Activated charcoal is an excellent cleanser. The extremely absorbent nature of the carbon alone is powerful, but when coupled with other ingredients like soaps, it becomes a potent cleaner. It's no wonder that it ends up in everything from toothpaste to soap.
There's just one problem: activated charcoal gets everywhere. The powdery black stuff is seemingly impossible to contain for very long. It gets on clothes, it works its way into every crack and crease in the skin, and it seems like it can stain practically anything.
So, can it stain your bathtub or shower? The answer is: it depends.
Different Tub Materials
The first major consideration you need to think about if you want to use charcoal soap is the composition of your tub.
Tubs generally come in three different materials. There are a lot of differences between them, and it should be fairly easy to tell them apart.
If the tub looks and feels like plastic, it's an acrylic tub. If it's harder and feels more solid, it's either enameled metal or porcelain. Use a magnet; if it sticks, it's enameled iron. If it doesn't, it's more likely porcelain.
Acrylic is generally the cheapest of the three options and isn't as durable as the others. It's lightweight and easy to move and install, however, and often can be more customized to the surrounding area. Porcelain and enamel tubs are generally much larger and heavier, which makes them more solid and resistant to damage, but more expensive.
Different materials can react differently to different chemicals. So where does activated charcoal come into play?
How Charcoal Might Stain Tubs
There are basically two ways in which activated charcoal can "stain" a tub. The first is an actual stain, and the second is sticking.
What many of us think is a stain is more commonly something called surface cling. When you use activated charcoal, some of that charcoal can stick to the surface of your sink or tub. This is easy to deal with. All you need to do is rinse off the charcoal itself. It's not actually stuck to or staining the surface of your tub, it's just held in place by surface tension or charge, nothing mechanical.
In the case of acrylic tubs, they are fairly resistant but not impervious to stains. What happens most often, though, is stuff that builds up on the surface. It could be dust and dirt, it could be mildew or mold, or it could be calcium, lime, or another mineral from your water building up. In areas with hard water or highly mineralized water, this happens faster.
When this mineral buildup or organic crud is exposed to activated charcoal, the charcoal sort of binds to it. That's what you want it to do, really; when you're using activated charcoal soap, the charcoal is there to bind to and absorb the grime and toxins you scrub off, to carry it away.
When charcoal binds to minerals that are stuck on the surface of the tub itself, they can be hard to remove and seem like it has stained your tub. It's actually pretty similar to the dye tablets dentists can use to look for plaque on teeth. The charcoal isn't sticking to the tub itself, it's sticking to stuff stuck on the tub.
To remove this, you'll need to do a little more work, since you need to remove the minerals or grime stuck to the tub. This might involve a little elbow grease, or a chemical like CLR or another cleaning product to dissolve the minerals. Either way, when you're done, there will be no more charcoal on your tub.
True staining is rare and tends to only happen to damaged tubs. Most of these "stains" wipe right off. We'll talk more about that momentarily.
Porcelain and enamel tubs are both very similar since the material used to coat the metal in an enamel tub is porcelain. In both cases, your tub is basically coated with a smooth, durable surface of ceramic porcelain. This ceramic is very resistant to staining and damage, which is why porcelain is such a popular material for these kinds of purposes.
You'll typically see the same two issues with porcelain as you do with acrylic. Either the charcoal is sticking to the surface but just needs to be rinsed away, or it is sticking to something else that has stuck to the surface of the porcelain and simply needs to be scrubbed away. Either way, it's unlikely that charcoal is going to actually stain your tub.
Cracks, Grout, and Other Considerations
We mentioned briefly above that there are some other considerations that can cause staining but aren't due to the material of the tub.
Abrasions. First up, we have abrasions. Abrasions, or scratches, leave grooves and marks on the surface of a material. Porcelain is very resistant to this kind of damage, which is why it's so durable over the decades. Acrylic is much more susceptible to this kind of damage.
When your tub's surface is abraded, charcoal and other buildup can stack up in those cracks. If the abrasion is new, charcoal can work its way deep into the scratch. It's not stuck there, but it can be very difficult to get out without further scrubbing, which runs the risk of further abrading the surface of the acrylic.
It's possible to abrade porcelain but it takes a lot more effort, and porcelain is more likely to chip and flake off. If that happens, it'll be pretty obvious what the problem is, and you'll have more to worry about than just a little bit of charcoal stuck around the edges.
Cracks. Where acrylic gets scratches, porcelain gets cracks. Often times, they can even be so small that you don't know they exist until something happens to change the colors or widen the cracks so they're visible.
This is where charcoal might "stain" porcelain. If your enameled iron tub or your porcelain tub has cracks in it, like from a heavy impact, shifting of the building putting pressure on it, or damage from installation, those cracks might be wide enough for the tiny bits of charcoal to work their way into them.
Normally, charcoal isn't even going to do this. Things like dyes are more likely to stain porcelain. Charcoal would need to be left on a crack or worked into the area for it to suffuse the crack enough to stain.
Unfortunately, if this happens, there's not much you can do. You'll either need to coat the tub with a fresh layer of porcelain – which is a tedious and occasionally dangerous process due to fumes – or you'll need to replace the tub.
Tile grout. The next problem you might encounter is that your tub isn't the only surface in a shower that can stain. If your backsplash or tub surround is made up of tile, it's likely that the tile is held in place with grout. Grout is basically like cement; it's meant to cure and be water-resistant, and it shouldn't stain, but it can discolor over time. It can also wear away, chip off, break, and otherwise have issues.
The main problem with grout is that, as a cement-like substance, it's more porous than other substances in your shower area. This means it can absorb something like charcoal more readily than other substances. A lot depends on the kind of grout, how old it is, and if it has had any sort of treatments or damage over the years since it was installed. Just be careful with tile and grout when you're using a charcoal soap, and rinse it quickly rather than letting charcoal-infused suds linger.
Age. Another concern is that all of these substances age. Grout and acrylic in particular can become more susceptible to damage and staining over the years. Porcelain is less likely to degrade over time, but it can still be damaged if it's struck or chipped. The older your tub is, the more likely you might run into issues with charcoal lingering when you don't want it to.
Charcoal abrasion. Probably the biggest problem with charcoal in a cleaning product is the same problem you have in toothpaste. Activated charcoal is, basically, tiny crystals of carbon. It's extremely abrasive. This makes it very good as a material for polishing surfaces, including teeth.
The problem is, charcoal doesn't know when to stop. One of the most common warnings on charcoal toothpaste is to discontinue using it if your teeth become sensitive, and not use it on sensitive teeth. The reason is that, while it's polishing your enamel, it's doing it by grinding away at that outer layer of enamel to reveal the non-stained enamel beneath.
This same thing can happen to the surface of your tub when you use charcoal soap. However, it's not as much of a problem as we make it sound. Charcoal is abrasive, yes, but not when it's just dripped onto a surface and rinsed away. You need to have some pressure and scrubbing involved for it to dig into a surface. When you use a soap with charcoal in it, the soap is going to be more likely to abrade your skin – for gentle exfoliation – than it is the surface of your tub.
Just keep this in mind if you use activated charcoal as part of a cleaning product. It's good for abrading away stuff that is stuck on the tub's surface, like mineralization. It's also good at sticking to that stuff so you can see where it is to remove it since white minerals on a white tub can be difficult to see.
How to Clean Up Charcoal Stains
If you're afraid that your use of activated charcoal is leaving stains on your tub, don't worry too much; you can clean it up fairly easily in most circumstances.
First, make sure you rinse and clean your tub thoroughly every time you use a charcoal soap. The same goes for your sink when you use charcoal toothpaste or a charcoal-based cosmetic product you wash off. Any time you use charcoal, you want to make sure to rinse it away when you're done. If you're in the shower, this is pretty easy obviously.
If any charcoal is stuck to your tub and you want to clean it up, the first thing to do is get it wet and try to wipe it away. Something as simple as a washcloth or a paper towel should be enough to get the most surface charcoal off of a surface like a tub. It's as likely as not that the "stain" you see is really just a bit of soap scum stuck to the surface of the tub. You should be able to wipe it away easily.
If you can't wipe away whatever the charcoal is stuck to, you might need to use another chemical to try to loosen it up. Something like Dawn or another dish soap can work well to dissolve grease and other kinds of organic grime like mildew. Something stronger like CLR works well to dissolve mineralization. As an added bonus, you'll have a much cleaner tub when all is said and done.
If you still have charcoal stains after all of this, you might need to look into refinishing your tub. It's possible that the problem is deep abrasions or cracks with stuff stuck in them, and that's not something you can scrub away. You'd either need to resurface the porcelain or, as likely as not, replace acrylic. Still, though, that's not the fault of the charcoal. Charcoal doesn't really stain the way we think of stains, at least not something like a tub. Cloth material is another story, but that's an analysis for another time.