Size doesn't matter...

At least when it comes to soap.

I get a kick out of it when I overhear "but the other bars are bigger" in reference to other soapmakers I am with at a show. Why? Because when it comes down to it, unless a bar is unusually big, how long a bar lasts has more to do with the recipe, the process used to make it, and how long it's cured than the overall size of the bar.

Lets start with the recipe. Each oil used to produce a bar of soap contains a different set of fatty acids. These fatty acids, when saponified, bring a certain set of characteristics to the final product. The softer the final bar, the quicker it will melt when used. Almond and apricot both produce a soft bar, where palm and coconut make for a very hard bar of soap. For me, developing a recipe is a balance of making a hard bar of soap, that still remains conditioning to the skin. I want my customers to get their money's worth from very bar, but not sacrifice overall quality in the process. 

There are three main ways soap is made: cold process, hot process, and hand milled (sometimes called rebatched). In cold process the oils are combined with the alkali at a low temperature, usually about 110 degrees, stirred vigorously usually with an immersion blender, and left to finish saponification over three days or more. 

With hot process, the oils and alkali are combined and then cooked over a much higher heat. Here the saponification takes places much more quickly, and the soap can be ready to use in hours rather than days.

Hand milled soaps (like I make) typically start with an unscented cold processed soap that is milled down to look a lot like mozzarella cheese, and remelted. This remelting process helps reduce the amount of air in the soap, allows you to add fragrances and colors when the total soap pH is at its lowest, and aligns the molecules in a way that ends with a very hard bar of soap. Basically making the soap twice allows me to use a very conditioning recipe, that would be too soft if made only using a cold process method, but then gain hardness through the hand milling process. 

Finally how long a soap is cured will greatly influence how long a soap will last with use. When I make a batch of unscented soap, I have to make a choice on how much water to use in the recipe. In mine, I can use anywhere from 64 to 96 ounces of water. The more water I use, the softer the initial bar, and the more shrinkage as the bars cure. Think of beef jerky, you can start with a full pound of meat, but a few hours in the dehydrator leaves you with just 3 or 4 ounces of jerky at the end. Soap is similar. Over time, almost all the water will eventually cure out of the bar, ultimately changing the shape of the bar over time. All my soaps are cured for at least 4 weeks, and typically closer to 8, in a room that is constantly under dehumidification. They have lost at least 30% of their water before they make it to your home and they continue to loose it until you start using the bar. This is why the bars often look cupped, they are changing shape as they dry. Unfortunately, using a high percentage of water in a recipe can be an easy way to increase a bars weight and to keep it looking pretty over a longer period of time. A heavy bar, depending on how it is made, can be as much soap as my smaller bars, just have more water in them.

As you can see, choosing a bar of soap is much more complex then just looking at the overall size or weight of the bar.