What is bentonite?

Bentonite is a very fine particle clay that’s often used as a suspending agent in glazes. Glazes need clay in them to keep all the other materials suspended in the bucket. If your glaze recipe doesn’t have enough clay to stay suspended, it will “hard-pan,” forming a hard, impossible to mix layer on the bottom of the bucket. Bentonite is a very powerful suspending agent and 1-2% is usually enough to keep a glaze suspended.

The Warnings You Get About Bentonite

When I was in school and learning to mix my own glazes, I was warned: “Make sure you add bentonite to the DRY materials and mix them together BEFORE you add any water.”

It was good advice.

The reason for the warning is when bentonite gets wet it swells and gels and sticks together and becomes tricky to incorporate into the rest of the glaze.

If you were to weigh out some bentonite, dump it into a wet glaze and start mixing, the bentonite would clump together. You would have to sieve your glaze a couple of times to try to break those clumps apart so the glaze could be homogenized again.

Nobody wants to do that, right?

But what if you can’t add the bentonite to your dry materials because you’ve already added water to your glaze…? Keep reading, friend.

A New Way to Mix Glazes

When I first learned to mix glazes, we would weigh out all the dry materials into a bucket and then add the water (adding wet to dry).

Whenever I made a glaze that contained bentonite, taking my teacher’s advice, I would add the bentonite as the last dry ingredient and then stir it in with the other dry ingredients before adding the water.

This separates all of the bentonite particles from each other. When the particles are separate, they can’t stick together so they incorporate into the glaze like they’re supposed to.

The downside to this process is it produces a lot of dust in the air, which is a potential respiratory hazard. The dust from glaze materials is harmful to the lungs and avoiding dust clouds is a good studio practice.

Then I heard that some people start mixing their glazes with the water already in the bucket. They add the dry materials directly to the water (adding dry to wet). This reduces the amount of dust in the air by wetting the dry materials right away.

Dry powder being dumped onto more dry powder creates a dusty environment. But dry powder dumped into a bucket of water is exposed to the air for a shorter amount of time. Once it’s wet, it’s no longer a respiratory hazard.

Even if you’re wearing a respirator (which you should be!) anything you can do to reduce the amount of dust released into the air will be better for you and everyone around you.

I happily adopted this new, healthier way of preparing glazes. My health is very important to me. But then what about the bentonite…?

How to Add Bentonite to a Wet Glaze?

(photos and instructions down below)

Now that my procedure had changed (because – safety first!) I could no longer mix the bentonite with the dry materials first to separate the bentonite particles so they couldn’t clump together. I mean, I could but I didn’t want to create unnecessary dust clouds.

Since I was now adding dry to wet, I didn’t know how to get the bentonite into the glaze. I asked around on various ceramic forums to see what other people were doing.

What I found out was, you can add the bentonite directly to plain water! Thank you internet! It’s so simple and convenient, I don’t know why we aren’t taught it from the beginning. At least, I wasn’t.

Some people say they add the bentonite to the water and then high-speed mix it right away. Others let it sit overnight.

I do it a bit differently. I add the bentonite and wait a couple minutes.

As I said earlier, bentonite clumps together if you add it to a wet glaze and try to mix it right away. That’s because when the bentonite particles are partially wet, they attract each other and stick together.

But if you add bentonite to plain water and DON’T TOUCH IT, each of those tiny clay particles will become fully submerged with water on all sides. Once they’re completely wet, the risk of them sticking together is gone and they’ll float freely around the water.

The process is similar to slaking down dry clay, like when you’re reclaiming trimmings. If you add dry trimmings to a lot of water and leave them still, the clay will absorb the water and all the particles will break free from each other, creating a soft layer at the bottom that can be mixed into a smooth slip.

But if you add a little bit of water to a bunch of damp or dry trimmings and then start mixing them around, the clay will stick together and become lumpy.

The key is completely dry clay coming into contact with water and staying still until it can “slake down.” The same is true for bentonite.

Adding Bentonite During the Glaze Mixing Process

So now, when I mix a new batch of glaze that contains bentonite, I calculate how much bentonite is needed first and weigh it out.

I start with water in the bucket and then I add the bentonite to the water. (I usually highlight the bentonite on my recipe page so I don’t forget to add it first).

Then I leave the water undisturbed for a minute or two while I batch out the rest of the glaze recipe (meaning, convert the recipe from % to grams). By the time I’m done, the bentonite is slaked and ready to go.

One way to tell if the bentonite is finished slaking is, it will have all sunk to the bottom of the bucket. There should be no more bentonite floating on top.

And also, as the bentonite slakes down, bubbles will often appear on the surface. If it’s still bubbling, it’s not ready yet. If it has all sunk and stopped bubbling, you’re probably good to go.

You can high speed mix if you want, but my glaze mixing space is very small so I don’t have room for mixing tools on my counter while I’m weighing out dry materials. The mixing comes after all the materials have been added.

Adding Bentonite to a Glaze That’s Already Wet

If I need to add bentonite to a glaze after it’s been mixed (to fix a hard-panning issue, for example) I just weigh out the bentonite (usually 1% of dry glaze weight), sprinkle it into a small amount of water, and wait for it to slake down.

You want enough water to fully cover the bentonite. In this example, I added 50g of bentonite to 200g of water. That’s a 4:1 ratio of water to dry bentonite. I wouldn’t use much less water than that because you want the bentonite to fully submerge. (You don’t need to weigh your water, I’m just like that!)

(I realize the numbers on the scale don’t add up! It was teeter tottering between 249 and 250 and I snapped the photo during a light moment.)

It would have slaked down quicker if I had sprinkled the bentonite more evenly or if the water level was a bit higher. It usually doesn’t take 5 minutes, more like 2-3 mins. I just wanted to make sure for the purpose of the video that it was good to go.

After a couple hours, the bentonite will settle out, leaving a layer of clear water on the top, which I remove with a syringe. I do this because I don’t want to add a lot of extra water to the glaze, I just want to add the bentonite.

The bentonite has settled and the water sitting on top can be syphoned off.

Then I label the container with the amount of dry bentonite in grams and I’m free to add it to any pre-mixed glaze as needed.

Here’s a video of the process:

After adding the wet bentonite to a glaze, it’s important to mix the glaze really well with a high-speed mixer to make sure the bentonite and the glaze become completely homogenized.

One thing I have not tried yet, but may work just fine, is to weigh out the bentonite and add it directly to the water that’s sitting on top of the settled glaze.

Any hard-panned glaze will have a decent layer of water on top. You might be able to just sprinkle the bentonite directly into the glaze and wait for it to slake down. As long as you don’t start mixing right away, I don’t see why it wouldn’t work.

Let me know if you try it!

Have you struggled with adding bentonite to your glazes in the past? Do you have a different method of getting bentonite into your wet glazes? I’d love to hear your thoughts in the comments below.

Ceramic Story-Time with Sue

Watch the Video

In this video, I read the blog post live in my Facebook group and answer questions.

Learn to Mix Glazes from Scratch

If you’ve never mixed a glaze from scratch before and want to learn, I teach an online workshop called Glaze Mixing Essentials where I show you all the steps to mix a glaze and then test a base glaze with multiple colourants. Click the link for all the details.

Join my free community

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!

Free Guide to Repeatable Glaze Results