Alum, found at the grocery store, can form beautiful diamond shaped crystals.
The best part is, anyone can grow some within a few days at home!
It is also beginner friendly, easy to grow and completely non-toxic. You can have your first specimen in a week or two.
Of course, trying to grow extremely high quality crystals will take some skill and patience. But it can be done – and in this guide, I will walk you through the steps to do exactly that.
Before that, we need to get our alum. There are many types of alum. The one we want is potassium alum. It is normally used in pickling and also water purification, so you can probably find it under the baking section. Alternatively, you can also order them online. 250 g is more than enough for the first try.
This guide will be divided into 5 easy-to-follow parts as shown below:
- Preparing the growing solution
- Growing a seed crystal
- Growing a big alum crystal
- Drying and storing the crystal
- More cool activities with alum crystals
Preparing the growing solution
To grow our alum crystal, we first need to prepare the solution for it to grow out of. Think of it as something like salt water. When salt solution evaporates, salt crystals are left behind. Similarly, when alum solution evaporates, alum crystals are left behind.
- Dissolve 60 grams of alum powder in 500 ml of hot water. Stir the solution using a spoon until all of the alum dissolves.
- Filter the solution with a coffee filter to get rid of any dust. Tissue paper also works.
- Wait for the solution to cool to room temperature.
Growing a seed crystal
After cooling, sprinkle a few grains of alum powder into the solution. Tiny crystals should start forming in the solution within 30 minutes.
This is where my guide differs from others you can find online.
Do not be tempted to grow these small crystals. Leave the solution alone for 2 days. This step will ensure that you get more beautiful crystals in the end.
Low quality alum crystals that formed at the bottom of the container.After 2 days, pour 50 ml of the alum solution into a flat dish, and the rest of the solution into a transparent jar/cup. You only want the solution – leave the crystals at the bottom of the original container behind.
Leave both the dish and the cup containing the solution in an undisturbed place. Beautiful crystals will start forming.
After a few days, your dish will contain many small, clear crystals. Once the crystals reach 0.5 cm in size, select the most beautiful crystal inside the dish.
Excited? I bet you are! Then, tie the crystal to a fishing line (or a string), taped to a stick.
Lower the seed crystal into the cup containing the remaining solution.
Place the cup in an undisturbed place, and all you need to do now is to wait!
Alternatively, if you don’t want to tie the crystal to a string (because it will get stuck inside), you can also grow your seed crystal at the bottom of the same cup/container. Here’s an example:
Growing a big alum crystal
As the solution evaporates, alum particles will be deposited on your seed crystal.
Therefore, it will slowly grow larger.
You can stop growing it when:
- You’re satisfied with its size
- The level of the solution has decreased so much that it’s about to expose the crystal
You never want the crystal to be exposed. Otherwise, its surface will no longer be smooth and clear.
An example of a crystal that waited too long until all the solution had dried up.Regardless, when you want to stop growing, take it out, and dry it using a piece of tissue paper (filter papers are better). Do not wash the crystal or you will cause it to re-dissolve.
And that’s it, you’re done!
Storing the crystal
If left in open air, your alum crystals might slowly turn white over time. This is because alum crystals have water molecules trapped inside their crystalline structure, and as that water dries up, the crystal turns white.
To prevent this, keep the crystal in an airtight container. Better still, keep it with some alum powder. The alum powder will sacrifice itself, and in turn, provide a stable atmosphere inside the container so that your main crystal does not dehydrate.
Alum crystals are not very hardy. It’s fine to hold them, but frequent handling will cause the crystal to be duller, and the edges less sharp. A coat of nail polish will provide good protection.
Also, the crystals are brittle and will crack when dropped. This makes them good decorations, but ill suited to be used as jewelry.
More cool activities with alum crystals
Growing alum crystals is fun. You can take this one step further by growing them on objects.
If you want to grow a single big crystal, just follow the procedure above, but place the seed crystal on the object of your choice. Here’s a beautiful specimen by Reddit user u/Higurashii who kindly shared it with me:
Other than that, you can also coat an object with a layer of small crystals. To achieve this, immerse the object in a supersaturated alum solution. Unlike the steps for growing large single crystals, this technique will cause lots of small crystals to form at once. It’s not what we want in the procedure above, but perfect for crystallizing objects.
Many people like to grow crystals on insects, tree branches or sculptures using this technique. Here’s a beautiful crystallized crab by taxidermist Jamie Keiman. Check out more of her creations on her Instagram page.
Because alum crystals refract light, you can even use them as prisms. One morning, I saw a ray of sunlight coming through the window, and I tried putting one of my crystals in its path. The result was beautiful!
Here’s a completely unedited photo of the refracted light.
And when I moved the crystal around so that the sunbeam hit it at different angles, specks of light danced around me.
That’s all for this guide. Hopefully you’ve found it useful.
Growing crystals takes time, but with patience and a little bit of luck, you can definitely create sparkling gems at home.
If you have any questions, feel free to put them down below.
Alternatively, check out my post on growing crystals with fertilizer. It’s even easier than alum, and produces spectacular results. Or take a look at how to grow crystals from table salt.
Happy growing!
Hi, Im trying to grow some alum crystals, ive got the alum dissolving in hot water, but whenever it cools, lots of white powder forms at the bottom of the container, and if I filter the white powder out, any alum I put in the solution dissolves and forms white powder within 2 hours or so of dissolving.
I attempted to reheat the solution and adding 5g/ 100 ml of alum, and after stirring, the alum was clearly fully dissolved, but when it cooled enough for me to try to sprinkle some alum in (for seed crystals), there was a layer of white powder on the bottom. Any extra alum I added dissolves and later forms white powder about 1-2 hours after dissolving. Ive used an entire 250g bag of potassium alum powder attempting to get this to work and I am stumped.
Any pointers as to what I could be doing wrong?
Is there a huge amount of white powder? Or is it just a thin later?
Don’t pour your solution away. Filter your remaining alum solution into a new container. Then it should not have any white powder. After that, heat the solution to evaporate off more water, until small crystal flakes start to form. Wait for it to cool. You’ve just made a saturated alum solution without white powder, that you can use to grow crystals with.
In short, since you’re having a problem with *adding more alum*, just prepare a solution and *remove excess water*. Both procedures achieve the same result: producing a saturated solution that is suitable for crystal growing.
Hey there Chase, thank you for the guides! You’ve inspired me to try crystal growing. I’ve tried to create the solution for the Alum, but struggled to see any crystal growth after half an hour or even a day or two.
After reading comments, my alum grains don’t seem to grow and just seem to be sitting on the bottom, maybe my solution may not be saturated enough, or it’s just balanced in its saturation (even after reheating it and adding more Alum once, as you suggest). Out of curiosity, I have a couple of questions:
1) Are there a certain number of times the solution can be reheated before it becomes ineffective? Or can it be reheated, cooled, etc. indefinitely until you notice the crystal growth?
2) If, for example, I started with a new solution, how accurate must the measurements be? Is it better to put more than you think, potentially creating an over saturated solution to ensure potential crystal growth, or is that not recommended?
Thanks in advance for any reply!
1) The solution can be used indefinitely. If you use tap water, repeated dissolving/evaporation cycles might cause the concentration of impurities to build up in the solution, and slightly affect subsequent crystal growth. But unless you’re using it many, many times, I wouldn’t worry about it. Furthermore, crystals will always form, it’s just that their shape might be different due to the impurities.
2) If like me, you’re doing it at home, it’s not possible to get an exact measurement to guarantee saturation. The difference between 0.1 g of alum can mean that either the crystal will grow quickly or slowly dissolve. Temperature will also affect this value. Therefore, I always make a supersaturated solution to ensure crystal growth, and let it slowly bring down the supersaturation. It is highly recommended!
Note: If you’re frustrated that crystals are still not forming after many tries, do test run with a massive amount of alum. Try to dissolve 25 g alum per 100 mL pf hot water. This will guarantee precipitation of alum crystals as they cool, and reassure you that the alum isn’t magically disappearing into the water. Then you can bring down the supersaturation from there.
Hi, Chase!
I’m preparing to make my first crystals and I have questions about the utensils I should use. Do you use plastic or glass? Are both fine?
Also, I’m confused in some steps. When we put our solution apart while we wait for the smaller crystals to form and while we are growing our final crystal, do we have to cover the solution in some way to avoid evaporation?
Thank you!
Both plastic and glass are fine. If you’re planning to hold hot water in them (when dissolving the alum), then make sure you use heat resistant material.
Yes, cover the solution when you put it apart. There are 3 reasons for this:
a) To prevent evaporation
b) To keep out dust
c) To discourage crystals from forming inside this solution
Good luck!
Could I smash a solid Alum to powder?
Sure you can.
Hey chase I’m at the step where you sprinkle some alum into the solution after it’s cooled and have left it for 1 and a half days and have seen no crystals forming at all is it because I have the bar closed or is there something I’ve done wrong?
How about the alum that you sprinkled into the solution?
1) If it is still there, and unchanged, your solution is exactly saturated. You can proceed to the next step, which is pouring the solution into a dish and waiting for seed crystals to form.
2) If it has disappeared, your solution is undersaturated. Reheat the solution and add about 5 g more alum per 100 mL water. Repeat the procedure.
Can I put the solution in the fridge for it to cool down quicker or is it better to wait?
You can do both. Quick cooling vs slow cooling doesn’t matter when preparing the solution. But temperature changes are very important when actually *growing* the crystal, because a sudden change in temperature might cause the crystal to dissolve or develop cracks.
Also (sincerely sorry for nagging again) after The solution has cooled down when I sprinkle the alum in do I close the jar afterwards or leave it open?
Haha it’s okay. Close the jar to keep out dust.
Hi Chase, awesome article, can’t wait to get started making crystals. I bought some alum but just realised it’s aluminium sulphate (can’t see any reference to potassium on the pack). Will this still work?
Hey Jesse. Just aluminum sulfate won’t work; you’ll need potassium aluminum sulfate.
P.S. If it’s cheaper, you can also buy potassium sulfate, and mix aluminum sulfate with potassium sulfate in a 4:1 mass ratio. The product is potassium aluminum sulfate, which is exactly what you need to grow crystals with.
Thanks! My seed crystals are almost ready. I’d like to add some red colouring at the next stage. Would you suggest preparing a saturated solution with the colour before adding it to the filled containers (e.g., 6g alum powder in 50ml food colouring, as per recipe)?
Yes, you can do that. It’s alright to add food coloring anytime during the growing process.
Whenever you’re doing something new (like adding food coloring), add it to a a solution and *test* to make sure that the solution is indeed saturated. Sometimes the addition of something else will change the saturation slightly. You can do this by dropping a few grains of alum inside to check whether they dissolve. If they don’t dissolve (and don’t become huge and ugly all of a sudden), then you know it’s safe to add your seed crystals.
All the best!
Can you use a spray can of clear acrylic on them to seal? It seems like nail polish would take a long time on a large piece and leave brush marks.
Yup, sure you can.
Hi Chase, I love this article, lots of good information here.
Do you know if your method would work with
Aluminium Potassium Sulfate Dodecahydrate? This is the only thing available to me at the moment.
If so, how much of it will I have to mix in 500 ml of water?
Thank you!
Thanks! Actually, the alum I was referring to is exactly aluminum potassium sulfate dodecahydrate. So, just use the amount specified in the guide and you’re good to go!
P.S. There are actually other types of alum like ammonium alum and chromium alum, all of which have similar growth characteristics but different solubility. Potassium alum is the most common one.
Thank you Chase. Growing crystals is turning out to be a lot of fun.
Just a few more beginner questions if you don’t mind!
– I’m making different crystals using copper sulfate, alum powder, MAP and borax. Will they melt in summer?
– Do I need to regularly coat them in clear nail polish, or is once enough?
– Is there a way to grow crystals using Iron (II) sulfate powder?
Thank you very much!
Sure!
-No, the crystals will not melt. They will only dissolve in water. However, higher temperatures might cause drier air, which will cause the crystals (except MAP) to dehydrate and become white. This can be prevented with a coat of nail polish.
-Just one or two coatings of nail polish is enough. You don’t have to keep coating them.
-Yes, just dissolve the iron sulfate powder in water, add a little dilute sulfuric acid (to prevent iron hydroxide from forming), and use the same procedure that I showed in one of my articles.
I’m interested in growing some crystals that will be safe to cast in epoxy. Also, some that could be colored with food dyer or something? GREAT site though, I just stumbled on it this morning!
Yes, you can cast the crystals in epoxy. To color the crystals, just add a large amount of food coloring to the solution. However, the coloring will just give a light tint, because crystallization is a process that naturally excludes impurities (and dye). If you want colored crystals, I suggest you just grow crystals out of a compound that has the desired color. For example, chrome alum crystals are naturally purple and have the exact same shape as normal alum crystals.
Hey Chase:
We tried this at home (using someone else’s instructions) for my son’s science experiment. We did not have any crystals the next morning, it just looked like I dissolved alum at the bottom of the jar. I did notice there were some spec of dirt in the bottom of the jar my son chose and we tried to remove them but couldn’t get them to easily come out b/c they were so small. Could that have caused the crystals not to form? I’m going to try your method and see if it works. My son needs them by Monday. Today is Thursday! Thanks for a great tutorial!
Hey there. I don’t think the dirt is the problem. Maybe you guys didn’t dissolve enough alum into the solution, hence the solution is not saturated? A solution that is not saturated will not form crystals.
Try sprinkling some alum powder into your solution. If the grains of powder start growing, great, the solution is saturated, and you can proceed with growing them. If the powder disappears (dissolves), it means your solution is undersaturated. To fix this, just reheat the solution and add about 10% more alum. After that, you can do the test again. Good luck!
I use potassium alum crystal as a deodorant. Can I smash the leftovers to a powder and use that? It feels like such a waste to just throw them away…
Yup. It’s a perfect source of alum. Then you can tell people you grew shiny crystals from deodorant haha
How would you make these crystals coloured?
The easiest way is to add food coloring. It won’t completely color the crystals, but there will be a light tint of color to them. Alternatively, you can grow chrome alum crystals (potassium chromium sulfate). They have the exact same structure and can be grown using the same procedure, but they’re dark purple instead. Since this purple color is natural and inherent to the crystal, it will turn out much better.
Hi Chase. Very nice article, crystal and photos, congratulations. I have a question, sometimes in the alum solution, mold or bacteria appear that at some point eat the crystal and it stops growing and loses its luster. is there a solution to this?
The easiest way is to prepare 2 solutions. Use one solution to grow the crystal. Once you see the slightest signs of mold forming, move the crystal from the 1st solution to the 2nd solution. Then, boil the 1st solution briefly to kill the mold. When the 2nd solution starts to mold, you can swap them again.
You can also try dropping a piece of copper metal into the solution, as copper is toxic to microorganisms. Some people have successfully dealt with this by adding small amounts of iodine to their solution.
Thank you very much for the interesting information. all three sound good. I will try iodine first, it looks easy but I’m afraid the color in the solution will not change. I have had an alum crystal for 14 years as a student. it is as beautiful as then. I like to use an amorphous quartz base and have the crystal grow on it. this makes it look like by nature. unfortunately since I saw mold in my solutions I was disappointed and did not bother again. I hope to see good results with your 3 tips. thanks again Chase. !
I have a student who would like to cite your web article in a bibliography. Can you please send me the information so we can do this properly?
Sure. I’ll email you the details.
Hi Chase, do you have any suggestions for suppliers to get bulk material? I want to do a lab in a university class. The best I’ve found is 10 – 30 lbs of food grade on ebay. Thanks
Hey Joe. I’ve never bought alum in bulk before, so I don’t know of any bulk suppliers. 30 lbs is a decent amount – you can prepare 100 L of saturated solution with it. For crystal growing purposes, good for 150 individual students.
made a solution with 100 g of alum potash salt and 400 ml of water and it has been 2 months but it is only 8mm X 6mm .Can you suggest how can i grow it faster and to size of 1.5 cm X 1 cm
How much time will it take
It really depends on the conditions where you live and the size of your container. I recommend doing 2 things to speed it up:
a) Make sure there are no small crystals growing at the bottom of your container
b) Move the setup to a place which is more exposed, like on a bookshelf (but not outdoors).
There’s also a method where you prepare new supersaturated solution and add it into the growing cup, but there’s a big risk it will make the crystal opaque and ugly, so I don’t recommend it.
Expect to wait another month to reach 1.5 cm^3.
I want to display my crystals. Could I give it a coat of clear nail polish? Or would that start dissolving it?
Nail polish is a common way of preserving crystals. It should be fine.
It almost looks like you added Borax to prevent the octahedral structure from taking over the cubic structure completely. Did you add anything to get that effect or did it just occur naturally?
Good observation! Unfortunately, no I did not. Maybe it was because I was using food grade alum, and impurities inside it affected the growth of the crystals in a similar way to borax.
Can you coat the crystal in a clear
nail polish to preserve it?
Absolutely. That is indeed a good way of preserving the crystals.