why is my kitchen soap cracking?

[Guspuppy]

I mean the soap sitting in a soap dish that I use every day - both the hand soap and also the dish (bar) soap. They each get used daily so why do they crack as if they are excessively drying out after being wet? I have several bars in the shower that do not get used every day (I rotate among them) but it IS humid in there, and they don't crack. Nor does the soap in the dish on the bathroom sink, also used daily. So what's the difference? Why does the same soap in a different room crack?

ETA: the hand soap in the kitchen is lard based and the dish soap is 100% coconut 0% SF. The bathroom soaps are all different, the current sink soap is a salt bar and the ones in the shower are lard, salt, and a shea butter one that was my first soap ever.

 

[Scooter]

...the dish soap is 100% coconut 0% SF. The bathroom soaps are all different, the current sink soap is a salt bar and the ones in the shower are lard, salt, and a shea butter one that was my first soap ever.

OK this is just wild, I noticed exactly the same thing about my dish soap just this evening. And my dish soap is also a 100% CO salt bar...20% SF in my case. My shower soap is a 4-oil vegetarian soap with not a lot of salt in it.

Yeah, why is our dish soap cracking, lol?

Scooter

 

[BattleGnome]

I wonder how much glycerine we was away with each use. My bathroom soap (red mica and honey m&p) has cracks in it but they are "filled in" with what looks to me like a lighter blueish color. I was thinking it was some sort of glycerine deposits.

 

[DeeAnna]

Cracking is caused by repeated wetting and drying of a soap that is not completely consistent in composition. If you've ever seen streaking/mottling in your soap, you've seen this non-uniformity -- some parts of the bar are higher in oleic and linoleic soaps, other parts are higher in stearic and palmitic, etc. (I'd say most handmade soap is like this, but the effect is not always visible.)

The more soluble bits of the soap soak up water faster than less soluble parts. These wetter areas become mushy and weaker and tend to dissolve and wear away faster during use. The "bluish" look is mushy damp soap lying in the developing crack.

If the soap bar isn't used hard enough to wear down the stronger, less soluble soap along with the softer, more soluble soap, the bar can eventually crack. This tendency to crack is going to be more obvious in soaps that are in less humid areas and/or those that are not used often so they fully dry out between uses.

It's like a wooden board long exposed to the weather -- the surface of the board will "check" (make fine cracks) along lines of weakness for much the same reasons as soap.

In commercial soap, these cracks tend to form on the narrowest ends of the soap due to the way the soap is made by extrusion. In handmade soap, the cracks can be anywhere on the bar, but I normally see them more along the longer edges and face of the bar of soap I have at my shop sink. This bar isn't used often so it drys out fully between uses, and the air in the shop isn't particularly humid.

 

[Guspuppy]

If the soap bar isn't used hard enough to wear down the stronger, less soluble soap along with the softer, more soluble soap, the bar can eventually crack. This tendency to crack is going to be more obvious in soaps that are in less humid areas and/or those that are not used often so they fully dry out between uses.

This is why I am so curious about it! The soaps are used frequently every single day, and the 100% CO dish soap is rubbed with a green scrubby every day to do the dishes with, so I would imagine the harder areas are being worn down just as fast as the softer areas. As for the other lard-based soap that is cracking, a bar from the same batch is in the basement where it gets used maybe once a month, and it is very dry down there... so why is it not cracking?! It's just a mystery!

 

[DeeAnna]

The composition of the soap as well as the conditions of use all have something to do with the tendency of a bar soap to crack, but that's about as far as I can go to explain or troubleshoot the matter. Cracking doesn't happen to me often, but I have seen it occasionally and know exactly what you're talking about.

 

[Scooter]

This tendency to crack is going to be more obvious in soaps that are in less humid areas

Ah. And with the colder days of late fall, plus turning on the heat, the humidity in my house has plummeted. That is OK. This same soap was too sweaty a few weeks ago. I think I would rather have the cracking rather than the sweating anyway.

Scooter