# Bath Bombs in Depth



## StevenRS11 (Apr 17, 2015)

First, a little bit of background. My wife discovered Lush bath bombs a while ago and has been totally obsessed with them since. Some of that may have rubbed off on me too, heh. The thing is, they aren't cheap, but more importantly, I like to make things.

     So I learned how to make bath bombs! I figured it would be a good thing to share my experience with anyone else trying the same thing. I come from a chemistry and engineering background with no prior experience in making cosmetic stuff, so my perspective might be a bit different, too.

I'll start with the basics first, and then follow up with more in depth explanations of each part. 

*Basics:*

     A bath bomb, at its most basic, is a mixture of both a solid, powdered acid and base. They are kept from reacting with each other because they lack a common solvent, aka water. Generally, citric acid is used as, well, the acid, and baking soda as the base. Without going into the chemistry of it right now, you need about 3 parts of citric acid to every 4 parts of baking soda by *weight*.  


    These two components and various other additions like oils, colors, scents, moisturizers, and surfactants are bound together through various means into a solid shape. Honestly, I am no expert on perfumery (maybe someone else could post which scents they think go well in bath bombs) but I do know that it’s easy to add too much. As for oils, I have found coconut oil works very, very well for several reasons (coming soon!). 


    If you want bubbles, I highly recommend Sodium Lauryl Sulfoacetate, or SLSA. It’s a much less aggressive surfactant than SLS, and honestly, I think it makes superior bubbles. My wife has pretty sensitive skin, and she said that she couldn’t even tell (besides the mountains of fluffy bubbles) that I had used a surfactant. 


    You should also add some ‘filler’ material, like Epsom salts, corn starch or even your favorite bath salts. (Or all of them!) Your basic bath bomb should have around half the amount of filler as it has citric acid + baking soda. Adding more filler makes it slower, and less filler makes it faster. No filler, well, think less “relaxing bathing experience with bubbles” and more “foamy fireworks in the tub”.


Adding a very small amount of water to the dry mix lets it clump up, and then all you need to do is press it firmly into a mold and let it dry. 

     The resultant solid shape gets dropped into water, the reaction starts, and then we all stand back and watch the magic like little kids seeing our first vinegar-baking soda volcano. Fiiiiiizzzzzzyesssss!


  Coming up, *Chemistry*!


  Or, “How to Train your Bath Bomb” if your memories of HS chemistry are full of demonic moles with eyes made of swirling numbers and a fur coat of arcane symbols.


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## galaxyMLP (Apr 17, 2015)

I also have a chemistry background (chemistry bachelors, work in a chemical plant). There are quite a few of us with the chem know how here! Its quite useful when making soaps/cosmetics. I look forward to the chem-y explanation!


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## shunt2011 (Apr 17, 2015)

There are also many posts on Bath Bombs, Bubble Bars and Bubble Scoops that are based off of Lush products.  I make Bubble Scoops/Bars.  People love them especially if you use some Lush Dupes fragrances.  We folks love to experiment in many different ways with many different ingredients.  You will find a lot of very intelligent, knowledgeable folks here.  Also very helpful.  Welcome!!  Look forward to your thoughts on  things as well.


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## StevenRS11 (Apr 17, 2015)

galaxyMLP said:


> I also have a chemistry background (chemistry bachelors, work in a chemical plant). There are quite a few of us with the chem know how here! Its quite useful when making soaps/cosmetics. I look forward to the chem-y explanation!



Oh man, now the pressure is on, heh. Whispers to self *don't mess up don't mess up*.

Its funny, really, when I was younger I used to make fireworks, and many of the tricks used in that apply here too. Things about controlling the reaction rates of various layers, pyrotechnic stars, all the good stuff. I am even using some of my old equipment (thoroughly cleaned, of course) again!

So far I've gotten little 'bath bomb boats' to swim around pretty quickly, and another that spins around like a pinwheel. I am about done with the chemistry part, too, and Ill post it pretty soon, along with some pictures.


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## happyshopper (Apr 18, 2015)

I have been researching bath bombs as I want something my son and I can make together, he loves bath bombs so will enjoy using them. Looking forward to your chemistry explanation as I would like to be able to explain to my son the chemistry behind it so he can learn as well as make something awesome 

I was planning on keeping it simple (and cheap) but you say its not recommended to use just citric acid and baking soda? Do I have to to use a filler as well?

A good tip I did read was to use those play balls you find for kids as moulds, we have some of those which I will cut in half to use.


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## galaxyMLP (Apr 18, 2015)

No need to feel you can't mess up! No judgment here! Ive never made a bathbomb before and I really wanted to try my hand at it soon. So this will be a helpful post.


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## The Efficacious Gentleman (Apr 18, 2015)

No added pressure, but there is also a chap on the forum who makes pyrotechnics! You're in good company


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## snappyllama (Apr 18, 2015)

I'm waiting for the day when we get to witness the two hobbies combined to make a true bath "bomb".  That's right, a M60 (the firecracker name - if childhood memory serves me) encased in a citric acid/baking soda bath fizzy.  Light her up, throw her in the tub and get ready for a fragrance explosion.


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## StevenRS11 (Apr 18, 2015)

snappyllama said:


> I'm waiting for the day when we get to witness the two hobbies combined to make a true bath "bomb".  That's right, a M60 (the firecracker name - if childhood memory serves me) encased in a citric acid/baking soda bath fizzy.  Light her up, throw her in the tub and get ready for a fragrance explosion.



You know, I bet you could have a hard shell of dextrin-bound inert filler with a hollow core half full of loose citric acid + bicarbonate and make that work. Maybe have a cotton "fuse" that would let enough water in through the side to start the reaction but not let air out too fast. Once it built up pressure, well, :mrgreen:.



happyshopper said:


> I have been researching bath bombs as I want  something my son and I can make together, he loves bath bombs so will  enjoy using them. Looking forward to your chemistry explanation as I  would like to be able to explain to my son the chemistry behind it so he  can learn as well as make something awesome :smile:
> 
> I was planning on keeping it simple (and cheap) but you say its not  recommended to use just citric acid and baking soda? Do I have to to use  a filler as well?
> 
> A good tip I did read was to use those play balls you find for kids as  moulds, we have some of those which I will cut in half to use.



You don't have to use a filler at all, no. Personally I think they fizz a  little to fast without at least 25% filler, but its not necessary.  Also, the fillers are generally cheaper than citric acid, especially  Epsom salts.


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## StevenRS11 (Apr 18, 2015)

*The Chemistry Behind Bath Bombs*
                As I mentioned earlier, the primary reaction that makes bath bombs work is the acid-base reaction between citric acid and baking soda, or sodium bicarbonate. This produces a salt, (tri)sodium citrate, water, and carbon dioxide. It looks like this-
C6H8O7 + 3NaHCO3 --› 3CO2 + 3H2O + C6H5O7Na3
 
​                 On the left side, we have the reactants, and on the right, the products. According to this, we need one molecule of citric acid per three molecules of baking soda. To convert a ratio of molecules to a ratio of grams (cause it’s hard to count molecules) we need to know how much the molecules actually weigh, or their ‘molar mass’. A ‘mole’ is just a (exceedingly large) number, by the way. For a more detailed explanation of moles and such, Wikipedia has a good entry on them. Long story short, Citric acid weights 192 grams per mole, and baking soda weighs 84 grams per mole. Since we need 3 times as much baking soda as citric acid, we multiply 84 * 3, getting 252 grams.

             Now, 192/252 does not reduce to ¾, which is the ratio I gave earlier. The reason being is that our skin is slightly acidic with a pH a little over 5, so bathing in a basic solution isn’t the best thing. So, we add just a little extra citric acid to make sure we use up all of the baking soda.

These two chemicals form the basic fizzing components of bath bombs. In practice, it fizzes fast and makes bigger bubbles. The amount of filler you use can vary the rate at which you bomb actually fizzes, as well as the particle size of your various components. Before I get into that, though, there is a second compound that some bath bombs use as an acid, cream of tartar or Potassium Bitartrate (K2C4H4O6).  

The reaction between cream of tartar and baking soda is similar to the Citric Acid + Baking soda reaction as they both produce carbon dioxide bubbles, but the cream of tartar reaction is both slower and produces less CO2 by volume of reactants. Its equation looks like this-

 NaHCO3 + KHC4H4O6 → KNaC4H4O6 + H2O + CO2
 
​ You can see that there is only one CO2 molecule on the right produced, as opposed to 3 for the previous reaction. It also proceeds a lot slower because cream of tartar is far less soluble in water than citric acid and because the reaction is even less energetically favorable than the reaction between CA and Baking Soda. As for mass ratios, cream of tartar’s molar mass is 188 g/mole, close enough to citric acid’s 192 that we can just say its 1:1 by mass. The bubbles produced by this reaction tend to be very, very small, resembling more of a smooth white cream than actual foam. Bath bombs composed of this tend to dissolve slower, as well. 

There  is also a second base that some bath bombs use, called sodium carbonate. I  know solutions of it are more alkaline than bicarbonate, but I haven’t had enough time  to experiment with it to really tell you how it affects the bath bombs. It  *seems* to fizz slower and it apparently makes a great water softener,  but I would hold off using it in large amounts because it is quite basic  and is used as a cleaning product. It may be fine, but I'm not sure.  I’ll add this info when I figure it out, as well as I experiment with the various  other compounds used in commercial bath bombs. 

            You can vary the amount of citric acid vs cream of tartar in bath bombs to produce bubbles anywhere in-between the two extremes, along with different amounts of filler to change the speed it dissolves. You can get really creative and use different layers of mixes in the same bath bomb to make dynamically changing effects, releasing colors or other stuff at different times. You can even use very fast mixes to generate thrust, making bombs that swim around!  

            Before we talk about that, though, we need to figure out how to bind our loose powdered mix into a solid shape. In the next section I will talk about the various ways I have found to bind the powder together, and how particle size of the reactants affects the actual bomb.


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## galaxyMLP (Apr 18, 2015)

I feel like I'm reading a good how to book. I'm excited to hear more!


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## squeakycleanuk (Apr 19, 2015)

This is a really interesting thread, thanks Stephen  I'm a complete novice with bathbombs and my attempts so far are in definite need of improvement, so this is very helpful


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