Designing your own soap recipe: purpose, acids, oils

A soap recipe is a list of oils, but the oils are only the means. What decides the bar is the fatty acids they carry, and what decides the acids you want is what the bar is for. This guide works in that order, then shows three designs taken all the way through the calculator.

First, make a recipe someone else has tested

Watson makes a point in Smart Soapmaking that is worth taking seriously: most failed batches come from people designing recipes before they have made one that works. Making soap and designing soap are separate skills, and learning both at once makes it impossible to tell a formula problem from a technique problem. Make a tested recipe, such as our classic olive, coconut and palm bar, until the process is dull. Then change it.

Start from what the bar is for

Every decision below follows from one question: what should this bar do? The table shows where this site’s own recipes sit for common purposes. Treat them as starting points.

From the recipes on this site. Hair, shaving and liquid soap have their own guides.

From purpose to fatty acids

The fatty acids guide explains what each acid does. For design, four groups carry nearly everything.

Lauric and myristic acid set the cleansing. Coconut, babassu and palm kernel oil are each about two-thirds lauric plus myristic, so every 10 % of one of them adds about 7 points of cleansing. The usual range of 12–22 therefore means roughly 20–30 % of a lauric oil. More than that cleans harder and can dry the skin: pure sodium laurate irritates skin on its own, and the other chain lengths in a blend are gentler.

Palmitic and stearic acid set the hardness and the life of the bar. Palm oil, tallow, lard, cocoa butter and shea butter carry them, and each adds roughly half its own percentage to hardness, cocoa butter a little more and lard a little less: 25 % palm oil adds about 12 points. Watson’s rule of thumb is at least 40 % solid fat in the blend, which these oils and the lauric oils together provide.

Castor oil props up the lather. It is 85–92 % ricinoleic acid, and each 5 % of castor adds about 4 points to bubbly, creamy and conditioning at once. Most recipes use 5–10 %; much more gives a tacky bar.

Liquid oils fill the rest, and their kind decides the shelf life. Oleic-rich oils such as olive, high-oleic sunflower, avocado and macadamia keep well. Oils rich in linoleic and linolenic acid, such as sunflower, soybean, hemp and linseed, make soft bars that go rancid sooner; keep them to a minor share, and read the iodine value guide for why.

Cable suggests starting a new design from three bases, a hard fat, a liquid oil and a lauric oil, and layering others in afterwards. It is a good way to keep the first draft simple enough to understand.

Checking a draft in the calculator

Enter the oils as percentages, set the superfat and the water, and read the results in this order.

1. The qualities, against your purpose. The shaded bands are a convention. A low-cleansing bar is supposed to sit below the cleansing band. Ask whether each number is where your purpose needs it.
2. Shelf life. Check the iodine value, and more directly the linoleic and linolenic percentages in the fatty-acid profile.
3. The uncertainty band. Open “How well is this pinned down?”. If the worst-case superfat is close to zero, raise the superfat, or ask your supplier for a certificate of analysis for the oil with the widest range; the SAP guide explains why.
4. The warnings and the method. Water that is too dilute, an additive that uses lye, a fragrance at its limit: the calculator flags them, and the method lists the order of work.

When a quality sits outside its range, the calculator suggests swaps: one oil for another at the same weight, from your stock if you keep it here, or from every oil in the database, nearest to the ranges first. The lye is worked out again once you make the swap. The suggestions rank oils against the convention and nothing else, so judge them as a maker would. Some candidates are free fatty acids or hydrogenated oils that behave very differently in the pot, some are oils whose figures are disputed, and the ranking knows nothing of price or scent. If the usual ranges are the wrong target for a bar, as they are for a low-cleansing one, a Maker account can save your own ranges as a target, and the swaps then aim at those instead.

Three worked designs

All three are for 1000 g of oils. The lye and water are the calculator’s figures.

1. An everyday bar, made gentler

The starting point is the 40/30/30 bar: 400 g olive, 300 g coconut and 300 g palm oil at a 5 % superfat and a 33 % lye solution. It takes 144.1 g of sodium hydroxide and 292.6 g of water, and scores hardness 45, cleansing 21, conditioning 51, bubbly 21, creamy 25, iodine 52 and INS 161. It is a good first bar, and its recipe page says plainly that it is slightly drying, which fits a cleansing figure at the top of its range.

The purpose is the same bar, kinder to dry hands. So cut the lauric oil: coconut from 30 % to 20 % takes about 7 points off cleansing. That also costs hardness, so trim palm only a little, to 25 %, and put the difference into olive oil. Add 5 % castor oil to hold the lather, and raise the superfat to 6 %. The result is the gentler everyday bar: 500 g olive, 200 g coconut, 250 g palm and 50 g castor oil at a 6 % superfat and a 33 % lye solution, which takes 137.5 g of sodium hydroxide and 279.1 g of water.

Everything the change was meant to do shows in the numbers: less cleansing, more conditioning, lather held up by castor, a little less hardness, and a wider margin on the superfat.

2. A low-cleansing bar

The purpose is a bar for people who find ordinary soap drying. In the recipe that becomes a limit on lauric acid, which describes the composition and makes no medical claim. Lauric oils are held to 10 %: 550 g olive oil, 200 g shea butter, 100 g coconut oil, 100 g rice bran oil and 50 g castor oil, at an 8 % superfat and a 33 % lye solution. The calculator gives 127.7 g of sodium hydroxide and 259.2 g of water, and hardness 28, cleansing 7, conditioning 69, bubbly 11, creamy 26, iodine 72 and INS 122.

That is outside the usual ranges on five of seven counts, on purpose. The swap panel will offer ways to pull every figure into range, such as babassu or another lauric oil in place of the rice bran, and each of them undoes the design. This is the case targets exist for: a target that sets cleansing at, say, 5–10 and leaves blank the figures you do not mean to judge measures the recipe by what it is for.

Two consequences of the design are worth planning for. With 55 % olive oil and a low hardness, the bar is soft for longer and wants a full cure before it is judged. And across the published saponification ranges of its oils, the calculator shows the superfat anywhere from 5.1 % to 11.0 % at a nominal 8 %. A certificate of analysis helps most for the oil there is most of: a measured value for the olive oil narrows that to 6.7–9.6 %, where one for the shea only brings it to 5.4–10.4 %.

3. A luxury bar, and what the luxury buys

The luxury bar spends money: 300 g olive oil, 200 g shea butter, 150 g avocado oil, 150 g coconut oil, 100 g argan oil, 50 g castor oil and 50 g macadamia nut oil, at an 8 % superfat and a 33 % lye solution. It takes 129.9 g of sodium hydroxide and 263.6 g of water, and scores hardness 32, cleansing 10, conditioning 64, bubbly 15, creamy 26, iodine 68 and INS 130.

Now replace the avocado, argan and macadamia with olive oil: 600 g olive, 200 g shea, 150 g coconut and 50 g castor. That version takes the same 129.9 g of sodium hydroxide and scores hardness 31, cleansing 10, conditioning 66, bubbly 15, creamy 25, iodine 67 and INS 131. Every figure is within two points. Avocado, argan and macadamia oil are, like olive oil, mostly unsaturated acids, so in the fatty-acid numbers they behave as olive oil at several times the price. Whatever they bring beyond their fatty acids, such as their unsaponifiable fraction or the story on the label, is not something these numbers can show. That is a reason to choose them deliberately, and to describe them honestly.

The swap panel has an opinion here too. Cleansing at 10 and INS at 130 sit below their ranges, and its first suggestion is babassu oil in place of the argan, which brings cleansing to 17 and INS to 144. Whether to take it depends on the purpose: for a rich, gentle bar, the low cleansing may be exactly what you wanted.

Iterating

• Change one thing at a time. Two changes in one batch leave you guessing which one did the work.
• Keep every version. A copied link keeps a recipe exactly; saved recipes keep their versions, and a Maker account shows each version and what changed between them.
• Make test batches small, 500 g of oils is plenty, and let them cure fully before you judge them. Soft soap at two weeks can be a good bar at six.
• Write down what you noticed as well as what you weighed: lather, feel, how the bar wore in the dish. Those notes are the evidence the numbers stand in for.

The calculator does the chemistry; the judging is yours. The superfat guide and the water guide cover the two settings every design also has to choose.