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Transparent soap calculator

Transparent soap is a hot process soap dissolved in alcohol, glycerin and sugar syrup. Give the oils for the lye; the solvents come out at Catherine Failor’s 60/40, to make it with, and at her 50/50, the most to add if a sample sets cloudy.

The oils

Filled in with the oils of our Transparent glycerin bar. Change them for a blend of your own.

%
%
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Total — %
The lye
Failor makes it at 0 % and corrects in the pot. Above 0 %, the rest of the lye is kept back as a finishing solution.

Sodium hydroxide in a 32.5 % solution, the strength of all Failor’s recipes: 2.08 g of distilled water to each gram.

fill in the oils and the superfat

Calculated at soapformulator.com, every value sourced. Check the figures against your own scale.

The formula, and where it comes from 
NaOH (g)       = Σ(oil × SAP ÷ 1000) × 0.7129 × (1 − superfat ÷ 100)
lye water (g)  = NaOH × 67.5 ÷ 32.5            # Failor's 32.5 % solution
soap           = oils + NaOH                   # her "actual soap"
                 60/40                50/50
solvents       = soap × 0.8           soap × 1.0
alcohol        = soap × 0.30          soap × 0.35
sugar          = 7 % of the batch     7.5 % of the batch   # 3.5 % with rosin
batch          = soap + solvents + sugar
glycerin       = 8.5 % of the batch   10 % of the batch
syrup water    = solvents − lye water − alcohol − glycerin
Fire first

Ethanol gives off enough vapour to catch fire at room temperature: its flash point is 13 °C (55 °F), and this method holds it between about 60 and 80 °C for an hour or more. The vapour is heavier than air, so it spreads across the worktop to a flame or a hot element and can flash back to the pot, and ethanol can burn with a flame you cannot see.

  • Electric heat only. An electric hob, or a hotplate plugged straight into the wall. No gas, no pilot lights, no candles and no smoking in the room.
  • A water bath and a sealed pot. Heat the soap pot inside a larger pot of water, never directly, under a tight lid or two layers of heavy plastic tied round the rim, with at least 5 cm (2 in) of room above the soap.
  • Only the alcohol you need in the room. Weigh it into lidded jugs before you start and put the bottle somewhere else: a fire that reaches a bulk container can flash back into it.
  • Air and attention. Ventilate the room so the vapour cannot build up, and never leave the pot.
  • A plan if it burns. Keep a dry-powder or carbon dioxide extinguisher within reach. If the pot catches fire, do not move it and do not throw water on it: turn off the heat if you can do so safely, leave the room, close the door and call the fire service.

In the pot, with your weights

Failor’s paste method, with the gel in a water bath as she advises for smaller batches. The transparent soap guide explains each step and what to do when one goes wrong.

  1. Set up for alcohol. Work on electric heat only, with no flame or pilot light in the room and a window open, and put a dry-powder or carbon dioxide extinguisher and a spare lid within reach. Set a water bath on the hotplate, weigh — of ethanol into a lidded jug and the — of the difference into another, and take the bottle out of the room.
  2. Make the lye solution. Stir — of sodium hydroxide into — of distilled water, with your face turned away from the steam: it heats to about 93 °C (200 °F). Let it cool to 57–63 °C (135–145 °F).
  3. Trace and gel. Bring the oils to the same 57–63 °C, pour in the lye solution and stick-blend to a light trace. Cover the pot and set it in the water bath at a gentle boil until the soap is translucent right through, at 77–82 °C (170–180 °F), then hold it there for about an hour more, stirring now and then.
  4. Test for spare lye. Put a drop of phenolphthalein on a small sample: Failor looks for the faintest pink. Deep pink means spare lye: give it another half hour, and if it stays deep pink, correct it with melted stearic acid once the alcohol is in. No colour at all means the soap is short of lye: stir in a quarter of the finishing solution, keep the pot hot for 15 minutes and test again, stopping at the first faint pink.
  5. Dissolve it in alcohol and glycerin. Turn off the heat. Standing back, pour in — of ethanol and — of glycerin, scrape every scrap of soap off the bottom and sides, seal the pot and simmer it gently in the water bath for about 30 minutes, until a spoonful held to the light shows no specks.
  6. Add the sugar syrup. Bring — of distilled water to the boil, stir in — of sugar until it dissolves, cover it and simmer it for 2 to 3 minutes, then stir it into the soap.
  7. Test on cold glass. Drip a spoonful onto the base of a glass chilled in the freezer. If it sets clear, carry on. If it is milky, add the difference from the table a little at a time, testing after each addition, and stop as soon as a sample sets clear. Failor keeps back some of that water, so that she can dissolve a little more sugar in it rather than add more liquid.
  8. Scent and pour. Let the soap settle, covered, until it cools to 60 °C (140 °F), skim it and stir in the scent, about 1 % of the batch: —. Pour, unmould once firm, and cure the bars for at least two weeks.

How the solvents are worked out

Failor counts the actual soap as the oils and the dry sodium hydroxide; the water of the lye solution she counts as a solvent. She works each recipe out twice. At 60/40 the solvents weigh 0.8 times the soap, and the alcohol 0.30 times; at 50/50 the solvents weigh as much as the soap, and the alcohol 0.35 times. The sugar goes on top. The glycerin and the sugar take her worked shares of the batch, 8.5 % and 7 % at 60/40 and 10 % and 7.5 % at 50/50, and the water to boil the sugar in is what the solvents leave once the lye water, the alcohol and the glycerin are counted. Her names for the two are not shares of the weight: with the sugar on top, the soap is about half of what is poured.

Her worked recipe

Failor’s example is 2 lb of palm oil, 2 lb of coconut oil and 1¼ lb of castor oil with 13 oz of caustic soda in 27 oz of water: 97 oz of actual soap. Her 60/40 table gives 29 oz of alcohol, 16 oz of glycerin, 13 oz of sugar and 5 oz of water to boil it in, 187 oz in all. Given her soap, this calculator gives 29.1 oz, 16.0 oz, 13.1 oz and 5.5 oz, 187.7 oz in all; the half ounce of water between the two is her rounding of the solvents, 77.6 oz, down to 77.

Her printed recipes do not all sit on this method. Basic Recipe 1, which our Transparent glycerin bar follows, uses less glycerin and more sugar, so its solvents differ from the ones above. For a blend of your own, or to make one of her recipes larger or smaller, she recommends working it out this way.

Transparent soap by the solvent method

Common questions

Why make transparent soap at 0 % superfat?
Free oil has nowhere to hide in a clear bar: it shows as a sticky, cloudy layer. So Failor works at 0 % and tests in the pot instead, with phenolphthalein after the gel and on cold glass before pouring, and corrects with lye solution or stearic acid. Weighing the lye at a small superfat and keeping the rest back as a finishing solution reaches the same end point from the side that cannot leave spare lye.
Which alcohol should I use?
Ethanol at 95 % (190 proof), sold as grain alcohol or rectified spirit. Isopropyl alcohol is no safer, with a flash point of 12 °C (53 °F); it dissolves soap less well and leaves the bar less clear, so Failor uses it for a quarter to a third of the ethanol at most, and only at 99 %.
Why work out two formulations?
Because more solvent is not better. Each extra gram of alcohol, glycerin or sugar makes a softer bar, so Failor starts at the lean end and adds solvent only until a sample sets clear. If it is clear at 55/45, she stops there, and what she added becomes the recipe.
Why is my transparent soap soft or sticky?
Usually too much solvent, most often glycerin or sugar, which the glass test cannot show: the drop sets clear and the slab stays soft. Failor remelts it and stirs in melted stearic acid with half its weight of lye solution. A sticky, cloudy layer on a clear slab is the opposite fault: too much oil, or too little lye.

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Goes well with

All tools
Lye calculator NaOH, KOH or dual lye, water three ways, additives, fragrance, mould sizing — every number explained
Melt and pour How much base fills a mould once fragrance and additions take their share, and the sums for a rebatch

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