Why are corners softer ?

[Elise]

Hi !

I have been wondering for a while now : why are the corners of my soaps soft for longer ?
It was particularly obvious with the last soap I made. It has 30% shea butter in it. I unmolded after 24h. The top was hard, the sides were also firm and didn't stick to the mold at all. So I turned the mold upside down and let it out. Once unmolded I noticed the bottom corners as well as the ones at both ends of the mold were very very soft and didn't hold their shape at all. They were soft like summer time room temps shea butter. While couldn't make an indent on the top when I tried.
I noticed that with all my recipes even if not always to the same extend. Is that the same for you ? Do you know why is that ? Does the corners saponify slower ? Is it a temperature thing ?

 

[lenarenee]

I haven't thought about it before. Maybe the corners just aren't exposed to as much air since they're touching the mold. Heat could also be a cause since there's less mass and fewer molecules saponifying in the corner area. Also...corners are "weaker" anyway compared to the center of a loaf of soap where it's the thickest and has the most mass resulting in more resistance against pressure.

 

[Elise]

I haven't thought about it before. Maybe the corners just aren't exposed to as much air since they're touching the mold. Heat could also be a cause since there's less mass and fewer molecules saponifying in the corner area. Also...corners are "weaker" anyway compared to the center of a loaf of soap where it's the thickest and has the most mass resulting in more resistance against pressure.

Yes but the flat bottom and the sides also aren't exposed to air. A agree that corners are weaker per se but I find that it even has a different texture in my case.
Maybe we'll never know why exactly... i'm asking but am aware that this could be a question without answer.
Do you also find your soap corners are soft for longer than the rest of the soap too?

 

[AliOop]

I tend to think it is mostly related to the amount of heat, as well as the level of trace. If my soap doesn't completely gel, or if I pour at a really thin emulsion, that's when my corners are soft and mushy.

Without external heat applied to the mold, the center of the soap is always going to get the warmest, because it is most insulated. The corners have the most exposure to cooler air, coming from several directions (as opposed to just one direction like the flat sides). That means they can be less likely to get as warm, and therefore not as hard.

 

[DeeAnna]

The corners of my soap sometimes do not get quite warm enough to gel. When that happens, the corners are soft like clay, but the rest of the soap loaf is firm. Sometimes I can even see a slight color difference.

If I think that's likely to happen -- usually in winter when my home is cooler -- I cover the ends of my long mold with light towels to keep the ends a bit warmer and lay a thick towel on the counter and set my mold on that.

 

[lenarenee]

Yes but the flat bottom and the sides also aren't exposed to air. A agree that corners are weaker per se but I find that it even has a different texture in my case.
Maybe we'll never know why exactly... i'm asking but am aware that this could be a question without answer.
Do you also find your soap corners are soft for longer than the rest of the soap too?

True, but my thinking was the density of the mold wall is higher than that of the soap - so maybe air could reach it - i.e. evaporation of water from the main body of the soap might make the soap edges firmer IF water them moves from the edges to the body. (diffusion). Strictly my half educated theory.

I really didn't pay attention to the corners of my soap after removing them from the mold, but other than the fact corners are smaller, therefore more delicate, I can't say I've noticed they are significantly softer than the soap body. But - I also avoid gelling if possible and since I use a lot of lard and soap cold....I'm usually successful.

 

[lucycat]

I gel my soap but some, especially my main mint soap has a hard time gelling in the corners. The temperature within the mold is higher than the air outside the mold. The corners have two mold sides so they it is more difficult for them to warm up as much as the center.

If I am not careful, I can have overheating problems in my milk soaps. I end up with the center of the bar having a whitish discoloration pattern. It is in the bars from the center of the log, never the pieces closer to the end, because the end bars just didn't get as hot.

 

[melinda48]

This is why I tend to pop my loaves in the freezer before UN-molding. It seems to help with the corners.

 

[Shelley D]

I found I had softer soaps when I was working with high OO recipes ~ soft edges and corners, and had to leave my soap in the mold longer until it firmed up. Now that I'm adding more butters to the mix, I really don't have that problem. Also, I don't gel ~ I pour into the mold(s) and let it sit at room temperature.

Do you add any harder butters like kokum or cocoa or maybe any coconut oil? What's the rest of your recipe like (with the percentages)?

 

[DeeAnna]

...Also, I don't gel ~ I pour into the mold(s) and let it sit at room temperature....

It's not strictly necessary to always add heat or cover the mold to get soap to gel. My soap generally gels even when I don't cover the mold -- most of the time the mold sits out on a counter in my pantry.

 

[Sirtunes]

Hi !

I have been wondering for a while now : why are the corners of my soaps soft for longer ?
It was particularly obvious with the last soap I made. It has 30% shea butter in it. I unmolded after 24h. The top was hard, the sides were also firm and didn't stick to the mold at all. So I turned the mold upside down and let it out. Once unmolded I noticed the bottom corners as well as the ones at both ends of the mold were very very soft and didn't hold their shape at all. They were soft like summer time room temps shea butter. While couldn't make an indent on the top when I tried.
I noticed that with all my recipes even if not always to the same extend. Is that the same for you ? Do you know why is that ? Does the corners saponify slower ? Is it a temperature thing ?

I recently moved from loafs to making individual bars. I noticed the oils are consistently thinner at the bottom when I remove the bars from the mold and the soap is softer . However, when I remove the bar from the mold, the hardening accelerates faster and the bar quickly gets more consistent . For this reason, I try to get the individual bars out of the mold asap after 24 hrs. I don’t know if this is helpful to you, but wanted to share my experience with you. I hope it helps.

 

[Elise]

I found I had softer soaps when I was working with high OO recipes ~ soft edges and corners, and had to leave my soap in the mold longer until it firmed up. Now that I'm adding more butters to the mix, I really don't have that problem. Also, I don't gel ~ I pour into the mold(s) and let it sit at room temperature.

Do you add any harder butters like kokum or cocoa or maybe any coconut oil? What's the rest of your recipe like (with the percentages)?

Most of my recipes are indeed high in olive oil, as I like using as much locally sourced oil as possible. I'm aware that it makes my soaps take longer to firm up and I take it into account when unmolding. But what questioned me is that sometime the soap gets quite very hard, like "I don't want to wait too much longer before cutting" hard, and the corners are still very very soft.
The consensus seems to be that they get colder faster and that makes sense to me. I don't think my soaps gel, but it still makes sense that were its warmer, saponification happens faster.

I do use coconut oil (between 15 and 25%) and shea butter (30% for my last soap). I bought cocoa butter but haven't used it yet. It's a project for 2025 !
I'm sure it will make the whole soap harder and I use the freezer trick sometimes. This was more about understanding what happens

Thank you @DeeAnna , @Shelley D , @melinda48, @lucycat, @AliOop, @lenarenee for chiming in !