How to Make a Saturated Epsom Salt Solution to Flocculate your Glazes

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.

  1. It increases the viscosity of the glaze (making the glaze thicker)
  2. It helps to keep all the other particles suspended in the bucket
  3. 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.

  1. Are you trying to thicken your glaze?
  2. Does your glaze need help staying suspended in the bucket? Is your glaze hard-panning?
  3. 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:

  1. 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.
  2. Fill the container with water.
  3. Add some Epsom salts and stir.
  4. Watch as you’re stirring to see if all the salts are dissolving.
  5. If all the salts dissolve and disappear, add more salts and repeat.
  6. Once you have a layer of salt sitting on the bottom of your container, you know you have a saturated solution.
  7. Always check for undissolved salts before flocculating. More salt can dissolve over time. If there aren’t any, add more Epsom salts and stir.
  8. 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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If you love learning about and discussing glazes, I’d like to invite you to my free social learning Facebook group called Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes and fixing various glaze issues. Please join!

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