This post has been updated. It was originally published on 8/18/2018.
One of the most common problems in all of chemistry and materials science: stain removal. From a scientific perspective, oil on the garage floor, red wine on a white rug, and chewing gum on your pants seat all present different cleaning challenges (at least until self-cleaning clothes hit the market). Let’s look at how stains work, and how we can use chemistry to remove them.
Ground rules
A stain depends on two factors: the contaminant and how it interacts with the material it hits. Most household stains are surface ones, where the contaminating substance flows into the gaps of a material, such as the fibers of a cloth or the pores in a concrete floor, and become trapped there.
Another type of stain is due to a molecular reaction. For example, those notorious yellow marks appear on the armpits of your white shirt because your sweat reacts with the aluminum chloride in your antiperspirant. (If you’re actually sweating yellow liquid, you have more than stains to worry about.) In other cases, a dye can form covalent bonds with a material to create visible marks, as when you tie-dye clothing. Fortunately, those stains are extremely rare, and usually deliberate.
In all cases, your first step should be to pretreat the stain as quickly as possible. This means wiping up any excess spillage and then putting down something that will start to draw out the stain. For example, you could dab—never rub—at an untreated stain with a paper towel or other absorbent item, allowing capillary action to draw up the liquid. Then dampen the mark with cold water, which generally helps keep stains from setting.
At this point, once you’ve pretreated the stain, you can apply a stain remover (and, depending on the substance, even rub the mark with it). But you need to choose your cleanser wisely.
Cleaning toolkit
After you’ve pretreated the stain, you need to look at the specific substance you’re dealing with. That’s because different types of contaminants respond to different cleansers. Here are a few you should know.
The most common stain removers are surfactants. Examples include dish soap, laundry detergent, and many industrial suds. Surfactant molecules have two polar ends, one hydrophilic (or water-friendly) and one hydrophobic. These cleaners work a bit like velcro: One end attaches to a water molecule, one end attaches to a molecule of the stain, and off they swirl. However, the same reaction allows surfactants to remove dyes as well as stains from your clothes, so read the label before using this type of stain remover.
[Related: Master odor removal with a little help from science]
Another approach is to use oxidizers. These substances don’t remove the staining substance itself; instead, they blast the part of a molecule responsible for color, called a chromophore, with oxygen molecules. Strong oxidizers knock chromophores out completely, which is one of the reasons why you don’t put bleach in your dark loads of laundry. But gentler oxidizers like sodium percarbonate (better known as OxiClean) or weakly-concentrated hydrogen peroxide can bleach some surface stains while leaving intact deeper-seated chromophores like dyes. That said, be careful with organic fibers like wool or silk, as some oxidizers can weaken or even dissolve them. Test weaker oxidizers on these fabrics before you deploy them on a large scale.
Also popular, especially for organic stains, are enzymatic cleaners. These use sets of enzymes to break down the molecules of the staining substance. Lipases, for example, break down lipids like olive oil. Many of these are more environmentally friendly than their harsher brethren. But these cleaners can vary widely in concentration and the type of enzymes used. In some, the labels won’t disclose which enzymes are in the mix; instead, they list an “enzymatic blend” as an ingredient. Unlike surfactants, you might have a harder time finding these stain removers in a pinch, such as when you’re traveling or away from home. To purchase this type of cleaner, check the label of a commercial stain remover for terms such as “enzymatic action,” and look closely at which types of stains it promises to treat.
Finally, for the toughest of the tough stains, you’ll need solvents like rubbing alcohol and acids like white vinegar. These harsher cleansers let you dissolve, grind off, or corrode away the tenacious color. When you do use vinegar, you’ll want to dilute its concentration with water, such as adding one part vinegar to two parts water.
So now that you have the tools, let’s get rid of those stains!
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How to remove red wine stains
You may not think of your red wine as a dye, but it’s full of anthocyanins, which are common natural dyes for textiles. Even worse, when you find them in wine, they’re dissolved in alcohol and water, making your glass’s contents the perfect cocktail to ruin any cloth they touch.
When possible, use a surfactant to remove the stain on the surface. That may not be enough, so also apply an oxidizer to knock out the chromophores. For example, you might start with dish soap and follow it up with hydrogen peroxide.
How to remove honey and syrup stains
Syrups like these mostly consist of sugars, which love water. A little H<sub>2</sub>O should pull them out. In most cases, you can just scrape off the goo and then dab at the stain with cold water. If a mark remains, try a light surfactant as well.
How to remove coffee stains
Just as coffee left to cool will change in taste and consistency, so will a coffee stain. If you get a drop of coffee on your shirt, then blot it up, turn the cloth around, and run cold water through the back of the stain. This turns your clothes into a sort of reverse filter, pulling the coffee out of the material.
If the stain has dried, at this point, all you’re really dealing with is the chromophores. So do what the coffee professionals do: Grab some sodium percarbonate. At high concentrations, this substance cleans out industrial-grade coffee machines, so a solution of ¼ cup sodium percarbonate to 2 cups water should easily destroy your stain.
How to remove chocolate stains
Chocolate is a one-two punch. It’s both oily, so you’ll need a surfactant, and organic, since the sweet treat includes vegetable oil (such as cocoa butter) and various cocoa solids.
That means for best results, start with soap and then follow up with an enzymatic cleaner. Use plenty of cold water. Once you’ve pretreated a chocolate stain, this is one of the rare occasions where you should rub the mark to loosen it.
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How to remove tomato sauce and ketchup stains
With tomato stains, time is of the essence. Ripe tomatoes get their red color from tannins, which are also used as dyes. They have enough sticking power to stain plastic. So the second the sauce hits the linen, rug, or other material, apply cold water. If you didn’t notice that stray ketchup squirt right away, older stains can still benefit from treatment with warm water and an enzymatic cleaner.
How to remove stain sweats
Fortunately for your laundry bill, you can tackle sweat stains with multiple approaches. We recommend tailoring your approach with the item itself. Dyed delicates, for example, should respond well to surfactants. But with light-colored, sturdy items, save yourself some energy: Make a paste of baking soda and warm water and rub it into the stain. Let it dry for up to two hours and then wash it out.
By the way, the latter technique will also work with any mineral stains (including depleted uranium!).
How to removes blood stains and other biological materials
Anybody who lives with a baby animal (including baby humans) quickly becomes familiar with the many fluids and semi-solids that escape our bodies—and the seemingly-permanent marks they leave on everything. The good news is, if the stain is fresh, water will likely erase most of it, once you clear away the offending matter. Any leftover traces should quickly yield to enzymes.
You may want to use the enzymes a few times, and then use a solvent like rubbing alcohol to clean whatever’s left. If you’re concerned about applying a solvent to a delicate item, use a weaker solution available at drugstores, or dilute your rubbing alcohol with one part water to one part alcohol.
How to remove grass stains
Grass is a bit tougher than other biological stains, because, much like red wine, it’s a fairly effective dye—especially on light-colored clothing. Start with an enzymatic cleaner, preferably one specifically designed to tackle these kinds of stains. If there’s some stain left over, or enzymatic cleaners don’t meet the bill, try an acid, such as diluted vinegar, to pull the stain out of the fabric.
However, before you apply any vinegar to dyed cloth, put a small amount on a clean area of the garment that people won’t see, wait for a few minutes, and then wash it out. This will tell you whether the vinegar reacts with the dyed clothing. If it damages the material, then you may want to consult with a dry cleaner instead of tackling the stain on your own.
How to remove grease and oil stains
Amateur chefs and gearheads alike end up smearing grease on their clothes. To capture this hydrophobic substance, you’ll need a surfactant like dish soap. Apply a small dab to the area, wet it, gently rub it in, and wash it out.
[Related: This is the one power cleaner you need for your kitchen]
For really tough grease stains, look for industrial soaps. They use the same action, but have stronger pull. However, for delicate garments that aren’t colorfast—that is, where the dye hasn’t fully permeated the fibers—industrial soap may strip dye as well as grease out of your clothing.
How to remove ink stains
Since ink is supposed to be permanent, you need to resort to solvents for this type of stain. (This is why your parents run for the hairspray; it used to be loaded with alcohol, although you won’t find that solvent in most modern formulas.) To lift it out, you need to bust out a rag and some alcohol. Soak a sturdy cloth rag that you won’t mind ruining in the solvent; as the ink dissolves, it’ll transfer to the rag. After you’ve dabbed the stain a few times, try applying an oxidizer to take the color out of what’s left.
How to remove mud stains
Mud will require everything in your toolkit, because dirt contains a mix of everything. First, dry out the mud completely and brush off as much as you can. Then hit it with a surfactant and some cold water. After that, if the fabric can take vinegar, apply some of the acid to eat away at other parts of the mud. Anything left will succumb to enzymes. We also recommend using psychology: Make whoever is responsible for the mud stains do all this; it’s a superb way to stop those stains before they start.
These are just the most common stains, of course, but what’s most important is the toolkit. Once you understand both how the stain works, and how different cleaners clear it away, the battle against contaminants is (almost) won.