Soap is a hostile place for microbes. The pH is high, and the surfactant that cleans your hands is not a friendly environment for anything trying to live in it. So a bar does not spoil in the sense that food spoils.
An unopened handmade bar is at its best for roughly a year and stays perfectly usable well beyond that. What ages is not the soap but the oils in it, which can slowly go rancid and show up as orange freckles and a waxy, crayon-like smell. A bar that looks and smells normal is fine, however long it has been in the cupboard.
What actually goes wrong in an old bar of soap?
Rancidity, and only rancidity. Most of my bars carry oils on top of the glycerin base, coconut and sweet almond, and you can see them on the label. Those oils are part of what you are paying for, and they behave like any oil in your kitchen. Given enough time, warmth, light and air, they oxidise.
Soapmakers call the result DOS, for dreaded orange spots: small rust-coloured freckles that appear on the surface, sometimes in one patch, sometimes scattered. They are harmless to wash with. They are also the honest sign that the bar is past its best, and they usually arrive with a faint smell of old crayons or old nuts that is hard to unsmell once you've noticed it.
How long does a handmade bar keep?
The useful frame is the shelf life of the oils in it, not a date stamped on a wrapper. Bars built mostly on stable fats — olive, coconut, palm, shea, cocoa butter — hold up for a year or two without complaint. Bars carrying a large share of delicate oils such as sunflower, hemp or rice bran are the ones that freckle first, sometimes inside a year.
Two other things fade faster than the soap does, and neither is spoilage. Scent goes first, especially essential oils in the citrus family. Colour goes second: a natural botanical colour dulls with light exposure, which is a different phenomenon from the batch-to-batch colour variation I've written about elsewhere. An old bar that has lost its perfume is still soap.
Why is my new bar of soap sweating?
Because glycerin is a humectant and the air in your bathroom has water in it. Glycerin pulls moisture from the air to the surface of the bar and it collects there as clear beads, which is why the phenomenon is called glycerin dew. It appears overnight in muggy weather and it looks alarming the first time.
It is not a fault, not mould and not the bar going off. Blot it dry with a cloth, move the bar somewhere with moving air and it stops. If it keeps recurring, the room is the problem, not the soap. A closed bathroom in a Florida August is a chamber built for this.
Does glycerin soap sweat more than other soap?
Yes, noticeably. Every bar I make starts from a clear glycerin base, and glycerin is exactly the ingredient that attracts water. So on a muggy night my bars can bead up sooner than a drugstore bar sitting in the same room.
The practical version: if a bar is wet in the morning, blot it and give it some air before you assume anything is wrong with it. I explain the base itself in why I make my soap with a clear glycerin base.
How do you tell a bar that's finished from a bar that's just old?
Use your nose first and your eyes second. Three checks, in order of how much they should worry you:
- Smell. Waxy, crayon-ish, faintly like old nuts. That is rancid oil, and it is the one real reason to stop using a bar.
- Orange freckles. Not dangerous, but they mean the same process is underway. Use that bar next rather than saving it for later.
- A dull, faded or slightly dusty surface. Cosmetic. Usually light exposure. Rinse the bar under the tap and carry on.
What is not a warning sign: beads of clear liquid after a humid night, or a colour that has calmed down since you bought it.
When should you throw a bar away?
Rancid smell, or visible mould, which requires something else in the bar to feed on, usually a fresh botanical that was pressed into the surface rather than suspended inside. Those are the two. Everything else on the list above is a bar you can keep washing with.
The one thing that genuinely shortens a bar's life is not time in a cupboard at all; it is water in a dish, and that belongs in the article about making a bar last. Store the spare ones dry, use the freckled ones first, and buy in quantities you'll actually get through, which is an argument for one bar at a time rather than a year of stock. The Loofah Soaps and the Chamomile Soap all start from the same glycerin base, so the same rule goes for every one of them: somewhere dry, with air moving past it.
Cosmetic products. Not intended to diagnose, treat, cure or prevent any disease.