Why Do Bath Bombs Sink or Float?
by Bold Apps
Posted on Jul 16, 2018
The idea for this post has been turning around in my head for some time, as it was one of the biggest challenges I had when developing my own recipes. To be fair, I am still tweaking and I am very close to getting exactly what I want for a floating bath bomb, but I had a very precise set of requirements and expectations I wanted my bath bombs to have.
With that, I’d like to share what I’ve learned along the way…
Before I go into the factors that contribute to a sinking or floating bath bombs, I would like to share some perspective that might ease your expectations and frustration:
Not all bath bombs are meant to float, depending on their purpose or intention!
Ask yourself, "What do I want my bath bomb to do?"
For example, if your bath bomb’s main purpose or intention is to give a luxurious experience to your user, it’s loaded with lots of butters, salts, and other yummy, soothing ingredients, and it doesn’t float: That is okay! If it’s beautiful to look at, dried hard, and does all the other things you intend for it to give to your user, then don’t worry if it doesn’t float. Please don’t continue to pull your hair out trying to make it be something other than its primary intention.
On the other hand, if your bath bomb’s main purpose or intention is to entertain with spinning, lots of colors, foam, glitter, and “bath art,” then yes, you DO want it to float.
This can be frustrating to grasp, especially when it seems that everyone on Pinterest, Youtube, and Facebook are only making bath bombs that put on a show.
Not that it doesn’t mean that you can’t have both purpose and entertainment in your bath bombs. If you manage to have both in a reliable product that you are happy with, then go for it! For me, I have found it easier and less frustrating to focus my energy on one or other purpose(s) for my recipes. I also feel this gives me more diversity for my products.
Lastly, another thing to consider is not everyone wants a foaming, sparkly bath bomb experience; they’re more interested in the soothing or luxury experience. I have found that younger users often want the “show” and the older users want the luxury or splurge.
Many people say the only way to get a bath bomb to float is how you mold/pack it – there is no recipe for a floating bath bomb – but that is not entirely true. Ingredients play a HUGE part in that mystery. In this post, we are going to talk to the type of ingredients used and their density.
This concept is pretty much common sense. If you make a bath bomb that contains a lot of heavy ingredients like butters and salts, it’s not going to float without some help. There are other binding ingredients that you can add to help bath bomb “floatage” and I will talk about that next.
This is where it might be a little heavy to understand, but once you do, a light bulb will go off in your head. First I need to give credit to Irene at Body Bonbon for being the first to put this information out there. I had just started thinking about how some ingredients weighed more than others and contributed to floating/sinking bath bombs when I stumbled upon her post “Bath Bomb Buoyancy – Thoughts”. Once I read her post and applied what I had already knew, I was able to tweak my recipes to get a floating bath bomb.
I knew many other people added binders like cornstarch, tapioca starch, and arrowroot powder to their recipes to help their bombs float, but I didn’t want to go down that path. This decision was two-fold: One, for some reason, adding those binders to my recipes even is the tiniest amount didn’t work for me or my end-product expectations. I didn’t like how they turned out, how they felt, or their reliability. Two, the stubborn streak in me knew that floating bath bombs could be made without all those binders and I wanted to figure out why.
Basically, the ingredients you use in your bath bomb have different densities during the creation process: particle, bulk, poured, wet, and tapped. If you think about it, we are basically taking several types of materials, mixing them together, wetting them, packing them into a mold, and then letting them dry. Each step in this process creates a different density. The trick is to understand the right combination to make a bath bomb float.
Here’s the various density definitions:
- Particle density (or true density) – This is the density of one particle of an ingredient. For example, one singular grain of citric acid.
- Bulk density – This is the density of a lot of particles in a mass. For example, a cup of citric acid.
- Poured density – This is the density of a bulk mass poured into a vessel/container. For example, sprinkling your bath bomb dough into a mold versus packing it in.
- Wet density – This is the density of a bulk mass when it contains moisture. For example, your bath bomb dough weighs more when it contains a liquid versus when it is dry.
- Tapped density – This is the density of a bulk mass when it is packed versus when it is poured. For example, packing your bath bomb dough into a mold versus sprinkling it.
So, with that in mind, here’s the particle (or true) and bulk densities of some of the typical bath bomb ingredients (again this info was borrowed from Body Bonbon):
- Sodium Bicarbonate – 2.2 g/cm3 and 0.80 g/cm3
- Citric Acid – 1.66 g/cm3 and 0.769 g/cm3
- Kaolin Clay – 2.6 g/cm3 and 0.801 g/cm3
- Cream of Tartar – 1.05 g/cm3 and 1.05 g/cm3
- Cornstarch – .08 g/cm3 - .10 g/cm3 and bulk average of 0.62 g/cm3 - 0.77 g/cm3 (tightly packed 0.63 g/cm3 and loosely packed 0.54 g/cm3)
- Isopropyl Alcohol – 0.785 g/cm3
- Coco Butter Liquid - .92 g/cm3
- Most Oils - .92 g/cm3
- SLSA – 0.971 g/cm3
- Water – 0.995 g/cm3"
So, what can we take away from this?
Obviously, some of these ingredients have a higher particle/true density than water, but at the same time have a lower bulk density than water. And although a single grain of citric acid may be denser than water, a group or a mass of those grains are less dense and weigh less due to the air pockets trapped in between those grains!
For example, if you were to weigh a cup of sodium bicarbonate and a cup of citric acid, you will find the cup of citric acid weighs less. This is because the sodium bicarbonate grains are smaller than the citric acid grains and; therefore, have less space in between each individual grain for air pockets.
Another thought: kosher salt and table salt. Kosher salt has bigger, individual grains compared to table salt, which means it has bigger air pockets in between each grain. A cup of kosher salt weighs less than a cup of table salt.
So, the key to making bath bombs that float when considering ingredients is to include as many ingredients that have a lesser bulk density than water. This is why cornstarch may help a bath bomb float and cream of tartar may make it sink. It’s not that cornstarch makes a bath bomb float, it’s just that it's bulk density is less dense than water. I found in my own recipes that decreasing the amount of cream of tartar and increasing other less dense ingredients helped me find a happy, floating spot.
Additionally, this is why it is so important to let your bath bombs fully dry after making them. Since oils and water have close-to or the same density as water, if your bath bomb hasn’t completely dried, then those spaces in between those particles haven’t had a chance to dry into air pockets. Those air pockets are a big key in “floatage,” which is what I will talk to next when discussing molding and drying time.
It is true that technique for molding/packing bath bomb dough contributes to “floatage” or “sinkage”. If you pack your dough into a mold as tight as you can and your bath bomb comes out and dries like a cannonball, then it probably won’t float!
Remember density! The denser your bath bomb materials are pressed together, the less air pockets there are in between their ingredient particles. You want to incorporate as much air into your dough and bath bombs as possible, so go gentle on molding. This can be done by lightly sprinkling your dough into each mold half and gently cupping the material with your hand to create a small mound and keep it in place. Do the same with the other mold half and then gently press them together.
- No packing
- No twisting the mold halves together
- No Pecs of Thunder…you shouldn’t have to press hard!
Lastly, think about purpose and intention. If your bath bomb is not intended to entertain or float, feel free to pack tighter. A tighter packed bomb will be sturdier and has the potential to give your user more “bang for their buck”.
The shape of your bath bomb plays another part in “floatage” due to water displacement. A round bath bomb will displace a smaller surface area in water compared to a flat bath bomb. The larger an object is, the more water it displaces. This also considers density, and takes into consideration the Archimedes principle of why things float or sink, but that is getting really deep. Water displacement principals along with density are why ships that weigh several tons are able to float and not sink.
Additionally, a round bath bomb has a smaller, compressed mass in which to support air pockets compared to a flat bath bomb. A flat bath bomb has more surface mass to distribute those air pockets. So, flat, boat-shaped bath bombs have a better chance at floating than round ones.
For some reason, there is the expectation that our bath bombs must dry rock-hard within 24-hours of being made and be able to float! I know there are some crafters out there who are able to do that, but the truth is, you will do yourself and your bath bombs a huge favor by letting them sit for a few days to dry.
Remember what I said in Part 2 about ingredient density? Oils, water, alcohol, etc. have close-to or the same density as water. When we make a bath bomb, we are trapping those liquids in between the grains/particles of our other ingredients. The bath bombs need time to dry those liquids in order to contribute to those wonderful air pockets. Air pockets gooood! Wet bath bombs baaaad! A wet bath bomb will weigh heavier than a dry one.
For example, as an experiment I made a bath bomb that weighed 7.2 oz. upon initial creation. At the end of 3-4 days drying time, the weight of that bomb decreased to 6.7 oz. That’s almost a half ounce less! I know that doesn’t sound like much, but for me and my recipes, I know that a couple tenths-of-an-ounce difference will make my bath bombs sink or float.
Another perspective: When bath bombs companies make their bath bombs, they are left completely in their mold for 24-hours to dry before being removed. After that, who knows how long it takes for that bomb to be packed, shipped, stocked, purchased, and eventually used in someone’s bathtub? I’m betting that a Bubbly Belle bath bomb at its earliest doesn’t get used for at least 10 days to 2 weeks after creation. Now THAT’S some drying time!
So do yourself and your bombs a favor by letting them sit to dry for a few days before using.
Well, that concludes my thoughts on bath bomb "floatage". I hope this has been helpful, and I couldn't have come to this point without the hard work and input from the talented bath and body community out there. Keep it up everyone!
(source: http://www.justtherightsize.net/2017/01/bath-bombs-why-they-sink-or-float-part-1.html, http://www.justtherightsize.net/2017/02/bath-bombs-why-do-they-sink-or-float.html, http://www.justtherightsize.net/2017/02/bath-bombs-why-do-they-sink-or-float_3.html)