Why does bar soap require cure while liquid doesn't?

[JayJay]

Hi Soapers,

I apologize up front if this is a repeat question. I tried to find the answer on existing threads but didn't really see a detailed discussion.

I have read that liquid soap is ready to use the same day. CP solid soap is technically soap after gel or within 24 hours (no gel) right? HP solid soap is technically soap after cook right? Why is it necessary for solid soap to cure, while liquid soap is okay to use as soon as there is no zap? Is there something different about KOH? Is it the water? What's the deal?

Now, I have seen on this forum that curing LS improves the soap, but that many people use it immediately. I have never seen anyone on this forum say the same about bar soap.

I trust the advice on this forum, so please don't take this as questioning the advice that has been given. I am legitimately interested in understanding the underlying theory behind the advice.

Thanks in advance for your help.

 

[Dorymae]

Curing does a few things, a bar becomes harder, the soap becomes more mild and in many cases lather is enhanced.

All of these things are very important to bar soap, but are less important in a liquid soap mainly because of the water added to it. The first I think is obvious, liquid soap is not going to become harder. The second can be helped with a cure, however most liquid soaps are made with more mild oils to begin with, and so becomes less important. (Castile or olive oil based liquid soaps are popular, but you rarely see a coconut liquid soap that would be used on a body - they are mainly cleaning soaps). Which leaves the third - lather. I believe this is where most of the benefit would lie, however remember that the more soluble a soap is the more it lathers, because you are getting more soap. Liquid soap is already diluted and so the amount you use is more than a bar soap would give you anyway, and most people use either a pouf or a wash cloth with liquid soaps which also helps it to lather.

So in conclusion, does it have to be cured? No, but would it benefit from a cure? Probably but only you can determine if the benefit is negligible or not.

 

[Susie]

There is a post by DeeAnna, I think, that explains this. I am apparently too sleepy to find it right now, though. Best I can remember is that NaOH makes a crystalline structured soap that changes over time. It loses ...... some element to the air. IDK what exactly. Hopefully she will be around soon to answer. You may want to use Google or another search engine to search like this: soapmaking forum deeanna cure.

 

[The Efficacious Gentleman]

There is no cure for deeanna!

It's the same with shaving soaps to a great degree - they are pretty good straight away when you use a lot of KOH, but there is improvement in time as they are more solid than a liquid soap.

 

[DeeAnna]

"...Why is it necessary for solid soap to cure, while liquid soap is okay to use as soon as there is no zap?..."

I have to talk about solid soap first to answer your question, so here goes --

Any solid soap, including the Gent's softer shave soaps, need time for three things to happen -- to finish saponifying, to evaporate excess water, and to develop a crystal structure. Even if your recipe is designed to make a long lasting, mild, lathery bar, you will see the best of these qualities when you use a soap that is given enough time to fully cure.

The first aspect of cure is to complete the last lingering bits of saponification. When you unmold a soap, the saponification might be 99.9% done, but it's good to allow that last 0.1% time to finish before using the soap. Anyone who has eagerly bathed with a freshly cut bar of soap and gotten some skin irritation has learned that lesson. (... Of course I've NEVER done that! ...) A few days should be enough to meet this goal for a properly made soap.

The second aspect of evaporation is the most visible process to us humans. Evaporation causes a bar of soap to lose weight, shrink somewhat in size, increase in hardness, and become less soluble in water. Y'all can discuss this one to death without my help.

The third aspect -- development of crystalline structure -- is a process that most people can't see and so they tend to ignore or discount this as not being important.

The crystal structure of soap is made of soap molecules that tend to join up to form large sheets. (The soap molecules form other shapes as well such as balls and hot dogs, but please bear with my simplification.) Ideally, these sheets stack one on top of the other, just like slices of bread in a triple-decker sandwich.

In this ideal structure, the spaces between the sheets are filled with a alkaline liquid made of water, glycerin, dissolved soap, etc. This liquid is much like the filling in my sandwich. Some of the alkalinity of this liquid is a natural consequence of lye soap being lye soap, and some may be bits of excess lye that will be saponified and neutralized during cure as mentioned above.

When a handcrafted soap is newly made, the soap structure is disorganized (small wonder after being wildly beaten up by a stick blender!) and there is a lot of free liquid floating around. This disorganized situation is a bit like a dry sponge lying in a puddle of water. You want the sponge to soak up the water to clean up the mess, but the process isn't instantaneous -- the sponge first has to get damp and then it can absorb the water. Same with soap -- the alkaline liquid needs to be "soaked up" and trapped within the soap structure, but it takes time to create an organized structure that can soak up the liquid.

A crystal structure of sorts is developed in commercial soaps by drying soap flakes under vacuum, extruding the flakes through dies, forcing the soap into a mold under pressure, and sometimes by (French) milling. Commercial soaps are also sometimes also given a certain amount of time to quietly cure on their own.

Since we don't normally have the ability to artificially dry, extrude, mold, or mill our handcrafted soaps, our soaps can only be cured by the application of time.

So on to liquid soap --

As Susie pointed out, liquid soap doesn't have a crystal structure, and it doesn't need to dry. A "cure" time for liquid soap is mainly to allow time for the last bits of saponification to finish up and allow any particles that might be in the soap to sequester (settle out).

I have to disagree about a liquid soap not being harsh -- it all depends on the fats used to make the soap and the amount of dilution to get the final product. Since liquid soap can be made of any fats, including coconut oil, it can be as stripping and harsh as any bar soap -- perhaps more so since the superfat in liquid soap is usually low. Dilution can modify harshness -- more water in the diluted soap will reduce the cleansing action.

 

[skayc1]

Curing is allowing excess water to evaporate from the soap. Which you would not want to do to your. Liquid Soap.

 

[Susie]

Curing is far more than allowing water to evaporate from bar soap. The qualities of the soap and lather are so different as to not even resemble what it was like at even the end of the first week.

When I first started researching making liquid soap, the conventional wisdom was to allow soap paste to cure a couple of weeks before dilution. I did try this. Then I tried one week. Then I tried not allowing any cure time whatsoever. There was just no difference in the quality of the soap, whatsoever. This was completely opposite of my experience with bar soaps.
My experience on curing bar soap was that the standard 4-6 weeks was not enough. I much preferred 8 weeks minimum.

 

[JayJay]

Thank you Susie, Dorymae, TEG, DeeAnna, and Skayc1.

It takes a village to explain to a newby!

Dorymae-- About the soft oils-- This is exactly ones my reasons for wanting to make liquid soap. Winter is coming so I want to try some OO heavy soaps in hopes that they will be kinder on my skin. My other reason is the lack of cure time!

Susie-- Thanks for explaining and trying to help me to find the answer. I could tell that your recollection of the DeeAnna answer was on the tip of your brain. That happens to me a lot. LOL. I appreciate your willingness to help!

TEG--I am sure that I will have TONS of questions for you in my soaping future when I am ready to try shave soaps. I am trying to tag and store these nuggets of information that you share as I see them so that I won't have to ask more questions in the future.

DeeAnna -- thanks for breaking down the elements of curing in that way because I didn't really get it before. I have started reading Kevin Dunn's Scientific Soapmaking. I am doing one initial read before I try doing the problems and experiments. One of the things that I have read so far is that cure time is finished when the bar stops losing weight. I knew, from this forum, that there are other factors involved, but I didn't understand the mechanisms involved in those factors. So thanks for helping me to put more pieces together. Hopefully as time progresses, I will have enough pieces of the puzzle to help me know what I am doing!

....soooooo... I have more questions. Does most of the crystalline structure form before the bar is done losing weight? Are there some recipes that need more time for the crystalline structure to form (due to the fatty acid structure)? I am thinking about Castile bars. Although I suppose long cure time could be needed to evaporate more water (to counteract the slime issue). Does the structure form more easily when there is more water in the bar? (before evaporation)

Sorry for the barrage of questions. I'm not even sure how to ask them. Thank you for your patience with me!

Maybe some of my questions will be answered as I read more of this book. But as it stands now, I am only processing a small percentage of what I have read so far. Science was not my strong suit in school. I am determined to learn, however!

 

[Susie]

JayJay, some of us are "why" learners. I am one, also. I can assure you that I had plenty of questions, and still do. I learn something new every single day on here.

You may think you are only finding out answers for yourself, but there are even newer people reading this that are also "why" learners that you are helping. Just keep on asking, and we will keep on trying to help.

 

[JayJay]

That makes me feel much better. Thanks Susie.

You're right. I am a "why" learner. I was the 7 year old kid in school who always asked the additional questions that had the other students falling asleep and the teacher wanting to stuff chalkboard eraser into my mouth in order to shut me up.

..."but WHY does gravity pull things to the center of the Earth? ...WHY does the Earth spin on an axis?"... "What do you mean 'it just does'?" :?

 

[DeeAnna]

"... (1) Does most of the crystalline structure form before the bar is done losing weight? (2) Are there some recipes that need more time for the crystalline structure to form (due to the fatty acid structure)? (3) I am thinking about Castile bars. Although I suppose long cure time could be needed to evaporate more water (to counteract the slime issue). (4) Does the structure form more easily when there is more water in the bar? (before evaporation)..."

You're gonna have to accept some "I don't knows" here, JayJay. I'm not a soap scientist -- I'm just an avid learner too.

1. I don't know. I suspect a lot of crystallization happens while the soap is drying down -- the first 4-8 weeks -- but I also believe there can be more gradual structural changes that go on long after the initial dry-down period. I've had a few soaps that just don't lather well at 4 weeks, for example, but lather considerably better at a year.

2.Soaps made with fatty acids that are short and straight (short-chain saturated FAs) are more likely to get better organized and get the job done faster than longer straight FAs (longer chain saturated FAs) and even faster compared with FAs that are bent (monounsaturated, polyunsaturated FAs). Another issue is that most soaps aren't pure soap made with just one FA, so that complicates matters. Most research is done on a pure (one FA) soap, and sometimes that research data doesn't always generalize to a more typical soap made with a mixture of FAs.

ETA: Don't ask about additives like the pumpkin puree -- questions about that and carrot puree both come up regularly about this time each year. Additives like food purees can really interfere with crystallization!

3. As far as the notorious Castile slime thing -- that's another can o' worms than just the issue of curing the soap. A soap with a high oleic content want to become a thick semi-solid gel (aka slime) over an unusually wide range of water content, unlike soap with a moderate to low oleic acid content. See http://www.soapmakingforum.com/showthread.php?t=56101 especially Post 32.

4. Do you mean like wrap the soap up in plastic wrap and let it "crystallize" without allowing evaporation? If so, I don't know. I suspect the two processes -- crystallization and evaporation -- may work better if both happen at the same time. So there's a bit of a synergistic thing going on. But that's just my opinion.

"...I was the 7 year old kid in school who always asked the additional questions...."

And I'm much the same kind of inquisitive person, although I'm far, far too much of an introvert to ask many "why" questions of others ... I just dig in and figure it out on my own.

 

[JayJay]

Thank you DeeAnna. You answered my questions very well.

I appreciate your help!

I think I have sufficient understanding to stop bugging people for the moment.

Have a wonderful week everyone! Happy soaping.


... Looking forward to the day when I can start answering some questions instead of asking all the time.

 

[FreeBird]

I have used the discounted water method for most of my 15 years of soapmaking. I learned in the very beginning, from a group of very experienced soapers which included more than one chemist, that allowing the gel as a part of the complete saponafication process is a major key in its success. There are also some combinations of oils that are not meant for this process.:wink: There also seems to be a misunderstanding that discounting water means there is no curing time necessary. This is NOT true. But what is true is that there is a shorter curing time required. I am not trying to convince you it is a better soaping process. We all have our preferred ways that work. I am just asking not to disregard it as a legitimate process.

Birdie

 

[JayJay]

I have used the discounted water method for most of my 15 years of soapmaking. I learned in the very beginning, from a group of very experienced soapers which included more than one chemist, that allowing the gel as a part of the complete saponafication process is a major key in its success. There are also some combinations of oils that are not meant for this process.:wink: There also seems to be a misunderstanding that discounting water means there is no curing time necessary. This is NOT true. But what is true is that there is a shorter curing time required. I am not trying to convince you it is a better soaping process. We all have our preferred ways that work. I am just asking not to disregard it as a legitimate process.

Birdie

What do you mean when you say "the discounted water method" ? How much do you discount and which combinations of oil don't work with it?

 

[FreeBird]

JayJay, DWCP is a method of cold process that considers the strength of the lye solution (lye vs. water). I highly advise that you have some experience in basic soapmaking and be comfortable with it. Marina at AquaSapone and Roxanne at River City Soap both have excellent presentations of the method. Marina, though, is considered the "mother" of the method. She still has a yahoo group for it.
As far as oil combinations that do not work I can not give you an exhausted list. I have a few recipes that I frequently make that have never worked with DWCP. One has both avocado and castor in it though not a huge amount of either. I do not have a clue why. So really it is, individually, trial and error.

 

[The Efficacious Gentleman]

Ah, okay - so it's actually just using the "lye solution" option on the calcs (which most non-beginners do) and using a fairly strong solution?

 

[galaxyMLP]

There is conflicting information on the first link you provided. First he/she says

"When a recipe is correctly formulated and the soap mixture is properly mixed, insulated and able to completely saponify within the first 24-48 hours, discounting water speeds up the curing time. Fully saponified soap only needs curing in order to dry out, and using less water from the start allows for soaps that are ready to wrap or use within a relatively short time - sometimes as little as one or two weeks only."

Then in the warning section its (click the link at the end of the first explanation, then click the "important warnings link on the next page)
"Water discounting does not "magically" eliminate curing times. Good soap needs to cure for at least 4 weeks, no matter which method has been used to make it - and if you cure discounted water soap even longer, you will always find your soap only gets better with age."

I also think that (IMHO) the "water discount" (really its the lye concentration method) it better for a beginner in a lot of ways. A lot of problems from using too much water can arise and its been proven time again. Yes, with known acc. FOs and EOs you want more water but otherwise its better all around to use concentration.

 

[DeeAnna]

This idea of using a "water discount" vs "full water" is nothing new here on SMF, although I frankly think this terminology is totally unnecessary. We don't need to be making the chemistry any more mysterious than it already is!

The odd lingo of "water discount" and "full water" and "water as % of oils" is more confusing and misleading than it is helpful. Speaking as a chemistry geek, I honestly would never use this terminology, except these concepts seem to be ingrained in the handcrafted soaping culture. And I definitely don't think "discounted water" soaping has to be some mysterious rite reserved only for experienced soapers.

***

Beginners usually use the default "38% water as % of oils" (which means the lye solution concentration can range anywhere from 26% to 33% depending on the recipe). They then run into problems with too-fast trace, too-slow trace, emulsion failures, "lye pockets", and even full separation in the mold, and they don't understand why. And rightfully so, since what they've been taught is "right" is really not. If they find us here at SMF, we get them straightened out.

There are several reasons why some recipes work better with a higher lye concentration and others work better with lower lye concentrations.

A big reason is the relative proportion in a recipe of short-chain saturated fatty acids vs. the unsaturated fatty acids and longer chain saturated fatty acids (FAs).

Recipes higher in lauric and myristic acids (short chain FAs) tend to reach trace faster. If you want to slow the saponification reaction for a CP (cold process) recipe that is high in these FAs, then use more water -- say 28% to 30% lye solution.

Recipes that are low in these short chain FAs tend to reach trace more slowly. To speed up the saponification reaction for this type of recipe, use less water -- say a lye concentration from 33% up to 40%. For a castile (100% olive oil soap) you might go as high as 50% lye concentration.

And that leads me to the trouble that can come from using the "water as % of oils" setting. By calculating the water content based on the oil weight, recipes that have a higher % of short chain FAs will call for less water. And vice versa for recipes that have a higher % of longer chain saturated FAs and polyunsaturated FAs. This is exactly the opposite of what works best! Better to use lye solution concentration (or water:lye ratio) to get recipes that behave more predictably and consistently.

Other reasons why some recipes work better with a higher lye concentration than others are the presence of sugars, alcohol, eugenol (found in clove and other spices), floral fragrances, and other ingredients that tend to accelerate the rate of saponification. Temperature can also increase the rate of saponification -- hotter tends to raise the rate, cooler tends to slow the rate. The degree of mixing is important too -- enthusiastic stick blending or whisking raises the rate, hand stirring with a spoon or spatula tends to slow things down. I'm sure there are others, but these are the biggies.

 

[JayJay]

Thanks Dee. That was very informative. I was curious about the term "discounted water method" because I thought is was a prescriptive technique of discounting water.

I never played with the water discount button because I was warned that newbies should not try it. I use 34% lye solution on Castile (which apparently is still a liberal amount of water). I soap pretty warm (120 degrees) , so my Castile traced really quickly at this level, but I looking to start soaping at lower temperatures because I have had some overheating lately with various recipes. Now that you have explained the short and long chain distinction, I can make more educated choices about temperature.

Thanks everyone for the information!

 

[sonoransoapnmadness]

I'm late to the party here but I wanted to go back to the idea of "mildness" from an extended cure. I'm in an FB group where someone had to beat her chest that she's a chemist and knows that soap can't be made milder after saponification is complete. To me, that ignored what else might be happening to the oils themselves. So, now my question is, does the crystalization of the soap make for a "milder" bar, and if so, how is that mildness judged or calculated? Thanks!