Isopropyl alcohol
Isopropyl alcohol at 91 to 99 %, misted over freshly poured soap, pops surface bubbles, helps melt-and-pour layers stick and is widely used to hold off soda ash. It is highly flammable, so it is sprayed well away from any flame or hot surface.
- H225 Highly flammable liquid and vapour
- H319 Causes serious eye irritation
- H336 May cause drowsiness or dizziness
Harmonised classification in Annex VI of Regulation (EC) No 1272/2008 (CLP), index number 603-117-00-0.
Identity
Isopropyl Alcohol
- Formula
- C3H8O
- CAS
- 67-63-0
- EC
- 200-661-7
- Also called
- isopropanol, propan-2-ol, IPA, rubbing alcohol
Guides
Why makers spray it
On cold process soap the spray goes on straight after pouring. Anne-Marie Faiola's Soap Crafting mists the top with 91 % isopropyl alcohol every 30 minutes for the first 90 minutes, three times in all, and calls it an optional step against the light white dusting of soda ash; her later Pure Soapmaking uses 99 % and says the alcohol forms a barrier between the soap and the air. She leaves it off tops dusted with mica or scattered with herbs, because it makes them soggy.
On melt-and-pour soap it does two quick, visible jobs. David Fisher sprays freshly poured base to clear the bubbles on its surface, and sprays each set layer before pouring the next, at about 54.5 °C (130 °F), so that the layers stick to each other.
Strength matters because the rest of the bottle is water. Faiola points out that 91 % alcohol is not the same as most standard rubbing alcohol, though the two are usually sold side by side. Look for isopropyl alcohol, or propan-2-ol, at 91 % or more on the label.
What the evidence says
For bubbles and layers the effect is immediate and easy to see. For soda ash the evidence is experience rather than measurement: the spray is widely used and widely reported to help, but we know of no controlled test, and how it would work is not established. Faiola's barrier explanation is plausible and untested. The measures with a clear reason behind them are covering the top, which keeps carbon dioxide away from the unreacted lye, and keeping the loaf warm so the lye is used up sooner; the soda ash guide sets them side by side.
Alcohol will not remove ash once it has formed. Soda ash is sodium carbonate, which dissolves readily in water but not in alcohol, so steam or a quick rinse takes it off and a spray of alcohol does not.
Fire and fumes
Isopropyl alcohol is highly flammable, H225 under its harmonised EU classification. Its flash point is about 12 °C (NIOSH), so at room temperature it always gives off enough vapour to catch fire, and a mixture of 2 to 12 % in air will burn (International Chemical Safety Card). The vapour is about twice as heavy as air, so it spreads along a worktop and across the floor towards a gas hob, a pilot light, a candle or a heater. Spray it well away from all of them, never into a warm oven or onto soap that is about to go into one, and keep the bottle closed between uses.
The same classification lists H319, serious eye irritation, and H336, drowsiness or dizziness from the vapour. Spray away from your face with a window open, and keep the mist light: it is a finishing touch, not a soak. If it gets in an eye, rinse with plenty of water for several minutes.
Store it cool, well closed and away from heat, in a labelled bottle out of reach of children; a spray bottle of clear liquid on a shelf looks like water.
Not the same as ethanol
Transparent soap is made by dissolving soap in ethanol with glycerin and sugar, and the two alcohols are not interchangeable there. Catherine Failor replaces only a quarter to a third of the ethanol with isopropyl alcohol, and only at 99 %, because it dissolves soap less well, leaves the bar less clear and smells. Neither is the safer alcohol: ethanol's flash point is 13 °C and isopropyl alcohol's 12 °C. The transparent soap guide covers the method and its fire precautions.
Sources
- Regulation (EC) No 1272/2008 on classification, labelling and packaging of substances and mixtures (CLP) , European Union (2008) . Annex VI, index 603-117-00-0, propan-2-ol: Flam. Liq. 2, H225; Eye Irrit. 2, H319; STOT SE 3, H336; precautionary statement P210, keep away from heat, hot surfaces, sparks and open flames.
- Isopropyl alcohol (CID 3776) , PubChem, US National Library of Medicine . The harmonised classification and the ECHA C&L notifications.
- NIOSH Pocket Guide to Chemical Hazards: Isopropyl alcohol , US National Institute for Occupational Safety and Health . Flash point 53 °F (12 °C).
- NIOSH Pocket Guide to Chemical Hazards: Ethyl alcohol , US National Institute for Occupational Safety and Health . Flash point 55 °F (13 °C).
- International Chemical Safety Card 0554: Isopropyl alcohol , ILO and WHO (2020) . Flash point 11.7 °C closed cup; explosive limits 2 to 12 % in air; relative vapour density 2.1; boiling point 83 °C; no open flames, sparks or smoking; eyes rinsed with plenty of water for several minutes; store fireproof, cool and well closed.
- Soap Crafting , Anne-Marie Faiola, Storey Publishing (2013) . 91 % isopropyl alcohol sprayed every 30 minutes for 90 minutes, an optional step against soda ash; 91 % differs from most standard rubbing alcohol; no alcohol on mica or herbs, which it makes soggy.
- Pure Soapmaking , Anne-Marie Faiola, Storey Publishing (2016) . 99 % rubbing alcohol misted on the top; the alcohol creates a barrier between the surface of the soap and the air.
- The Complete Photo Guide to Soap Making , David Fisher, Creative Publishing International (2018) . Melt and pour: a light spray of rubbing alcohol removes bubbles and helps layers adhere; the next layer poured at about 130 °F (54.5 °C).
- Making Transparent Soap , Catherine Failor, Storey Publishing (2000) . Isopropyl alcohol for only a quarter to a third of the ethanol, and only at 99 %: it dissolves soap less well, gives a less clear bar and smells.
- Commission Regulation (EU) No 231/2012 laying down specifications for food additives: E 500 (i) Sodium carbonate , European Union (2012) . Freely soluble in water, insoluble in ethanol.