Unsafe slime? How bad is borax, really?

Slime is a fun bit of chemistry that anyone can do – but how safe is it?

It’s August, which means it’s the school summer holidays in the UK and, as is traditional, it’s been pouring with rain. This has left many a cabin-fevered child searching for ways to amuse themselves.

Start hunting around the internet for things to do and it’s not long before the concept of the “kitchen science” experiment turns up. There are actually loads of these, and it’s even possible to do some of them without permanently damaging anyone’s eardrums, dusting every surface with cornflour and leaving a parent rocking in the corner muttering “why did I encourage this?” over and over to themselves.

Which brings me to slime – surely the go-to fun science experiment. What’s not to love about taking some of that white, runny PVA glue found in gallon bottles in school classrooms everywhere and magically turning it into glorious, gloopy slime? Add some food colouring and you can even have coloured slime! Add glitter and… well you get the idea.

Many YouTubers love this stuff. A quick search for “make your own slime” turns up pages and pages of videos, giving instructions as to how to do just that.

In fact, it seems that slime-making is currently a bit of a craze, with children all over the world making all kids of different types. There’s unicorn slime, rainbow slime, fluffy slime – you name it. Brilliant, you might think, a whole generation of youngsters interested in chemistry. What’s not to like about that?

Well, as a few news reports have recently pointed out, there might be a problem if children are handling lots of borax, or certain other chemicals.

Polyvinyl alcohol

Slime, you see, is a really nice example of polymerisation – the same process that goes on when plastics are made. PVA glue, the usual starting material, is a polymer itself. The letters PVA stand for polyvinyl alcohol (its systematic name is poly(1-hydroxyethylene)), but literally no one calls it that, not even A-level chemistry teachers forced, kicking and screaming, to follow IUPAC naming conventions).

PVA is a long chain of carbon atoms with alternating CH2 groups and alcohol, OH, groups. As anyone who’s ever handled it will know, it’s quite runny. Thick, yes, but still runny. Basically, it’s a liquid.

But if you mix it with borax, aka sodium tetraborate, some magic happens. And when I say magic, I mean chemistry. The chains of atoms become linked together (essentially via hydrogen bonds), and as a result the new substance is a lot more solid. But it’s not quite solid. At least, not in the sense of something that keeps its own shape. No, this is weird, peculiar, stuff that sits somewhere in between solid and liquid.

Borax joins the chains of PVA together.

There’s something tactilely pleasing about slime. Put it in your hands and it feels cool and slightly moist – your fingers slide over and through it with a sort of squeaky sensation. Leave it alone for a few minutes and it flows to take the shape of its container, forming a perfect, mirror finish on its surface. Tip the pot over, and it will gradually creep toward the edge.

It is safe to handle. Here are my hands, handling it (we made this at the March for Science in Bristol back in April). You will notice that my skin is not falling off.

It’s white unless you dye it. We went for red, which is pleasingly disturbing.

I did, though, wash my hands after I took that photo. And that’s because, while the PVA is pretty harmless (as you know if, like me, you spent your primary school days painting your hands with glue just so you could peel it off later) the borax isn’t. At least, not entirely.

Before I go any further, let’s be clear: lots of things aren’t “entirely” safe. Most of the cleaning products in the average kitchen and bathroom have warning levels of varying degrees of severity on them, and we don’t think too much about it. Even things that are designed to be in contact with skin, like hand soap and shampoo, usually have warnings about eye irritation and statements like “if irritation occurs, discontinue use”. Even water is deadly in the wrong context (don’t try inhaling too much of it, for example). So when I say not entirely safe, I don’t mean to suggest that panic needs to ensue if your child has so much as looked at a borax solution.

Borax has traditionally been used in several household products, although admittedly more in the US than in the UK. Most people know it as a laundry additive, where it softens water, brightens whites and inhibits the growth of the bacteria and fungi which can make clothes stinky.

It’s not considered a lethal compound, in the sense that you’d have to eat a large quantity – far more than anyone might reasonably consume by accident – before it became deadly, and you’d almost certainly throw up long before then. Borax can irritate the skin (but see note at the end), and inhalation of the dust is well known to irritate the lungs. This is more of a concern for people working with borax on an industrial scale day in and day out – but it could become an issue if, say, someone were making slime every single day using large quantities of borax (not recommended).

Then there’s another concern. If borax is exposed to hydrochloric acid, it forms boric acid. Long-term exposure to boric acid can cause kidney damage and fertility problems, both in men and women. It’s also potentially teratogenic, which means it could cause harm to an unborn child. Borax and boric acid are not the same thing but, of course, our stomachs contain hydrochloric acid. Therefore, if you swallow borax, you’re effectively exposed to boric acid.

Frequent exposure to borax might cause skin irritation (see note at end)

These risks are the reason borax was added to the Substance of Very High Concern (SVHC) candidate list on 16 December 2010, which is the first step in restricting use of the chemical within the European Union. As far as I can establish, it’s still a “candidate”, but the European Chemicals Agency substance information card does state that borax may “damage fertility or the unborn child”.

Now, the chances of achieving the levels involved in “long-term exposure” from occasionally handling borax solutions are slim to none. It’s safe to handle dilute borax solutions (see notes at the end). Indeed, borax is even approved as a food preservative in the EU (E285). To put it into context, alcohol (ethanol) also causes organ damage and is a known teratogen and a carcinogen (which borax isn’t) and that turns up in all sorts of things we’re regularly in contact with, everything from antiseptic hand gels to mouthwashes to drinks (and it’s also approved as a food additive, E1510 – which is good news if you like liqueur chocolates).

I personally have no concerns about handling dry borax in small quantities to make up solutions myself. However, I wouldn’t let children do that part. Once made I’d consider the solution safe, so long as children were supervised and weren’t doing anything really silly like drinking it. I’d also tell children to wash their hands after handling the slime and, if I thought they had sensitive skin for any reason (eczema, say) I’d suggest plastic gloves.

Borax is easy to buy online.

Because of the European Regulations, it theoretically shouldn’t be that easy to get hold of borax in the UK. But I found it for sale on Amazon.co.uk. The listing says that it “can only be purchased by Professionals and by trade and business users,” (sic) but I ordered some and there were no checks. A plastic bag full of borax powder (the decahydrate, Na2B4O7.10H2O) arrived within a few days.

Most of the news reports doing the rounds have involved children suffering from severe skin irritation. For example, in February this year a woman from Manchester posted photos of chemical burns on her daughter’s hands online as a warning to other parents. However, looking into the details of that story it turns out that she wasn’t using borax. In fact, she used fabric detergent “as an alternative”.

Take a look at pretty much type of fabric detergent and you’ll find hazard warnings, usually indicating it’s corrosive and definitely saying “keep out of reach of children”. Those are there for a reason. Fabric detergent is designed to remove grease and  stains. In other words, to break down fats and proteins, and guess what your skin is made of? Yep. Don’t get neat fabric detergent on your hands. Even if your skin isn’t particularly sensitive, it’s almost certainly going to irritate it.

Fabric detergents are usually labelled corrosive.

Bottom line: don’t use fabric detergent as a borax alternative to make slime, because there’s a real risk that enough of it could get onto your (or your child’s) skin that it could irritate.

When it comes to borax itself, if I understand things correctly, it’s not actually restricted in the EU – including the UK – yet. (I might have this wrong – do correct me if you think I have.) It’s not something you can pop to the supermarket and buy, but as we’ve established you can buy it online fairly easily.

Borax solutions are extremely unlikely to cause harm, if used sensibly (boron chemist David Schubert agrees, see note at the end). But, once again: if you’re doing this experiment it’s best not to let children make up the solution – an adult should do that part.

A sensible quantity is about 1 gram of borax in 25 millilitres of warm water (for those without a metric scale: one level teaspoon of borax in half a cup measure of water). This will actually polymerise quite a bit of PVA – you don’t need that much. I recommend making the borax solution in a labelled plastic cup which you should throw away afterwards. Don’t leave it anywhere where someone might mistake it for their drink! Once the solution is made just add a little bit to some PVA in another plastic cup, give it a good stir with a spoon or a lolly stick, and the magic (chemistry) will happen. Add food colouring if you like (be aware that it can stain!) and enjoy the slimy goodness. (See additional note for teachers & technicians at the end.)

Do supervise any and all slime-making, don’t let children handle slime all day, every day, and if you know they have sensitive skin, make them wear plastic gloves. Make them wash their hands before they eat or drink anything.

If a child has made slime somewhere else, at a party or a science club, say, and they bring it home, again, there’s no need to worry. They can play with it perfectly safely. Don’t let them leave it on a radiator, though. That will end in disaster.

I am not a fan of the “it might be a bit dangerous, so no one should ever try it” mentality. I mean, that’s just no fun, is it? But I’m also not a fan of unnecessary risks – because trips to hospital are equally no fun. So if you want to try this experiment, I’ve summarised my guidance in this graphic.

Stay safe with slime by following this guidance

And if you want a even safer slimy experiment, and you can bear the mess, I suggest mixing cornflour with just enough water to make a thick paste in a shallow tray. Then let your kids stick their fingers in it, bounce things off it, and generally play with it. (Check out this link to find out more about why it behaves as it does.) I’m told it makes an even better mixture if you add basil seeds.

Have fun this summer, stay safe, and don’t eat the slime!

Note for teachers and technicians:
This post is aimed at people who might be making slime at home, and hence not have easy access to CLEAPSS guidelines. Anyone doing the experiment with students in school should, of course, refer to their department’s risk assessments and policies. For the record, at the time of writing, CLEAPSS classify 0.2M or 40g/dm³ (or more dilute) borax solutions as “low hazard”.

Edit: 15th August 2017:
After I wrote and published this post I was contacted by someone who specialises in boron chemistry, David Schubert. Now, if anyone knows about boron safety, it’ll be the guy who spends all day writing risk assessments for boron-based chemicals! He told me that borax has been shown to be safe for skin contact. He also said that you absorb less boron through intact skin than you consume by eating a normal, healthy diet (boron is a naturally-occurring trace-mineral – nuts and pulses are good sources), and even provided me with a link to a research paper on the subject. I asked him about the high pH of boron solutions, since alkaline solutions can be irritating in general, and he told me that borax solutions are less alkaline than sodium carbonate and not at all irritating to skin. At this point I will stress that when we’ve seen reports of children suffering skin irritation after making slime, it hasn’t been clear exactly what they’ve been handling. It’s very likely they were adding other chemicals to their slime, and it was actually one of those causing the irritation. Perhaps they developed an allergy to something. It’s impossible to say. Either way, the bottom line is that borax solutions are pretty safe – there’s no need to worry. (Still don’t drink them though!)

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Do you really need to worry about baby wipes?

Never mind ingredients, just give me a packet that's not empty!

Never mind ingredients, just give me a packet that’s not empty!

A little while back I wrote a post about shampoo ingredients, and in passing I mentioned baby wipes. Now, these are one of those products which you’ve probably never bought if you’re not a parent, but as soon as you are you find yourself increasingly interested in them. Yes, I know, reusable ‘wipes’ are a thing. But after dealing with a nappy explosion at 2am in the morning, I’m willing to bet that more than one parent’s environmental conscience has gone in the rubbish bin along with a bag of horror they never want to see again, at least for a little while.

But which wipes to buy? The cheapest ones? The nicest-smelling ones? The fragrance-free ones? The ones with the plastic dispenser on the top that allow you to easily grab one wipe at a time? Or not, because those bulky dispensers produce yet more plastic waste? Or just whichever brand you grabbed first at the all-night supermarket at some unpleasant hour that’s too late to be night yet too early to be morning?

All of the above at one time or another, probably. However, I’m going to suggest that one thing you can stop worrying about right now is whether or not your wipes are labelled ‘chemical-free’.

As I’ve explained before, everything is made up of chemicals. By any sensible definition, water is a chemical, and thus the claim that Water Wipes® (“the world’s purest baby wipe”) are “chemical free” is simply incorrect.

These wipes are not, actually, chemical-free.

These wipes are not, actually, chemical-free.

In fact, Water Wipes® aren’t even, as you might imagine, made of some sort of non-woven fabric impregnated with plain water. No, they contain something else: grapefruit seed extract.

Well, that sounds natural, I hear you say. It does, doesn’t it? Grapefruit, that sounds fresh. Seed, well seeds are healthy, aren’t they? And the word ‘extract’ is very natural-sounding. What’s the problem?

Let’s start with what grapefruit seed extract, also called GSE, actually is. It’s made from the seeds, pulp and white membranes of grapefruit. These ingredients are ground up and a drop of glycerin is added. Glycerin, by the way, is otherwise known as glycerol, or propane-1,2,3-triol. It’s naturally-occurring – it’s one of the molecules you get when you break up fats – and it’s usually made from plants such as soybeans or palm (uh oh…), or sometimes from tallow (oh dear…) or as a byproduct of the petroleum industry (yikes! – I wonder if the manufacturers of Water Wipes® enquired about the nature of the glycerin being added to their product…?)

But anyway, back to GSE. Like all plant extracts, grapefruit seed extract is stuffed full of other chemicals that occur naturally. In particular, flavonoids, ascorbic acid (vitamin C), tocopherols, citric acid, limonoids and sterols.

citric acid synthetic vs natural

Can you tell the difference?

So… in short, not chemical-free at all. Not even a bit. The problem here is that, in marketing, the term ‘chemical-free’ is used to mean something that only contains ingredients from ‘natural’ sources. But this is meaningless. Take citric acid, for example. (E330 by the way – E numbers don’t mean something’s deadly, either. In fact, quite the opposite.) There’s no difference between citric acid extracted from a grapefruit and citric acid prepared in a laboratory. They both have exactly the same atoms and the same molecular formula and structure. They both react in the same way.

They’d both be classified as corrosive in high concentrations, and irritant in low concentrations. This isn’t even “might” cause irritation. This is absolutely, definitely, positively WILL cause irritation.

Wait, hang on a minute! There’s a potentially corrosive chemical in the ‘chemical-free’ baby wipes, and unsuspecting parents are putting it on their baby’s skin?!


But before anyone runs off to write the next Daily Mail headline, let’s be clear. It’s really not going to burn, alien acid-style, through a new baby’s skin. It’s not even going to slightly redden a baby’s skin, because the quantity is so miniscule that it quite literally has no corrosive properties at all. It’s the same logic as in the old adage that “the dose makes the poison“.

This is where we, as consumers, ought to stop and think. If a fraction of a drop of citric acid is harmless then…. perhaps that small quantity of PEG 40 hydrogenated castor oil or sodium benzoate in most (considerably less expensive, I’m just saying) other brands of baby wipes isn’t as awful as we thought, either…

Indeed, it’s not. But what sodium benzoate in particular IS, is a very effective preservative.

Grapefruit seed extract is marketed as a natural preservative, but studies haven't backed up this claim.

Grapefruit seed extract is allegedly a natural preservative, but studies haven’t backed up this claim.

Why does this matter? Well, without some sort of preservative baby wipes, which sit in a moist environment for weeks or months or even years, might start to grow mould and other nasties. You simply can’t risk selling packets of water-soaked fabric, at a premium price, without any preservative at all, because one day someone might open one of those packets and find it full of mould. At which point they would, naturally, take a photo and post it all over social media. Dis-as-ter.

This is why Water Wipes® include grapefruit seed extract, because it’s a natural preservative. Except…

When researchers studied GSE and its antimicrobial properties they found that most of their samples were contaminated with benzethonium chloride, a synthetic preservative, and some were contaminated with other preservatives, some of which really weren’t very safe at all. And here’s the kicker, the samples that weren’t contaminated had no antimicrobial properties.

In other words, either your ‘natural’ grapefruit seed extract is a preservative because it’s contaminated with synthetic preservatives, or it’s not a preservative at all.

If you're worried, just use cotton wool pads and water.

You can always use cotton wool pads and water.

If you’re worried that baby wipes may be irritating your baby’s skin – I’m not claiming this never happens – then the best, and cheapest, thing to do would be to simply follow the NHS guidelines and use cotton wool and water. It’s actually easier and less messy than you might imagine – packets of flat, cosmetic cotton wool pads are readily available (and pretty cheap). Simply dip one in some clean water, wipe and throw it away. It’s really no more difficult or messy than wipes.

But if you’re choosing a particular brand of wipes on the basis that they’re “chemical-free”, despite the fact that other types have never actually caused irritation, you can stop. Really. Buy the cheap ones. Or the nicest-smelling ones, or the ones that come out of the packet most easily. Because NONE of them are chemical-free, and it’s really not a problem.

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Basic Chemistry


The other end of the pH scale.

When you start writing a blog it’s hard to predict what people will find most interesting. Inevitably, it’s not what you expected. For example, two of The Chronicle Flask’s most-read posts are about rhubarb and lemons. Perhaps people are more interested in fruit than I ever imagined. Or perhaps I’m getting a lot of hits from people mistakenly looking for recipes.

Or maybe it’s because both feature the ever-interesting topic of acids. In which case, I should probably write something else about acids.

So, this is a post about bases.

Just in case this spectacular bit of contrariness isn’t immediately obvious, bases – some of which are called alkalis (I’m coming to that in a minute) – are at the other end of the pH scale to acids. Acids are the things with a pH value of less than 7, and bases have pH values of more than 7. So basically (hoho), they’re the opposites of acids.

whysoblueI’m using the word base deliberately, and not just because of all the brilliant chemistry puns you can make with it. The more familiar word is probably alkali, but while all alkalis are bases, not all bases are alkalis.

Alkalis are often described as soluble bases. More precisely, alkalis are produced from the metals in group 1 (the ‘alkali’ metals) and group 2 (the ‘alkaline earth’ metals) of the periodic table. The more general term, base, applies to anything that can neutralise an acid. Chemists have another definition: a base is a proton (H+ ion) acceptor, while acids are proton donors (actually chemists have yet another definition, but the proton acceptor one is the one that gets trotted out most often).

The distinction between alkalis and bases does matter to chemists and the two types of substance usually look quite different – bases tend to come in solid lumps or powders (baking soda, for example) and alkalis are more likely to arrive as a solution in a bottle – but in terms of chemistry they both get involved in the same type of chemical reaction, which is neutralising acids.

Indigestion tablet advertWe make use of this all the time, whether we realise it or not. For example if you’re suffering from acid indigestion you probably reach for the indigestion tablets. An advertising campaign for a particular brand of these says that they “turn excess acid into water and other natural substances”. Those ‘natural substances’ are salts – presumably it was decided that the word ‘salt’ had too many negative connotations (which is probably true: how many people would pop a pill that promised to turn into salt in their tummy?) The main ingredient in the tablets in question is calcium carbonate; a base that reacts with stomach acid to produce calcium chloride. Which is definitely a salt, if not the one most people think of when they hear the word.

Tangentially, calcium chloride is also a food additive with the E number E509. It falls into the category of anti-caking agents, which is sort of funny when you think about it.

Anyhoo, that’s one place you use a base (rhyming now as well as punning, sorry). You’re actually making one yourself every time you eat, because your liver produces a substance called bile (bloggers love bile) which helpfully neutralises the acid your stomach produces. If it didn’t, your intestines would get damaged by that acid, so it’s important stuff.

Interestingly, in a lot of the older medical traditions (you know, swallow three leeches with meals, turn around three times under a full moon and bury a toad under a horseradish in a mock turtle) the body’s health depended on the balance of four ‘humors’, or vital fluids: blood, phlegm, ‘yellow bile‘ (choler), and ‘black bile‘. If you had too much of the last two, it was supposed to cause aggression and depression, and in fact the Greek names for them are the root of the words cholera and melancholia.

It’s interesting that in the 21st century many people are obsessed with ‘alkalinizing‘ the body (just check out the comments on that lemons post) when for thousands of years people have understood that too much alkali is probably a bad thing. Public understanding of science has really moved on hasn’t it?

soapBile does something else that’s really quite important in the body, it helps you to digest fats. Bases are generally really good at breaking down fats. This is another thing that’s been known for quite a while, ever since soap was first discovered about (sources vary quite considerably on this) six thousand years ago. Soap is made by a process of saponification, in which fats react with a strong base, usually sodium hydroxide (otherwise known as caustic soda, or sometimes lye). This breaks apart the fat molecules to make glycerol and carboxylate salts (they’re the soap bit). Because of this use, sodium hydroxide features in a famous, and rather gruesome scene, in the film Fight Club.


The fire diamond for NaOH

Because bases are so good at breaking down fats they’re actually surprisingly (or not, if you’ve just watched that Fight Club clip)dangerous, especially because they’re also quite good at breaking down proteins. Your skin is mostly fat and protein, so they can do quite a bit of damage. Remember fire diamonds? The one for sodium hydroxide has a 3 in the blue box, which means that short exposure could cause ‘serious temporary’ or ‘moderate residual’ injury – yikes.

Corrosive hazard symbol

Corrosive hazard symbol

The European hazard symbol is even more alarming, featuring a hand with holes being burned through it. Of course, acids have symbols like these too, but people sort of expect acids to do this kind of stuff. Whereas they’re often (unless they’re chemists) strangely unaware of the dangers of alkalis. For example there’s the a famous, and gruesome, story of the serial killer John George Haigh, who famously dissolved the bodies of his victims in oil drums full of concentrated sulfuric acid. It worked quite well, but he was caught eventually when the police searched his workshop and found sludge containing three human gallstones and part of a denture.

Sulfuric acid is a particularly powerful acid, and is undoubtedly incredibly dangerous stuff, but sodium hydroxide is not much safer. It will cause instantaneous and serious burns, and solid sodium hydroxide gets incredibly hot if it’s added to water. In fact, the water will quickly boil if you’re not careful.

In May last year American Carmen Blandin Tarleton was in the news because she had just received a face transplant. She needed it because her estranged husband had doused her with concentrated sodium hydroxide six years previously. She had undergone fifty-five operations before she made the decision to get the transplant. The pictures are really quite horrific. I won’t reproduce one here; you can see the result of the attack if you follow the link above. Tarleton has also written a book about her experiences. She was left blind and horribly disfigured, with burns to 80% of her body. Doctors described it as “the most horrific injury a human being could suffer”. Sodium hydroxide is not nice stuff.

It’s surprisingly, shockingly, easy to buy sodium hydroxide. Because it’s used in soap-making, you can get it quite easily. It’s even available on Amazon. And of course it’s an ingredient in lots of drain cleaners available in supermarkets. When they say you should wear gloves to handle this stuff, it’s definitely not health and safety gone mad. You really should. Even I would (and I’m really bad about wearing gloves).

So spare a thought for bases. They’re just as interesting, and certainly no nicer or safer than their acidic cousins. In fact, they’re so good at breaking down fat and protein that they could arguably be more dangerous. And next time you’re cleaning out your oven, do remember to wear your gloves.