We don’t use water from the tap.
It sounds like an exaggeration. It isn’t.
Tap water isn’t the same from one time to the next. The lime content swings. The minerals vary. It depends on the season, on the pipes, on whatever the waterworks is up to that day. Small differences… but in soap, small differences are big problems.
When lye is mixed with water, it reacts violently. The temperature rises to over ninety degrees. The water carries the lye into the fats and starts the saponification. That’s the water’s only job: to transport. And for that job, the water has to be clean. Not “clean enough.” Clean.
Distilled water is water without variables. No lime. No minerals.
No surprises. The same starting point, every time.
That means saponification proceeds the same way from batch to batch. That the soap you buy in March behaves like the one you buy in October. Same hardness. Same lather. Same result.
Here’s what often surprises people: most of the water disappears.
During saponification, the water reacts with lye and fat. During the curing period… the six weeks the soap rests… the remaining water slowly evaporates. The soap hardens. It gets firmer. What’s left is concentrated.
A finished bar of soap keeps only about fifteen percent of the water that was added at the start. The rest is gone. Evaporated over six weeks of patience.
It’s not romantic. It’s not something you make an Instagram story about. It’s craft.
And craft starts with controlling what you can control.
It’s the most boring choice we’ve ever made. And one of the most important.
Sources:
- Soap Queen, 2019: How Temperature Affects Cold Process Soap
- Lovely Greens, 2023: How to Cure Handmade Soap
- BrambleBerry: Water Discounting Cold Process Soap
- Soap Authority: Making Soap with Tap Water — 3 Things You Must Know
- Ultimate Guide to Soap: Distilled Water in Soap Making