Lye: sodium and potassium hydroxide
Updated
The ingredient that makes soap soap, that everybody is nervous about, and that is entirely manageable with three habits.
There is no soap without it
Every bar of soap ever made was made with an alkali. "Lye-free soap" either is not soap or is soap somebody else made the lye stage of.
Melt-and-pour bases are pre-saponified: the lye stage happened at the manufacturer.
And there is no lye left in finished, properly formulated soap. It was consumed by the reaction.
Sodium or potassium
Sodium hydroxide makes hard bar soap.
Potassium hydroxide makes soft or liquid soap.
They have different saponification values, so a recipe for one is not a recipe for the other, and potassium hydroxide is frequently sold at 90 percent purity, which has to be accounted for in the calculation.
Some liquid soap recipes use a blend, which needs a calculator that handles both.
Handling it
Sodium hydroxide is a strong alkali. It causes severe burns, and the burns develop with a delay, which is why the response is prolonged irrigation rather than waiting to see.
Goggles, not glasses. Gloves. Long sleeves. Closed shoes.
Lye into water, never water into lye, and never the other way round: the reaction is violently exothermic and adding water to solid lye can erupt.
Ventilation, because the fumes in the first minute are genuinely unpleasant and should not be breathed.
And keep vinegar away from your burn plan: flushing with large amounts of cool running water is the guidance, and neutralizing an alkali burn with an acid generates heat.
The safety data sheet is where the specific handling and first aid information lives, and it is worth reading once properly.
Buying it
Food grade or better, 99 percent or higher for sodium hydroxide, from a supplier who provides the SDS.
Not drain cleaner, which may contain other things.
Store it sealed, dry and high up, away from children and away from acids. It absorbs moisture from the air and clumps, and a clumped container is a weighing problem as well as a storage one.