Handling lye safely

Sodium hydroxide, the lye that makes bar soap, comes as small white beads or flakes that look like sugar and behave nothing like it. It burns skin, it can blind, and dissolving it in water makes a solution hot enough to scald. None of that makes soapmaking a dangerous hobby, because the risks are known in advance and the precautions are the same every time. This guide goes through them with the reasons behind each, and what to do if something goes wrong. Potassium hydroxide, the lye for soft and liquid soap, is handled in exactly the same way.

Why lye burns

Sodium hydroxide is a strong alkali, and the reaction that turns oil into soap in your pot works just as well on the fats in your skin. Lye saponifies them and dissolves the proteins that hold the tissue together. The damaged tissue softens instead of sealing over, so the alkali keeps working its way down for as long as it stays there. That is why the UK Health Security Agency warns that sodium hydroxide can cause deep, full-thickness burns, and why the treatment is so simple: wash the lye away, straight away, and keep washing.

Two things catch people out. A lye burn may not hurt at first, so the absence of pain tells you nothing. And lye solution feels slippery between the fingers, like soap, because it is turning the oils in your skin into soap. If anything on your skin feels slippery while you work, go to the tap.

Eyes are the serious case. Alkali can penetrate every layer of the eye, and UK clinical guidance treats any alkali in the eye as an eye emergency. Every other rule in this guide matters less than wearing goggles.

What to wear

What to wear, from a caustic soda maker’s handling advice and a glove maker’s test data.

Put all of it on before you open the lye, and keep it on until the soap has been unmoulded and cut. Anyone helping wears the same: Anne-Marie Faiola tells readers that a friend who helps them pour must “fully suit up”, gloves and goggles included. Raw soap batter is caustic too until the lye in it has reacted, which in a well-made batch takes a day or two. Tie back long hair.

Where to mix

Choose somewhere air moves: next to an open window with the cooker hood running, or outdoors on a still day. For the first minute or so, dissolving lye gives off a cloud of steam that carries a fine mist of lye with it, and that mist catches the throat. Stir with your face turned away and wait for the cloud to clear before you lean over the jug.

Mix in or beside the sink, on a clear, steady surface, well back from the edge. The sink contains a spill, the cold tap is your first aid, and a basin of cold water in it will cool the jug afterwards.

Keep children and pets out of the room for the whole session: the beads look like sugar and fresh soap batter looks like cake mix. Never leave a jug of lye solution alone, even for a minute. If the doorbell goes, either the door waits or the jug goes somewhere no one can reach it.

Lye into water, never water into lye

Dissolving sodium hydroxide gives off a great deal of heat. Pour the lye into the water and that heat is shared by all the water at once. Pour water onto a heap of lye and the first splash meets a lot of lye with very little water to carry the heat away: it boils where it lands and spits hot, concentrated lye out of the jug. In 2003 a soapmaking book was recalled in the US, through the Consumer Product Safety Commission, because its instructions combined the lye and the water in the wrong order and the mixture could bubble over.

Make mixing a fixed routine, the same every time:

1. Dress first. Goggles, gloves and sleeves go on before any container of lye is opened.
2. Water in the jug. Weigh cold water into a heat-safe polypropylene jug that holds at least twice the volume of the finished solution, and stand the jug in the sink. Cold matters: lye added to hot liquid, Grosso warns, takes it “dangerously close to or over the boiling point in no time”.
3. Lye in a dry cup. Weigh the lye into a separate dry cup to the gram, then close the lye tub at once.
4. Lye into the water. Tip the lye slowly into the water while stirring with a stainless steel spoon, face turned away, until no beads are left on the bottom.
5. Cool it where it cannot be knocked. Leave the jug standing in a basin of cold water in the sink, label it as lye, and keep it in sight until you use it.
6. Rinse straight away. Rinse the cup and spoon under running water, and wipe the scale and the worktop for stray beads.

Lye into water, in the sink, face turned away: goggles on, gloves under the sleeves, and the jug standing in cold water to cool.

Within moments of mixing, the solution can pass 90 °C (194 °F); Fisher and Failor both measured about 93 °C (200 °F). That is hot enough to scald even before you count the lye in it, and hot enough to soften the wrong kind of plastic, which is why the next section matters. Cool it in the basin of cold water. Never drop ice into the solution itself: that adds water the recipe did not ask for.

Milk, beer, juice and other liquids need their own methods, because sugars and proteins react with hot lye. The liquids guide and the milk soap guide cover them.

What lye can touch

Suitability from a labware maker’s chemical resistance chart, a caustic soda handbook and the UKHSA hazard compendium.

Look for the code 5 or the letters PP moulded into the base of a jug, and choose a thick, rigid one with a pouring spout. The equipment guide covers the rest of the kit.

If lye gets on you

Those times are minimums. The US Agency for Toxic Substances and Disease Registry’s guidelines for sodium hydroxide say to “flush exposed areas with water for at least 15 minutes” and to “flush exposed or irritated eyes with plain water or saline for at least 30 minutes”. Canada’s CCOHS asks for “at least 60 minutes” for skin and eyes alike, with “DO NOT INTERRUPT FLUSHING”, and the NHS says about an hour for any chemical burn, and to call 999. Lukewarm water makes a long rinse bearable.

Plan the first-aid route before you start: a tap you can get under at once, a clean towel nearby, and, as Grosso advises, the emergency and poison-centre numbers written up where you can see them.

Why not vinegar

The idea of pouring vinegar on a lye burn comes straight from school chemistry: an acid neutralises a base. On a person it is the wrong treatment. Neutralisation releases heat, right where the tissue is already damaged, and the minutes spent finding a bottle are minutes in which the lye keeps working. Water is already running, and it dilutes the lye, carries it away and cools the skin all at once. A caustic soda maker’s own handbook says not to use any kind of neutralising solution on a burn, least of all in the eyes, and ATSDR’s guidelines are blunt: “Neutralizing substances should not be used.”

The myth survives because older soapmaking books printed it, some older chemical data sheets still add “then rinse with dilute vinegar” after the flushing, and vinegar does have a proper use in soapmaking: wiping a worktop after a spill has been mopped up. Keep it for the worktop.

Spills and cleaning up

Dry beads. Sweep them up with a dustpan and brush, gloves on, into a dry plastic container, then wipe the spot with plenty of water. Beads bounce and cling to things by static, so check the floor, the scale and your sleeves before you take your gloves off.

Lye solution. Soak it up with paper towels or an old cloth and put them in the bin, then wipe the surface down with plenty of water. A final wipe of diluted vinegar on a hard surface is fine; industry handles spill residue the same way, with dilute acid and then plenty of water.

Batter and tools. Raw soap batter is caustic until it has saponified. Scrape the pot and wipe the tools with paper towels into the bin before you wash anything, or leave the pot and utensils somewhere safe for a day or two until the film on them has turned to soap, then wash them in hot water with your gloves on. Do not pour raw batter down the sink, where it can set in the pipe. A little leftover batter can go into a spare mould or a lined box: it becomes soap like the rest.

Never test a bar by touching it to your tongue. The old “zap test” means deliberately tasting something that may still be caustic; the soap pH guide covers the safer checks: phenolphthalein, a pH meter or titration.

Buying lye

Buy sodium hydroxide that is 99 % pure or better, as beads or flakes, from a soapmaking supplier or sold as food grade or technical grade. The label should say sodium hydroxide and nothing else. Clean lye is plain white; a tub with specks or a tint in it goes back.

Drain cleaner is only usable if its label says 100 % sodium hydroxide. Crystal drain cleaners are usually blends: the safety data sheet for one big brand’s crystals lists sodium nitrate, salt and aluminium granules with the lye, and aluminium reacts with lye, fizzing and heating up. Gels and liquids are mixtures too. None of those belong in soap.

Potassium hydroxide looks almost the same, and it takes about 40 % more of it by weight to do the same work. Use it where a recipe means sodium hydroxide and you get a soft mess; do the reverse and the soap is badly lye-heavy. Label every tub clearly. The lye calculator asks which one you are using and how pure it is, and the calculator guide explains both settings.

In England, Wales and Scotland the law has a say too. Sodium hydroxide, and anything with more than 12 % of it, is a corrosive product under the Offensive Weapons Act 2019, and since 6 April 2022 it may not be sold to anyone under 18. A seller who sells it online or by phone must check the buyer’s age and may not deliver it to a home or a parcel locker, so expect to collect it with ID, or to have it sent to a business address. Potassium hydroxide is not on the Act’s list.

Storing lye

Keep lye in the container it came in, with the lid screwed down between every scoop. Sodium hydroxide pulls water and carbon dioxide out of the air: an open tub clumps, and part of it slowly turns into sodium carbonate, so the lye gets weaker and the amount the calculator gave you is no longer quite right.

Store it labelled and locked away, or at least out of reach of children and pets, and apart from food, acids and anything aluminium. Grosso suggests a lidded, labelled plastic box, never the cupboard under the kitchen sink, and warns against a high shelf if you keep more than a few pounds, because a heavy tub is easily dropped. Never decant it into a food container or a drinks bottle, and never put a jug of lye solution in the fridge.

Lye solution made ahead to cool overnight needs the same care: a label, a place no one else can reach, and a tight lid. Susan Miller Cavitch covers hers because, left open, the air slowly weakens it into sodium carbonate.

Getting rid of lye

The simplest way to have no lye to get rid of is to mix only what the recipe calls for. If some solution is left over, keep it for the next batch in a tightly closed polypropylene or HDPE bottle, labelled as lye with its strength and the date, and store it like the dry lye. The calculator’s master batch setting weighs out a stored solution of any strength; the water and lye guide explains it.

Solution you will not use, and dry lye you no longer want, go to your council’s household hazardous-waste collection, not down the sink and not into the bin. Water companies ask for chemicals to go there, and some recycling centres accept chemicals only in their original, labelled container, so ask before you take a jar of homemade solution.

Handled this way, lye goes from its tub to the jug and from the jug to the pot, nowhere else, and a correctly calculated recipe uses all of it up. Next on the beginner path: the equipment you need, then choosing the oils for your first soap.