Bath Bombs: Why Do They Sink or Float? (Part 2)

For Part 2 of my topic, “Bath Bombs: Why Do They Sink or Float?” we are going to talk about ingredients. So, pour yourself a cup of coffee and get comfortable because we are going to go deep.

As a reminder from Part 1 of this topic, there are 5 factors that contribute to whether a bath bomb sinks or floats:

1. Ingredients
2. Ingredient Density
3. Molding
4. Shape
5. Drying Time

Okay, here we go.

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.

Ingredients

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.

Ingredient Density

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 (Lush does it) and I wanted to figure out why.

Photo courtesy Lush

So, let’s jump right in, shall we?

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!

Photo courtesy Slate

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.

Photo courtesy SheKnows

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.

Stay tuned for Part 3 where I talk to how molding, shape, and drying time contribute to a a floating or sinking bath bomb!