When you wash old copper pennies with vinegar, the pennies react with the vinegar to form a blue compound called copper (II) acetate. Now, if you let that vinegar dry, small, black crystals will be left behind in the dish. Those are copper acetate crystals.
It turns out you can grow much bigger copper acetate crystals. They are sleek, shiny, and look like pieces of polished obsidian.
Hi, I’m Chase. I’ve been growing crystals at home with household chemicals for 3 years now. From the very beginning, I’ve always wanted to grow a nice copper acetate crystal. It took me a lot of experimentation, but I finally managed to find a method to grow a gem that I’m truly proud of.
In this guide, I’ll share what I’ve learnt, and show you how you can grow your own perfect, black copper acetate crystal at home with scrap copper and vinegar.
Growing copper acetate crystals
To grow copper acetate crystals, you’ll need:
- 10 g of scrap copper
- A bottle of white vinegar
- 250 mL of 3% hydrogen peroxide
- Two large jars
- A big, flat container
- A small dish
- Nylon fishing line
Now, copper acetate is mildly toxic. It is not particularly dangerous when handled properly, and it’s fine to touch the crystals once in a while if you wash your hands afterwards. But avoid prolonged contact as it can cause skin irritation.
Also, it might not be a suitable experiment for young children, because you definitely don’t want them to eat it.
If you’re looking to grow safe and beautiful crystals with kids, check out my other guide on growing crystal clusters with fertilizer.
With that said, let’s get started.
Preparing the copper acetate
First, get your hands on some scrap copper. Anything that’s made of copper will do, although the most common sources are copper pipes and wires.
If possible, cut the copper up into small pieces.
Then, get your vinegar ready. Normal food grade white vinegar you can find at the grocery store is quite dilute – it has a concentration of 3-6%. This works fine, but the reaction will take longer.
To speed it up, you can get cleaning vinegar, which has a concentration of above 30%. This vinegar is much stronger, so be careful while using it.
Add 10 g of copper metal to a non-metal container.
Next, pour in 400 mL of 5% vinegar. If your vinegar is twice as concentrated, then you should use half the amount, and so on.
After that, pour in 250 mL of 3% hydrogen peroxide. It is a common antiseptic which can be found at pharmacies or bought online.
The hydrogen peroxide’s job is to quickly oxidize the copper metal into copper oxide, which readily reacts with the vinegar to form copper acetate. Don’t use a higher concentration; a messy, brown precipitate will form.
Upon adding the peroxide, the mixture will begin to fizz as oxygen gas is released by the hydrogen peroxide. Sometimes, the fizzing can be quite intense. So, add the peroxide slowly to ensure the reaction does not become too vigorous.
Within a few minutes, the solution should start turning blue, which is a sign that copper acetate is forming. It will steadily turn darker.
This is what my solution with added hydrogen peroxide looks like after 30 minutes.
At this stage, you have two options:
- Warm the solution on a hot plate at 70°C for 1 hour
Heating the solution will greatly speed up the reaction. However, it also releases lots of vinegar fumes, so you must do it outside or in a fume hood.
Also, don’t carry out this step if you decide to use very strong acid – the reaction can go out of control. Make sure to take the appropriate precautions when heating acid.
A much safer alternative is to:
- Leave the solution outside for 2 days (concentrated vinegar) or a week (normal vinegar)
This should give plenty of time for the reaction to occur.
Regardless of what method you choose, in the end, your copper acetate solution should be a very dark blue. Now, we need to further concentrate it by evaporating away excess water.
Pour the solution into a large, flat container to evaporate outdoors, in the shade.
Wait for signs that the solution is reaching saturation. Saturation is the point where the solution contains so much copper acetate that crystals start forming.
First, you will start to notice blue-green flakes appearing on the walls of the container, just above the water level. Then, small, black specks will form on the surface of the solution. Some of it might also grow on the pieces of leftover copper at the bottom.
Those are actually mini copper acetate crystals.
When those specks appear, congratulations, your solution is now saturated!
You’re now ready to move on to the next stage.
If you have any problems in this stage, refer to the troubleshooting section at the end of this article.
Growing copper acetate seed crystals
From the previous step, you should have around 250 mL of saturated copper acetate solution. Of course, if you want more, just increase the corresponding quantity of copper, vinegar and hydrogen peroxide.
Filter 200 mL of the solution into a jar, close it, and leave it aside for now.
Transfer the rest of the solution into a small dish. To this same dish, add about 1 mL of vinegar.
After that, place it in a cool, undisturbed area like a storeroom or the inside of a drawer, and wait for a few days.
This will provide time for copper acetate crystals to start forming from the saturated solution. You will need those small crystals as “seeds” to grow bigger ones.
While you’re waiting, here’s a fun experiment you can do:
Place a small amount of copper acetate solution on a piece of plastic or glass and let it dry.
Here’s what it should look like after a few hours:
Aren’t the patterns just beautiful? Those dendrites look like rivers of blue frost creeping up a window on a cold winter’s day.
Here’s a microscopic view of those crystals, taken under polarized light by Cora A. Harris, who’s currently researching crystallography. She kindly allowed me to feature them here.
Anyway, let’s get back to our copper acetate seed crystals.
Within the week, your dish should have formed a bunch of small, black copper acetate crystals.
Notice how they’re much neater than the ones you saw previously. This is due to slow growth in an undisturbed environment.
You can leave them inside the dish and wait for them to grow bigger, but there’s a problem.
The crystals are growing on the bottom of the container. This means that instead of being symmetrical in shape, they will be flat on the bottom. Also, left alone, the crystals will start to grow into each other.
To fix this, first cut about 20 cm of nylon fishing line, and tie one end of it to a stick. Poke the other end of the fishing line into the copper acetate solution.
Adjust the line until it touches a nice crystal. Since there are so many, you can pick any one that is more convenient for you.
Once the line and the crystal have good contact, leave them alone for 24 hours.
Observe the fishing line in contact with a seed crystal. They will get stuck together within 24 hours.
By then, the crystal should have grown into the fishing line, which means that they are now stuck together.
Now, you want to suspend the seed crystal in the 200 mL of solution that you stored in a jar earlier. Just place it like this:
Loosely cover the top with a piece of foil / plastic wrap / paper to slow down evaporation and keep out dust. This step is very important to ensure your crystal grows nicely by reducing the risk of cracking, imperfections and parasitic crystals.
All you have to do now is to wait.
Growing a big copper acetate crystal
At the beginning, your seed crystal is still small, so it is unable to “absorb” all the excess copper acetate that wants to crystallize out of solution.
This means that many other small crystals will form on the sides of the container or on the surface of the solution.
When they start building up, don’t bother removing them one by one. Just pour the solution into a new container, and transfer your main crystal there.
Some of these crystals might even decide to form on the fishing line. If they grow too close to your main seed crystal, they might merge and get stuck together.
This is no big deal if you want a crystal cluster. But if you want a single perfect crystal, you need to remove all other crystals before they grow into each other.
It’s easy to do so. Just place the unwanted crystal on the fishing line under some running tap water, and rub them off with your fingers.
As time passes, your copper acetate crystal will start growing bigger. And as this happens, you should notice that fewer unwanted crystals form.
Here’s what my crystal looks like after two months:
The level of the solution will also start decreasing. Make sure the water level doesn’t become low enough to expose the copper acetate crystal.
If you want to continue growing the crystal, you just need to prepare more saturated copper acetate solution using the same procedure above to replenish the growing solution.
Otherwise, you can also choose to stop growing the crystal when you decide that it’s big enough. Just remove it from solution, and dry it with a piece of tissue/filter paper. Make sure you don’t wash it – copper acetate is soluble in water.
After that, you’re done! Enjoy the beauty that you’ve just created.
Even after having grown crystals for so long, I’m still amazed at how nature creates such flat faces and crisp edges. All the crystals on this page (and on my website) formed by themselves – they were not cut or polished in any way. And yet, their form is symmetric, their faces smooth as a piece of polished glass.
Now, if you think the crystal isn’t big enough, you can of course, continue to let it grow. Unlike some other crystals like Epsom salt or copper sulfate, copper acetate crystals grow relatively slowly. But to me, their appearance more than makes up for it.
Here’s a copper acetate crystal that sat in my storeroom, growing in solution, for half a year:
At that point, I had already refilled the solution 3 times.
In the end, I decided to stop its growth after 7 months. Black and glossy, weighing in at 21.5 grams, it is undoubtedly one of the most beautiful crystals I’ve ever grown.
And to think that such a gem was made from nothing but scrap copper and vinegar!
Growing copper acetate crystal clusters
I’ve already shown you how you can use copper acetate to make beautiful patterns on glass, and how to grow a large single crystal. But you can do more with it.
It’s actually quite easy to grow crystal clusters with copper acetate. A crystal cluster is simply a bunch of crystals stuck together. To do so, just let several seed crystals stick to the string, and wait for them to grow into each other. Then, repeat the same procedure above.
Here are some amazing clusters of copper acetate crystals by Julas xZiom.
You can even make copper acetate crystals grow on objects.
For example, if you dip a piece of copper into some saturated copper acetate solution and wait for a week, some tiny black crystals will form on it. Wait longer, and the piece will be covered in big, black crystals.
Here’s a piece of copper metal coated with small copper acetate crystals.
You can do the same for other (non-metal) objects. Indeed, many people crystallize insects, branches and sculptures to transform them into works of art.
Storing the copper acetate crystals
Copper acetate crystals contain water molecules as part of their crystal structure. When left in open air, they can dehydrate and turn bluish white.
A dehydrated copper acetate crystal cluster, after several months in dry air by Julas.
To prevent this, you can coat them in nail polish or varnish to serve as a protective barrier. Or you can just keep them in an airtight container – I’ve kept some copper acetate crystals this way for 3 years now, and they still look black and shiny.
Troubleshooting & FAQ
Why is the reaction so slow?
There are many factors affecting the rate of reaction. For example, your copper pieces might be bigger than mine, your place colder than mine etc.
Generally, used and tarnished copper will react faster than fresh, shiny copper. Also, if your copper has a layer of wax/coating protecting it, use some sandpaper to sand it off first.
Regardless, don’t be worried if your solution is not as dark blue as mine after say, 1 hour, or 1 day. The important thing is that your solution is blue, which is a good sign that at least some reaction is taking place. You’ll get there, sooner or later.
What is this cloudy stuff that’s appearing in the solution?
The cloudy stuff is a mixture of basic copper carbonate and hydroxide. When the copper acetate solution is not acidic enough, it can cause copper hydroxide to precipitate.
Left alone, the bluish goo will eventually cloud up your entire solution and stick to your crystals, like half dried paint.
The matrix of black crystals embedded in greenish blue goo can be quite pretty, but sometimes, you don’t want that to appear when you’re trying to grow a big, shiny crystal.
To prevent (or fix) this, just add more vinegar to the solution. It’s recommended to add more concentrated vinegar (>30%), because you’ll have to add less of it to achieve the same effect. The solution should clear within an hour.
Why are crystals not forming?
Copper acetate crystals only form from a saturated solution. It’s likely that your solution is not saturated enough. Continue letting it evaporate, and wait for the small, black crystals to form. They will, sooner or later.
This is what a saturated copper acetate solution looks like. You need to wait until there’s crystalline stuff floating on the surface or black specks at the bottom of the solution.
Will the string get stuck inside the crystal?
If you tie the crystal on the string with a knot, then yes, it will get stuck inside. But since the crystal is black, it’s barely noticeable once you cut it away.
If you poke the string into the solution to let a seed crystal grow directly on it, then you can just pull the string out after you’ve finished growing it.
Are these crystals fragile?
You can hold them just fine, but they are brittle and chip easily. This means that you can’t polish them. Also, you can’t wash them since they’ll dissolve in water, which makes it unsuitable for jewelry. Covering them with a coat of nail polish will help them resist wear and tear.
Is there another way to make copper acetate?
Yep. There are many other methods to make copper acetate. The one I mentioned above is just the most accessible for the average person. Alternatively, you can make copper acetate by:
- Reacting copper oxide with vinegar (acetic acid)
- Reacting copper hydroxide with vinegar
- Reacting copper carbonate with vinegar
- Reacting copper sulfate with baking soda to produce copper carbonate, then reacting the copper carbonate with vinegar
- Electrolyzing vinegar with a copper anode
Also, note that although old pennies (before 1982) are made of copper, newer ones mainly consist of zinc, so they will not work for the reaction.
What other crystals can I grow?
It’s amazing just how many different types of crystals you can grow with household materials. The easiest crystals to grow are Epsom salt, alum, and monoammonium phosphate fertilizer. All are non-toxic and beginner friendly.
I highly recommend growing monoammonium phosphate crystals, they are among the easiest and fastest to grow. Check out my guide here.
If you want to do a little more chemistry, then you can also try growing potassium ferrioxalate crystals, which are emerald green.
That’s all for this guide. If you have any questions, feel free to leave a comment below. If you enjoyed the guide, remember to share it with your friends. If you want to stay updated with more articles like this, consider joining my newsletter.
And as always, happy growing!