In order to understand how to get bath bombs to float we have to first understand floating itself. So, what makes something float? Basically the object you are placing in water needs to be less dense than the water it has been placed in. There are a lot of indepth videos on youtube covering buoyancy if you are interested:
Buoyancy & Density,
What Makes Something Float or Sink?,
Why Do Things Float?.
So how do we achieve this? There’s a simple way and a more complicated way but I won’t guarantee these will work for all of you because there are simply too many factors to take into account. I’m a firm believer that it’s not simply just a case of packing lighter and is actually combination of ingredients and/or packing. Why do I believe this? Simple. Because I pack my bath bombs very hard and the only time I have ever had an issue with sinking is when I’ve been dealing with the most basic of recipes. The experimentation and observations I’ve made have led me to believe it’s a combination of factors.
THE PACKING METHOD
Pack lighter. Yep. That’s it. Pack your bath bombs as light as you can without compromising on their structural integrity. Why does this work for some people? Well, because when you pack lighter you end up with more air inside your actual bath bomb.
It’s pretty simple but it won’t work for everyone. Some people find they have to pack so light that their bath bombs aren’t strong enough and noone wants a fragile crumbly bath bomb. So let’s take a look at the more complicated explanation:
THE COMPLICATED METHOD
So for this we look at density. There are different types of density:
Bulk Density – is the density of something like powders, soils, granules.
Poured Density – This would be the density of the powder if you just poured it.
Tapped Density – When the powder has been tapped or compacted in a specific way.
Now obviously the Poured Density of a material is going to be a lot lower than the Tapped Density and the Bulk Density is going to be a lot lower than the Particle Density, which I will explain below.
Particle Density is the density of the actual particles of the material, so it’s the true density of a material. This is higher than the bulk density of a material.
Another couple of things to note is that when you wet a material the bulk density changes once again. There’s also porosity of materials to take into account but I’m not even going to touch on that because my brain might just explode.
Now the problem with bath bombs is that we start off dealing with materials like bicarb and citric acid that have both a particle density and bulk density, which changes depending on whether it’s freely poured, tapped, compressed or wet.
Considering the fact we pour our ingredients into a bowl, then we wet them and compress them to varying degrees it’s going to be incredibly difficult to know exactly which of these densities to refer to when trying to mess with our recipes. If we use a liquid to bind our bath bombs that sets off the reaction between the bicarb and the citric, carbon dioxide is released, then we are probably again messing with the density of the end resulting bath bomb.
Taking this into account, I’m going to take a quick look at the individual ingredients you might find in a bath bomb and what their true and bulk density is. (These figures may vary slightly depending on where you look and where you get your supplies) I’m going to get my figures from various places using google and I will only put the true and bulk densities where I can find them easily.
Sodium Bicarbonate – 2.2 g/cm³ ~ 0.80 g/cm³
Citric Acid – 1.66 g/cm³ ~ 0.769 g/cm³
Kaolin Clay – 2.6 g/cm³ ~ 0.801 g/cm³
Cream of Tartar – 1.05 g/cm³ ~ Not sure. Less than 1.05
Cornstarch – tightly packed 0.63 g/cm³ ~ loosely packed 0.54 g/cm³
Isopropyl Alcohol – 0.785 g/cm³
Cocoa Butter Liquid – 0.92 g/cm³
Most Oils – 0.92 g/cm³
SLSa – 0.971 g/cm³ (I was only able to find one reference to the density of slsa)
Density of Warm Water – 0.995 g/cm³
See how greatly the True and Bulk densities for some of those ingredients varies? Which one do we refer to? Don’t ask me, we all pack differently and we all use different binders so I can’t really say.
But we do know that the basic ingredients that make up a bath bomb – Bicarb and Citric – are much denser than water. This is probably why when I make a basic bath bomb I find it sinks unless I’ve allowed it to dry out for a week or so. Once some of that water binder evaporates the bath bomb becomes much more porous and ends up floating. It also ends up crumbly because there was nothing other than the water holding it together
So what do we do then? Well, we want our bath bomb to be less dense than water so we incorporate more of those ingredients that we know are have the lowest density. They may not be less dense than water by themselves, but that’s okay, we are going to assume that some air is going to make it into our bath bombs regardless of how we pack and if we incorporate those ingredients and we are still getting sinkers, chances are we can get floaters by packing only a tiny bit lighter. Basically use as much of the lower density ingredients as you can without totally ruining your fizz.
This is probably why a lot of people think cornstarch makes a bath bomb float. It’s not technically the cornstarch, it’s just that cornstarch isn’t very dense. It’s much less dense than water. So if someone has a recipe that is sinking and they remove a little bicarb and replace it with cornstarch and it floats they are going to think that cornstarch is the answer to the floating dilemma, but their recipe was probably very close to being a floater without the cornstarch. Someone else might add cornstarch to their recipe and find it does nothing at all. Hell, I don’t even use cornstarch in mine and they float fine so it’s not any particular ingredient that’s going to do it unless you happen to be almost at the point of buoyancy already and just need that tiny push from an ingredient that’s much less dense than water.
I have used kaolin, slsa, cream of tartar and various oils in my bath bombs with no buoyancy issues. I use a 2:1 bicarb:citric ratio like most people and I grind my citric acid. If you are having loads of difficulty you could try lowering the bicarb and upping the citric then use some of those other fillers to slow down the fizz a little.
Another important factor is to make sure your bath bombs are fully dry before testing them. You may find a bath bomb from a new batch sinks but then test it a day or two later and they all float.
Anyhoo, those are just my thoughts on bath bomb buoyancy. As I said, I can’t guarantee any of this will give you foolproof floating bath bombs but hopefully it goes some way to helping you create them.