How to Make a Saturated Epsom Salt Solution to Flocculate your Glazes
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Flocculate… What Does That Even Mean?
You may have heard this term “flocculate” in reference to glazes and thought it was a funny word but you’ve never really known what it means.
Or you may have been told what it means several times and still don’t really understand it.
Or, maybe you understand what it means but you always mix up “flocculate” and “deflocculate” because you can’t remember which is which.
This sums up my own progression of understanding the word flocculate. I don’t know why but it took me so long to really get it. Nobody ever explained to me in a way that solidified it in my brain.
I made a point to read every article and excerpt I could find about it until I was finally like “Ohhh, now it makes sense!”
I could write pages on flocculation and deflocculation now. It’s a pretty complex subject when you get into the science, details and nuances of it but the basic principles are straight forward.
Flocculation affects how the clay particles in a glaze recipe interact with each other.
This is a very important distinction. Only the clay particles in a glaze are affected by flocculation. So your glaze recipe must first have clay in it (bentonite, kaolin or ball clay) for flocculation to work.
When a glaze is flocculated, the clay particles become attracted to each other and stick together instead of flowing freely past one another.
Here’s how it works:
Clay particles are flat and thin, like tiny playing cards. They have flat “faces” and thin “edges”. The edges and faces of a clay particle have a charge, positive or negative like magnets.
Positive attracts negative, but two positives or two negatives will repel each other.
The faces of a clay particle are negatively charged. The edges can be both negative and positive.
When you add Epsom Salts (magnesium sulphate) to flocculate a glaze, you’re introducing positive magnesium ions to the glaze. These positive ions attach themselves to the edges of the clay particles so now the edges are all positive.
Since the faces have a negative charge and the edges have a positive charge, the clay particles all start attracting each other and sticking together, edge to face, face to edge.
Positive edges of clay particles are attracted and stick to negative faces.
This forms an open, house-of-cards-like network of clay particles throughout the glaze bucket. The clay particles are bonded together, but there are lots of open spaces in between them.
This open network of clay particles does a few key things.
- It increases the viscosity of the glaze (making the glaze thicker)
- It helps to keep all the other particles suspended in the bucket
- It increases the time it takes for a glaze to dry after applying, promoting a smoother application
So, if you’re wondering whether your glaze needs to be flocculated with Epsom Salts, consider these three issues.
- Are you trying to thicken your glaze?
- Does your glaze need help staying suspended in the bucket? Is your glaze hard-panning?
- Is your glaze drying too fast, causing an uneven application layer?
If your answer to any of these questions is YES, then you may need to flocculate your glaze.
So to sum up what flocculation means, a flocculated glaze has clay particles that stick together, rather than move freely past one another.
A popular way to remember this is the clay particles “flock” together, like sheep flock together. Flock-ulate.
We didn’t talk about deflocculating in this post, but another mnemonic device I use to remember the difference between flocculants and deflocculants is that Darvan is a Deflocculant and a Dispersant – all starting with D.
Whatever it takes to figure this stuff out, right?
Read This Before Flocculating
Flocculating should always come AFTER measuring specific gravity. Measuring specific gravity is a way to calculate whether your glaze has the ideal amount of water for your application method.
You must get the water content right before determining whether your glaze needs to be flocculated.
If your glaze is too thin or won’t stay suspended because it has too much water, then flocculating isn’t going to help. In some cases, it will make your glaze application worse.
The only way to know whether your glaze has the right amount of water is by measuring specific gravity. It’s a quick and easy process that will improve the consistency of your glaze results.
If you’re not familiar with how to measure specific gravity, you can grab my free guide here.
Measuring specific gravity is the essential first step before you do any flocculating or deflocculating of your glazes. Once you know the water is where you want it, you can ask yourself the 3 questions above to decide if it needs to be flocculated.
What is a Saturated Epsom Salts Solution?
Epsom salts are my flocculant of choice. Other flocculants exist but Epsom salts are easy for me to find and they work really well.
Epsom salts, like the ones you can buy at the drug store and bathe in, are made of magnesium sulphate. If you live in another part of the world, they might not be called Epsom salts. Using the chemical name “magnesium sulphate” should help you find them.
A close up digital microscope shot of epsom salts on a green piece of paper.
To use Epsom salts as a flocculant, the salts need to be dissolved in water. This breaks up the bond between the magnesium and the sulphate so the magnesium ions are free to attach themselves to the clay particles.
A saturated solution is defined as a chemical solution containing the maximum concentration of a solute dissolved in a solvent. Additional solute will not dissolve in a saturated solution.
In our case, the solute is Epsom salts and the solvent is water. So a saturated Epsom salt solution will have the highest possible number of magnesium ions per volume of water.
When you add Epsom salts to water they’ll dissolve and seemingly disappear into the liquid. If you keep adding more salts, they’ll continue to dissolve until you reach the saturation point. At this point, no more salts will dissolve and they’ll start collecting on the bottom of the container.
And that’s how you know you have a saturated solution, when you have undissolved salts on the bottom of your container.
The reason why we want our Epsom salt solution to be saturated is because we don’t want to dilute our glaze with more water. Remember, we’ve already determined that we have the right amount of water by measuring specific gravity. If our Epsom salt solution is weak, we’ll be adding more water than necessary to the glaze.
The water is just a medium for carrying the magnesium ions into our glaze bucket, so they can attach themselves to the clay particles. We want maximum ions, minimum water.
When using the saturated solution to flocculate your glaze, you use the liquid on top and leave the undissolved salt crystals on the bottom.
I keep a labelled container full of saturated Epsom salts solution in my glazing area. If I ever notice there are no crystals on the bottom, I just add more salt to the mixture and stir.
How to Make a Saturated Epsom Salt Solution:
- Grab a container with a lid that you’ll store your saturated Epsom salt solution in. A clear container makes it easy to see if you have undissolved salt crystals on the bottom.
- Fill the container with water.
- Add some Epsom salts and stir.
- Watch as you’re stirring to see if all the salts are dissolving.
- If all the salts dissolve and disappear, add more salts and repeat.
- Once you have a layer of salt sitting on the bottom of your container, you know you have a saturated solution.
- Always check for undissolved salts before flocculating. More salt can dissolve over time. If there aren’t any, add more Epsom salts and stir.
- When flocculating, use the liquid on top and leave the undissolved salts on the bottom.
Do I Need to Use Hot Water?
If you use hot water while creating your saturated solution, the salts will dissolve faster and more salts will dissolve overall.
Solubility (the ability to dissolve) increases with temperature. But when the water temperature comes back down to room temperature, the solubility will decrease and some of the previously dissolved salts will precipitate out of solution. Meaning they’ll turn back into solid, undissolved salts.
This is perfectly ok because when we have undissolved salts at the bottom of our container, we know we have a saturated solution.
You don’t have to use hot water when making your saturated Epsom salt solution but doing so will speed up the dissolving process. I do recommend using hot water if you’re making the solution right before you’re going to use it.
But then it’ll cool over time and be room temperature every other time you use it. So if you only have cold water to start with, you’ll be fine.
What matters more is that you have undissolved salts at the bottom of your container when you go to flocculate your glaze. That’s how you know you have a saturated solution. (Have I driven that point home yet? Repeat after me… )
Re-crystallized Epsom salts. An undisturbed saturated solution will re-crystallize over time, espcecially with fluctuations of temperature.
How Much Epsom Salt do you Add to the Water?
You might want to know how much salt to add to a certain amount of water. You might think this would be something I would weigh out and measure carefully.
But I don’t. I just add more than I think I need and stir. If there’s undissolved salt on the bottom, I stop. If there isn’t, I add more salt.
Since I’m always using it and filling it back up again, and water will continue to evaporate and salt will continue to dissolve and precipitate, I’m pretty cavalier about it.
The truth is, it doesn’t normally take much Epsom salt solution to flocculate a glaze so even if it’s slightly undersaturated, it will usually give the glaze enough magnesium ions to become flocculated.
If you don’t like the idea of eyeballing it and would rather measure, then here’s what Wikipedia says:
-
- 26.9 g/100 mL (0 °C)
- 35.1 g/100 mL (20 °C)
- 50.2 g/100 mL (100 °C)
It would be a fun experiment to try it yourself and verify what Wikipedia says. Start with 100 mL (=100g) of water and 100g of dry Epsom salts in separate containers. Take the temperature of the water.
Add a spoonful of Epsom salts to the water and stir.
If all the salts dissolve, add another spoonful and stir. Once the salts stop dissolving, weigh how much dry salt you have left. Subtract that from 100 and you know how much Epsom salts you need to make a saturated solution with 100mL of water.
If you do this experiment, I’d love it if you would post your results in the comments section below or post in my free Facebook group. There are also some flocculating videos posted in the group that you may find helpful.
If this post helped or inspired you, please share it with others.
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Once you have your saturated solution of Epsom salts, how do you know how much to add to your glaze bucket? Also, what is “hardpanning”.
Start with a drop at a time for a small glaze test and a teaspoon at a time for a bucket of glaze. Hard panning is when all the particles settle into a hard layer on the bottom of the bucket and are very hard to mix.
Sue
:I always check the specific gravity for my glazes at the start of each session. If you flocculate your glaze, what happens when you return to it a month later and want to check the specific gravity again? Can you get an accurate reading from the already flocculated glaze?
Hi
Very interesting. One of my decorating glazes,stain is separating to the bottom
I m not sure what SG it should be..sorry if you mentioned it and I didn t see it.
Does flocculating a glaze with little or no clay still work? You only speak of clay particles being affected by charge.
Only the clay particles in a glaze are affected by flocculation. So your glaze recipe must first have clay in it (bentonite, kaolin or ball clay) for flocculation to work.
Sue
Also, what is a prudent, incremental amount to add to a glaze?
Start with a drop at a time for a small glaze test and a teaspoon at a time for a bucket of glaze.
Sue
When do you add Epsom salts versus bentonite to a glaze that has hard panned???
Hi Debra,
I add bentonite if the glaze doesn’t already have bentonite or at least 10% clay in it. If it has enough clay, then I know it just needs to be flocculated.
Sue
This was very interesting Sue! Thank you for sharing your knowledge so freely with the rest of us!
Thank you for sharing Sue
When I was in college we used zircopaz to stop hard panning. Is that not used any more? If so why not? Thank you
Hi Diana,
I haven’t heard of using Zircopax for hard-panning before. Zircopax is an opacifier (whitener) so you’d be changing the colour of your glaze if you used it for hard-panning. I wonder if you’re thinking of something else?
Sue
So useful! I wish I had read it before adding half a cup of epsom salt solution to a big bucket. Now my glaze is so thick that the spatula stands in it and it takes forever to dry on the pot.
Will my glaze application be too thick – it sure looks like it – ? Is there any way to counteract some of it?
Thanks –
Hi Priska,
You could try counteracting with a very small amount of Darvan, which is a deflocculant and the opposite of Epsom salts, which are a flocculant. You wouldn’t want to go back and forth like this all the time, but in a pinch it might help.
Sue
Hi Sue,
Your posts are so helpful! You might have answered this, but will adding Saturated Epsom Salts change specific gravity.
Thank you.
No, the only thing that changes specific gravity is changing the water content. Flocculating and deflocculating doesn’t affect specific gravity.
Hi Sue,
I just had to make up a saturate so measured it as I went. The figures you gave from Wikipedia were absolutely spot on! 35.1 at 20C.
Laurence
Great, thanks for confirming!
Sue
I think I have read that Epsom salts is not working as flocculant for glazes that have Kaolin only as clay ( works if ball clay is used). If kaolin is used as clay in the glaze then one may have to use vinegar as flocculant. What is your experience Sue? By the way….excellent articles and website, I cannot really empathise enough how helpful it is….you clearly have a very good understanding of what you describe….so much thank you….
I use Epsom salt solution to flocculate glazes with just kaolin in them all the time. Sorry for the extreme delay in response, I must have missed this comment.
Sue
Muchas gracias por compartir sus conocimientos!!!!!
This was a very helpful post – thank you! I used it today to fix a beloved glaze that was threatening to hardpan. Really appreciate the clear and complete info.
This was a very helpful post. Thank you I do have a question. With glazes that use Epsom salt in them, can you use a soft brick kiln or do you have to use a hard brick like salt and soda firing?
Soft brick kiln is perfectly fine. The epsom salts don’t react with the bricks like sodium in atmospheric firings.
supper helpful- thanks so
much for sharing your knowledge so
clearly!
How do you know when you have added enough floculent?
Thanks by the thousand.No one explained the physics behind the chemistry before!My teacher at school swore by the stuff but it never worked when I had those problems in my own studio years later.Now I know why.
Well that was 50 years ago.
Just wrapped an entire career built on tableware cone 10 R, limit formula,calculation, ASTM testing. Now I can actually take time to surf the net!
I used and still use Veegum as it was easy to calibrate , didn’t crystallize or mold plus,didn’t spot my porcelains.like bentonite. I always feared dumping soluble material into a glaze mix because I’d had fit and scumming problems that way with Gerstley Borate far back before calculation/computers gave me a way forward.BTW I am through making glazes in my retirement years.More like potions of crushed rock for my own amusement. Veegum allows for a great deal of amusement.
I have a question about using flucculant for slip. I am doing my first attempt at using slip for decorating. I saw a video stating that I need a flucculant for the slip. Is this flucculant good to use for slip as well (not only for glazes)? Thank you. Barbara