Buckle up for Day 6 of Blogtober!
It looks like soap. It smells like soap. So why won’t I let you have it yet?
Because looking finished and being ready to leave the workshop are two different things. Rather like me putting my coat on, then remembering fourteen things I need to do before I can actually leave the house.
What does “curing” mean?
First of all, let’s clear up what curing isn’t.
Soap is not slowly “turning into soap” for four to six weeks. The chemical reaction between the oils, butters and sodium hydroxide, known as saponification, is largely complete within the first 24 to 36 hours after I pour the soap into the mould.
Curing is what happens afterwards.
During the curing period, excess water gradually evaporates from the bars. At the same time, the internal structure of the soap continues to develop and become more crystalline. The result is a harder, longer-lasting bar with better lather and a more refined feel when you use it.
When soap is first cut, it is still quite soft, largely because of its water content. Water is an essential part of the soapmaking process because I combine my oils and butters with a solution of sodium hydroxide and water to make the soap. But we don’t want all of that water to remain in the finished bar.
A soft bar of soap simply won’t last very long in your bathroom. It will wear away much more quickly, which is no good for you and no good for your hard-earned money either. So, while the bars are curing, water is slowly evaporating and they become firmer and harder. The structure of the soap continues to develop too, helping to improve the lather and the overall performance of the finished bar.
That cure time is what allows the soap to become the product I actually want you to use.
And just to clear up another common misconception: curing isn’t about waiting for sodium hydroxide to “evaporate”. That isn’t how the chemistry works.
How do I cure my bars?
Once I’ve unmoulded and cut a batch of soap, the individual bars spend around four to six weeks curing before they are ready for sale.
Over the years I’ve collected hundreds of plastic stacking baskets for the job. Before I start cutting, I line enough baskets with tissue paper, then place each freshly cut bar inside with space around it so that air can circulate freely.
I usually fit two loaves’ worth of soap into each basket, and every basket is labelled with the soap variety, its batch code and the date it will be ready to use.
The baskets are then stacked away and left to do what soap does best at this stage: very little, very slowly.
How curing affects the weight of a bar
As the bars cure and lose water, they naturally lose some weight too. A commonly quoted rule of thumb is that a bar may lose around 10% of its original weight during curing, but in practice that can vary quite a bit from batch to batch.
That means I need to allow for that weight loss when I cut and cure the bars, so that once they are ready for sale they still meet the minimum weight stated on the packaging.
For many years I labelled my bars as 100g, but because curing isn’t completely predictable, I would occasionally end up with perfectly good finished bars weighing 98g or 99g. Technically, that meant I couldn’t sell them as 100g bars.
These days I label them as a minimum of 95g instead. I haven’t made the bars any smaller or changed the recipe, and most still weigh more than 100g when they are ready for sale. The 95g minimum simply gives me a sensible margin for the natural variation that happens during curing.
Why not sell it sooner?
Because I want you to receive a bar that performs properly, rather than one that has been rushed out of the workshop before it is ready. Admittedly, this can make stock planning rather interesting. If a particular variety suddenly becomes very popular, I can’t simply make another batch today and have it back on the shelf tomorrow. Every new batch still needs its full cure time.
That can occasionally mean a favourite soap is out of stock for a while, but shortening the curing period isn’t a compromise I’m willing to make. Waiting those extra weeks gives you a harder, longer-lasting bar that is properly ready to be used.
Why doesn’t commercially made soap need a long cure?
Commercial soap is usually produced very differently from traditional cold-process soap, which is why it can often be made, packaged and sent out much more quickly.
Mechanical processing: Many factory-made soaps are processed using heavy machinery that removes moisture and compresses the soap into a dense, hard bar. Milled soaps may be passed through rollers several times before being pressed into their final shape, so they don’t rely on weeks of air-drying to become firm.
Different formulations: Commercial bars contain ingredients designed to give them hardness, stability and a long shelf life. Some are technically detergent-based cleansing bars rather than traditional soap, so the manufacturing process is completely different from cold-process soapmaking.
Glycerin may be removed: During some large-scale soapmaking processes, the naturally produced glycerin is separated from the soap and used elsewhere. Because glycerin attracts and holds water, removing some or all of it also changes the moisture content and behaviour of the finished bar.
Handmade cold-process soap, on the other hand, is usually left much closer to the state it naturally reaches after saponification, so time does part of the work that industrial processing can achieve much more quickly.
So, although four to six weeks might seem like a long time to wait for something that already looks perfectly finished, that curing period is an important part of making good cold-process soap. It gives each bar the time it needs to become harder, longer lasting and ready to perform as it should.
It may not be the most exciting stage of soapmaking, but it is 100% worth the wait 😉
Vicki x