Bentonite is a very fickle, but extremely effective material and knowing how to use it is just as important as knowing when and why to use it. Here’s the science you need to know and a mixing tutorial to get you started.
Define the Terms:
Charge Attraction: The interaction between charged particles. Oppositely charged particles attract each other, while like-charged particles repel each other.
Colloidal Particles: Particles whose size is so small that they naturally disperse in a liquid and do not settle.
Disperse: To distribute evenly throughout a medium.
Hard Pan: When glaze materials settle out of suspension and form a hard-to-mix layer on the bottom of the bucket.
Suspension: A heterogeneous mixture of solid particles dispersed in water. Bentonite’s positively charged edges are attracted to negatively charged faces of other clay particles, forming an open, house-of-cards-like network throughout the suspension.
Swelling: Expanding in size when coming into contact with water.
What is Bentonite?
Bentonite is a fine-particle clay that is often used as a suspension agent in glazes. Bentonite has colloidal properties, which means the particles are so small and light that they disperse in water and remain in suspension rather than settle out. Clays like kaolin and ball clay are also suspension agents and are commonly found in glaze recipes. Without suspension agents, glaze particles will sink and settle, forming a hard, impossible-to-mix layer on the bottom of the bucket. This is called hard panning.
Most glazes need at least 10% clay in order to stay in suspension, mix easily, and prevent hard panning. If a glaze doesn’t contain sufficient clay, then bentonite can be added instead. Just 1% bentonite has a suspending strength almost equivalent to 5% kaolin or ball clay. So if your glaze contains 0–5% clay, then it will likely need 2% bentonite to keep it in suspension. If your glaze contains 5% clay or more and it’s having trouble staying suspended, 1% bentonite is probably all it will need to stay in suspension.
Bentonite also increases the hardness of a glaze after it dries. If your glaze is really powdery after it dries, which is often due to a lack of clay, adding bentonite will help to harden and bind the glaze layer.
Since bentonite has a very high shrinkage rate and a high iron content, I don’t recommend adding much more than 2% to a glaze recipe. The high shrinkage can cause problems with cracking as the glaze dries and the iron could cause undesired speckling in the fired glaze surface.
Warnings 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 this warning is that when bentonite gets wet, it swells and gels, sticking together and forming clumps that make it 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, making the glaze lumpy. You’d have to sieve your glaze several times to break those clumps apart so the glaze could be homogenized again. Occasionally the bentonite clumps will reform even after sieving. It’s a pain and a big waste of time. But what if you can’t mix the bentonite with your dry materials first?
A while back, I changed how I mix my glazes. I used to add wet to dry—where I would weigh out all the dry materials first and then add water. This made it easy to mix the bentonite in with the dry materials before adding the water. Unfortunately, this method creates unnecessary dust clouds and I wanted my studio to be as dust-free as possible, so I changed my process. I now add dry to wet—meaning I start with the water in the bucket, weigh out each dry material and add them directly to the water. This greatly reduces the dust in my studio because I’m not dumping dry powder on top of dry powder. All the dry powder gets wet instantly. Once it’s wet, it’s no longer a dust hazard.
This change meant I could no longer mix the bentonite with the dry materials first. How was I going to separate the bentonite particles so they wouldn’t clump together? Luckily, it’s possible to add bentonite to a wet glaze, but you can’t just dump it in like any other material.
Understanding How Bentonite Works
As I said earlier, bentonite clumps together if you add it to a wet glaze then try to mix it right away. That’s because when bentonite particles are partially wet, they attract each other and stick together, just like the clay we use to make pots. All clay particles generally have an opposite charge on the edges and faces that attracts them and holds them together like magnets in the presence of water. This attraction is what gives clay its plasticity so we can form and shape it. But if there is enough water in between clay particles, the attraction charge is weakened and they no longer stick together; they flow freely.
With bentonite, the swelling that occurs when the particles come into contact with water can prevent the particles from becoming wet on all sides. If bentonite particles come into contact with each other before they’re wet on all sides, they‘ll attract each other, stick together, and form a barrier that prevents water from fully surrounding each particle.
But, if you add bentonite to plain water and don’t touch it—just let it absorb, each of those tiny clay particles will become fully submerged, with water surrounding each one on all sides. Once they’re completely wet, the attraction charge is weakened, the risk of the bentonite particles clumping together is gone, and they’ll disperse and float freely around the bucket.
The process is similar to slaking down a dry clay body. If you add dry clay trimmings to water and let them sit still, the clay will absorb the water and all the particles will break free from each other. This creates a soft layer that can be mixed into a smooth slip. But if you add a small amount of water to a bunch of damp or dry trimmings and then mix 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.
Once you understand how it works, it’s easy to add bentonite to your wet glaze. Here are a couple of common scenarios where you would need to add bentonite to a wet glaze and the best way to do it.
Scenario #1: Adding Bentonite While Mixing a Glaze From Scratch
When mixing a new batch of glaze that contains bentonite, calculate how much bentonite is needed first and weigh it out. Start by adding the water needed for the glaze to the bucket and then add the bentonite to the water. Leave the water undisturbed for a minute or two. One way to tell if the bentonite is finished slaking down is that all of it will have sunk to the bottom of the bucket. There should be no more bentonite floating on top. Also, as the bentonite slakes down, air bubbles will often appear on the surface. If it’s still bubbling, it’s not ready yet. Once all the bentonite has sunk and any bubbling on the surface has stopped, give the water a quick stir and then start adding the rest of the glaze materials to the bucket.
Scenario #2: Adding Bentonite to an Existing Glaze
To add bentonite to a glaze after it’s been mixed, just weigh out the bentonite, sprinkle it into a small amount of water, and wait for it to slake down. You want just enough water to fully cover the bentonite. The bentonite will settle out, leaving a layer of clear water on the top, which can be removed with a syringe or by carefully pouring it off. You don’t want to add a lot of extra water to your glaze, you just want to add the bentonite.
You can keep a container of wet bentonite handy in your studio for whenever you need it. Make sure you label the container with the dry weight of bentonite in grams. After adding wet bentonite to your 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. Tip: If your glaze has a significant layer of clear water on top (which is common for glazes that have settled and require bentonite) you can weigh out the bentonite and sprinkle it directly onto the glaze water. Remember, don’t touch it. Wait for it to slake down and then mix it into your glaze.
the author Sue McLeod is a potter and studio technician in Victoria, British Columbia, Canada. She teaches online glaze classes and writes articles about glazes and studio tips on her blog at www.suemcleodceramics.com.