| 50% | 579 g | 495 g | 85% |
| 75% | 543 g | 438 g | 81% |
| 100% | 511 g | 392 g | 77% |
1,000 mL mold, 80/20 salt bar at 20% superfat and 33%, batter 0.95 g/mL, salt crystal 2.17 g/cm³. The volume column finds the oils whose batter plus salt fills 1,000 mL; the weight column scales the oils until batter plus salt weighs what 1,000 mL of plain batter would.
With no salt, the mold takes 669 g of oils. At 100% salt it takes 511 g, because the salt displaces only its own volume. Scale by weight instead and you'd make 392 g, filling 77% of the cavities. The reverse mistake, keeping the plain 669 g and adding the salt on top, overflows by the full 31%. Enter your mold's volume in the tool and it sizes the oils so the batter, salt included, fills it.
Brine soap: how much salt dissolves in lye water?
Soleseife, or brine soap, dissolves the salt in the recipe's water instead, so it ends up in the bar invisibly. The usual figures, "up to 25% of the water" or "a saturated brine is 26%", are plain-water numbers. Plain water does hold that much: 35.9 g of salt per 100 g at 20 °C. Lye water holds a fraction of it. Sodium hydroxide brings the same sodium ion as salt, and the common-ion effect drives salt out of solution as the lye gets stronger. The chlor-alkali industry, which makes lye from brine and has to get the salt back out of it, publishes the curve:
| Lye concentration | Salt it holds (wt % of the solution) | Most salt, as % of the water | Plain water, for comparison |
|---|---|---|---|
| 25% | 7.4% | 10.7% | 35.9% |
| 28% | 5.8% | 8.5% | 35.9% |
| 30% | 4.7% | 7.0% | 35.9% |
| 33% | 3.5% | 5.4% | 35.9% |
| 35% | 2.7% | 4.3% | 35.9% |
| 38% | 2.0% | 3.3% | 35.9% |
| 40% | 1.5% | 2.5% | 35.9% |
20 °C. Salt saturation in sodium hydroxide solution from OxyChem's Caustic Soda Handbook, Graph 7 (digitized; ±0.1 wt%), read at the soaper's lye concentration, the conservative reading. Plain water: CRC Handbook. "Most salt" turns that saturation figure into a percent of the recipe's water weight.
At an ordinary 33% the lye water holds salt at about 5.4% of the water's weight: for the classic bar's 292 g of water, 16 g of salt. Follow the "25% of the water" rule and you'd add 73 g, of which 57 g can't stay dissolved. That explains salt that won't dissolve, the cloudy slurry when lye goes into brine, and the sludge in the bottom of the jug.
Soap Queen's brine soap puts 0.5 oz of salt in 3.6 oz of water with 1.6 oz of lye (31% concentration); the lye water can hold 0.24 oz, so about 0.26 oz stays solid, and the tutorial notes that "a small amount may remain undissolved". Nerdy Farm Wife's recipe (43 g salt, 269 g water, 112 g lye) says to stir "until completely dissolved"; at 20 °C only about 20 g of the 43 can be. These are predictions from the solubility data, not measurements of those batches. The undissolved salt isn't dangerous (it goes into the batter as fine crystals), but it isn't a brine, and it's worth knowing the grit in the jug is salt, not lye.
Recipes argue about the order: dissolve the salt, then add the lye, or the reverse. The endpoint is the same solution either way; salt first just means the lye throws salt back out of solution as fine crystals, the "slurry". Temperature matters more than order. Freshly mixed lye water is hot, and hot lye water holds more salt (at 80 °C a 25% solution holds about 9% against 7.4% at 20 °C), so salt that seems to dissolve can crystallize again as the jug cools. Judge by the cool solution. The lye itself dissolves completely at any concentration up to 50%, so solids at these ratios are salt.
When to cut salt bars
Early, and while they're still warm. Salt bars go from sliceable to crumbling in a few hours, and the more salt, the sooner. Beyond that the advice is a spread, not a rule: published cutting times run from 20 minutes to 8 hours after pouring (Lovin Soap reports a batch ready at 25 minutes; others cut at 2, 3–4 or 4–8 hours), because the timing depends on the salt, the recipe, gel and the room. No calculator can predict it, this one included. Individual cavity molds sidestep the problem, which is why most salt bar recipes use them; in a loaf, check with a knife from the first half hour and cut the moment it's firm, with a smooth blade.
- Salt: fine sea salt or table salt. Coarse salt is scratchy and falls out; don't grind it, which makes sharp shards. Dead Sea salt is rich in magnesium chloride, which pulls moisture from the air; makers report the bars weep.
- Cure: the usual 4–6 weeks, many makers prefer 8; claims that salt bars keep improving for a year are common on forums and anecdotal.
- Measuring: by weight. USDA puts table salt at 6 g a teaspoon, so "1 tsp per pound of oils" is 1.32%. Converting at 5 g/tsp instead gives 1.1%.
The printable salt bar and brine sheet
SoapmakingToolbox.com · salt bars & brine soap
Salt bars per 1,000 g of oils (80/20 coconut/olive, 20% superfat, 33%)
| Salt, % of oils | Salt | Per lb of oils | Batch weight | Batch volume | Oils for a 1,000 mL mold |
|---|---|---|---|---|---|
| 25% | 250 g | 4.0 oz | +18% | +8% | 621 g |
| 50% | 500 g | 8.0 oz | +35% | +15% | 579 g |
| 75% | 750 g | 12.0 oz | +53% | +23% | 543 g |
| 100% | 1,000 g | 16.0 oz | +70% | +31% | 511 g |
Brine: the most salt lye water holds at 20 °C
| Lye concentration | Most salt, g per 100 g of water | Salt in the lye water (wt %) | Plain water holds, g per 100 g |
|---|---|---|---|
| 25% | 10.7 g | 7.4% | 35.9 g |
| 28% | 8.5 g | 5.8% | 35.9 g |
| 30% | 7.0 g | 4.7% | 35.9 g |
| 33% | 5.4 g | 3.5% | 35.9 g |
| 35% | 4.3 g | 2.7% | 35.9 g |
| 38% | 3.3 g | 2.0% | 35.9 g |
| 40% | 2.5 g | 1.5% | 35.9 g |
Salt: fine sea or table salt, weighed. Coconut 70–100%, superfat 15–20%. Cut early and warm; cavity molds are easiest. Brine data: OxyChem Caustic Soda Handbook, Graph 7. Salt never changes the lye.