Glazes and glazing are finicky. From materials to mixing, and from application to firing, the spectrum of failure to perfection is vast. Learn how to control one of the most overlooked aspects of better glazing, proper mixing.
Defining the Terms
Particles: The individual solid units of material that make up a ceramic material before it is shaped and fired. Key characteristics of ceramic particles include size, shape, surface area, and distribution. Particle properties directly affect packing density during forming, shrinkage and densification, mechanical strength and porosity.
Settling: The process in which the solid particles in a liquid glaze suspension sink to the bottom of the container over time due to gravity.
Suspension: In ceramics, glazes are slurries. They consist of water and undissolved powders kept in suspension by clay particles.
Water: The most important ceramic material, it is present in every clay body, slip, and glaze. It either helps a process happen or is actively involved in a change.
Glaze Results on the Thin Side?
As a studio technician at a busy pottery studio, it’s my job to mix and maintain twenty different studio glazes. I’m also the one studio users often go to for help when their glazes don’t fire as they had hoped. Every now and then someone will show me their finished piece where the glaze doesn’t quite look right. As soon as I see it, I can tell that the issue is glaze thickness. The glaze application layer is way too thin. There are two common reasons why the application could be too thin.
The glaze contains too much water, making it thin and watery in the bucket. (This means specific gravity needs to be adjusted.)
The glaze was applied too thinly.
Issue #1 is controlled by the technicians. Issue #2 is controlled by the users.
In order to determine which issue we’re dealing with, I’ll measure the specific gravity of the glaze. This process tells me whether the water content is too high or too low. When I measure the specific gravity and it’s where it should be, then I know the glaze was applied too thinly. If the glaze is brushed on or sprayed on, it could be too thin because not enough coats were applied. But a glaze that has the right amount of water and is applied by dipping is rarely too thin. It’s usually too thick because it was held in the glaze for too long. So how can it be too thin?
The number one reason why a dipping glaze is applied too thinly is inadequate mixing of the glaze in the bucket.
Glaze Particles Will Settle
Take a minute to think about what glazes are composed of. They’re solid particles suspended in water. Some solid particles are heavy and sink to the bottom of the bucket quickly. Others are light and take some time to settle. But all of our glaze particles are heavier than water and will eventually settle (1). An undisturbed bucket of glaze that’s been sitting long enough will have a layer of water on the top and solid particles sitting below the water. The heaviest particles will sink to the bottom, and the lighter particles will layer themselves above that, but below the water.
In order for the glaze to fire as intended, the proportion of particles applied to the piece must be equivalent to the glaze recipe. Glaze recipes are formulated very carefully so the glaze will melt at a specific temperature and will appear a certain way.
If you dip a piece of pottery into the top of the bucket when the heavy particles have sunk to the bottom, and the water has risen to the top, then the application layer will have too much water, not enough solid particles, and the proportion of solid particles will be out of whack.
The heavy particles, light particles, and water must be mixed together really well so the glaze becomes homogenized. This means that every inch of the glaze bucket has the exact same composition, top to bottom.
Use a High-Speed Mixer
The way to achieve a well-mixed bucket is with a high-speed mixer. I recommend high-speed mixing glazes at the beginning of every glaze session. Examples of high-speed mixers are a drill with a paint mixing attachment or a hand/immersion blender (2). I use a cordless drill for test-sized batches. For larger batches, I use a corded drill (more power) and a larger paint mixer attachment.
During a typical glazing session, I high-speed mix each glaze once, and then use a manual mixing device (like a rubber spatula) in between dips. The initial blending is important, especially if your glaze has been sitting still for a few days or more. If my glazing session takes more than a couple of hours, I will often high-speed mix again partway through, just to be sure the batch stays thoroughly mixed.
Don’t Have Access to a High-Speed Mixer?
If you have your own studio space with your own tools and your own glazes, I recommend purchasing a high-speed mixer if you don’t have one. Hand blenders are often abundant in second-hand shops, where they are fairly affordable.
Is it really necessary? Think about the difference between beating eggs with a fork versus with an electric egg beater. High speed is much faster and more effective. But, if you don’t work in a studio with access to high-speed mixing devices, and it wouldn’t be appropriate to bring your own, it’s up to you to use the mixing tools available to you and mix the glazes as well as you can.
The best way to do this is to keep on stirring! When you first think it’s been mixed enough, don’t stop. Keep stirring. Most people (myself included) are naturally inclined to stir a bucket of glaze for 10–20 seconds. I witnessed a lot of glaze bucket stirring, and also timed myself to see when I would naturally stop. I stopped at 20 seconds and would consider myself on the thorough side—making sure to scrape the whole bottom and the sides to mix it all in.
Turns out, this wasn’t quite long enough. How long is enough time? Only one way to find out.
A Stirring Experiment
In order to show our studio users the importance of mixing well, I took one of our studio glazes and did an experiment. You can do this experiment with your glazes, too.
I chose our Sapphire Blue glaze, which contains 4% rutile and appears very different when it’s thin versus thick.
I used a stirring stick provided by our studio—a 11/2-inch diameter dowel.
The glaze bucket is a 30-gallon, gray plastic garbage can. The bottom isn’t flat—it has a hump in the center with a moat around it (you know the ones).
The glaze batch quantity is around 20 kg (44 lbs).
I stirred the glaze for 10 seconds, going back and forth, side to side, front to back, over the hump, in the moat, clockwise, and counter-clockwise. It seemed like a fairly solid effort.
I stopped stirring and quickly dipped a test tile, holding it in for 6 seconds.
I immediately resumed mixing for another 20 seconds = 30 seconds total. I stopped, dipped another test tile for a 6-second dip, and started stirring again.
This time I stirred for 30 more seconds, for a total of 60 seconds. This felt like a really long time. My goldfish brain started to get bored, but I kept mixing.
When I finally got to 60 seconds, I dipped a third test tile for 6 seconds and called it a day.
How Long Should You Stir?
Figure 3 shows the fired results and a great visual example of the effectiveness of mixing: Left tile = 10 seconds, middle tile = 30 seconds, right tile = 60 seconds.
As you can see, 10 seconds is definitely not enough time to bring all of those heavier particles up to the top. It’s very thin: too much water at the top of the bucket.
At 30 seconds, it’s much closer to being well mixed, but when you compare it to the 60-second mix, you can see that it’s not quite homogenized yet. It’s a bit splotchy.
At 60 seconds, the glaze is quite uniform from the top of the tile to the bottom. This is ideal. If you want variation with the thickness, you can do that intentionally using different application methods.
After the initial mix, you generally won’t need to mix that much again throughout the glazing process, unless it’s a long glazing session. In between dips, 5–10 seconds of new mixing should be plenty. And yes, I do stir the glaze before each and every dip. It only takes a few seconds for the particles to start settling again.
Just Keep Stirring
So based on these results, at our studio, I recommend stirring each bucket for at least 60 seconds before glazing. The time can vary for different-sized buckets and different stirring devices.
Tip: Find a 60-second tune that you can sing or hum while you’re stirring. The intro to Queen’s “Bohemian Rhapsody” is pretty close. And so is the chorus of “Dancing Queen” by ABBA.
Now I have a visual example that I can show people when I suspect their glaze wasn’t mixed well enough. Seeing the visual results of this test is really helpful to demonstrate the importance of mixing well.
Remember, 60-second stirring is only applicable if you DON’T have access to a high-speed mixer. With a high-speed mixer, it’s even more effective and much faster! But if you’re going old fashioned . . . Just keep stirring!
the author Sue McLeod has been a studio potter since 2010 and worked as the ceramics studio technician at a community studio in Victoria, British Columbia, Canada, for 6 years. She now works full time from home, researching glazes in her home studio and teaching three online glaze courses: Glaze Mixing Essentials, Mastering Glaze Consistency, and The Art of Glaze Chemistry. She also runs a large, free, glaze-support community on Facebook titled “Understanding Glazes with Sue.” Read more ceramics articles and register for her online glaze courses at suemcleodceramics.com.
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Glazes and glazing are finicky. From materials to mixing, and from application to firing, the spectrum of failure to perfection is vast. Learn how to control one of the most overlooked aspects of better glazing, proper mixing.
Defining the Terms
Particles: The individual solid units of material that make up a ceramic material before it is shaped and fired. Key characteristics of ceramic particles include size, shape, surface area, and distribution. Particle properties directly affect packing density during forming, shrinkage and densification, mechanical strength and porosity.
Settling: The process in which the solid particles in a liquid glaze suspension sink to the bottom of the container over time due to gravity.
Suspension: In ceramics, glazes are slurries. They consist of water and undissolved powders kept in suspension by clay particles.
Water: The most important ceramic material, it is present in every clay body, slip, and glaze. It either helps a process happen or is actively involved in a change.
Glaze Results on the Thin Side?
As a studio technician at a busy pottery studio, it’s my job to mix and maintain twenty different studio glazes. I’m also the one studio users often go to for help when their glazes don’t fire as they had hoped. Every now and then someone will show me their finished piece where the glaze doesn’t quite look right. As soon as I see it, I can tell that the issue is glaze thickness. The glaze application layer is way too thin. There are two common reasons why the application could be too thin.
Issue #1 is controlled by the technicians. Issue #2 is controlled by the users.
In order to determine which issue we’re dealing with, I’ll measure the specific gravity of the glaze. This process tells me whether the water content is too high or too low. When I measure the specific gravity and it’s where it should be, then I know the glaze was applied too thinly. If the glaze is brushed on or sprayed on, it could be too thin because not enough coats were applied. But a glaze that has the right amount of water and is applied by dipping is rarely too thin. It’s usually too thick because it was held in the glaze for too long. So how can it be too thin?
The number one reason why a dipping glaze is applied too thinly is inadequate mixing of the glaze in the bucket.
Glaze Particles Will Settle
Take a minute to think about what glazes are composed of. They’re solid particles suspended in water. Some solid particles are heavy and sink to the bottom of the bucket quickly. Others are light and take some time to settle. But all of our glaze particles are heavier than water and will eventually settle (1). An undisturbed bucket of glaze that’s been sitting long enough will have a layer of water on the top and solid particles sitting below the water. The heaviest particles will sink to the bottom, and the lighter particles will layer themselves above that, but below the water.
In order for the glaze to fire as intended, the proportion of particles applied to the piece must be equivalent to the glaze recipe. Glaze recipes are formulated very carefully so the glaze will melt at a specific temperature and will appear a certain way.
If you dip a piece of pottery into the top of the bucket when the heavy particles have sunk to the bottom, and the water has risen to the top, then the application layer will have too much water, not enough solid particles, and the proportion of solid particles will be out of whack.
The heavy particles, light particles, and water must be mixed together really well so the glaze becomes homogenized. This means that every inch of the glaze bucket has the exact same composition, top to bottom.
Use a High-Speed Mixer
The way to achieve a well-mixed bucket is with a high-speed mixer. I recommend high-speed mixing glazes at the beginning of every glaze session. Examples of high-speed mixers are a drill with a paint mixing attachment or a hand/immersion blender (2). I use a cordless drill for test-sized batches. For larger batches, I use a corded drill (more power) and a larger paint mixer attachment.
During a typical glazing session, I high-speed mix each glaze once, and then use a manual mixing device (like a rubber spatula) in between dips. The initial blending is important, especially if your glaze has been sitting still for a few days or more. If my glazing session takes more than a couple of hours, I will often high-speed mix again partway through, just to be sure the batch stays thoroughly mixed.
Don’t Have Access to a High-Speed Mixer?
If you have your own studio space with your own tools and your own glazes, I recommend purchasing a high-speed mixer if you don’t have one. Hand blenders are often abundant in second-hand shops, where they are fairly affordable.
Is it really necessary? Think about the difference between beating eggs with a fork versus with an electric egg beater. High speed is much faster and more effective. But, if you don’t work in a studio with access to high-speed mixing devices, and it wouldn’t be appropriate to bring your own, it’s up to you to use the mixing tools available to you and mix the glazes as well as you can.
The best way to do this is to keep on stirring! When you first think it’s been mixed enough, don’t stop. Keep stirring. Most people (myself included) are naturally inclined to stir a bucket of glaze for 10–20 seconds. I witnessed a lot of glaze bucket stirring, and also timed myself to see when I would naturally stop. I stopped at 20 seconds and would consider myself on the thorough side—making sure to scrape the whole bottom and the sides to mix it all in.
Turns out, this wasn’t quite long enough. How long is enough time? Only one way to find out.
A Stirring Experiment
In order to show our studio users the importance of mixing well, I took one of our studio glazes and did an experiment. You can do this experiment with your glazes, too.
I stirred the glaze for 10 seconds, going back and forth, side to side, front to back, over the hump, in the moat, clockwise, and counter-clockwise. It seemed like a fairly solid effort.
I stopped stirring and quickly dipped a test tile, holding it in for 6 seconds.
I immediately resumed mixing for another 20 seconds = 30 seconds total. I stopped, dipped another test tile for a 6-second dip, and started stirring again.
This time I stirred for 30 more seconds, for a total of 60 seconds. This felt like a really long time. My goldfish brain started to get bored, but I kept mixing.
When I finally got to 60 seconds, I dipped a third test tile for 6 seconds and called it a day.
How Long Should You Stir?
Figure 3 shows the fired results and a great visual example of the effectiveness of mixing: Left tile = 10 seconds, middle tile = 30 seconds, right tile = 60 seconds.
As you can see, 10 seconds is definitely not enough time to bring all of those heavier particles up to the top. It’s very thin: too much water at the top of the bucket.
At 30 seconds, it’s much closer to being well mixed, but when you compare it to the 60-second mix, you can see that it’s not quite homogenized yet. It’s a bit splotchy.
At 60 seconds, the glaze is quite uniform from the top of the tile to the bottom. This is ideal. If you want variation with the thickness, you can do that intentionally using different application methods.
After the initial mix, you generally won’t need to mix that much again throughout the glazing process, unless it’s a long glazing session. In between dips, 5–10 seconds of new mixing should be plenty. And yes, I do stir the glaze before each and every dip. It only takes a few seconds for the particles to start settling again.
Just Keep Stirring
So based on these results, at our studio, I recommend stirring each bucket for at least 60 seconds before glazing. The time can vary for different-sized buckets and different stirring devices.
Tip: Find a 60-second tune that you can sing or hum while you’re stirring. The intro to Queen’s “Bohemian Rhapsody” is pretty close. And so is the chorus of “Dancing Queen” by ABBA.
Now I have a visual example that I can show people when I suspect their glaze wasn’t mixed well enough. Seeing the visual results of this test is really helpful to demonstrate the importance of mixing well.
Remember, 60-second stirring is only applicable if you DON’T have access to a high-speed mixer. With a high-speed mixer, it’s even more effective and much faster! But if you’re going old fashioned . . . Just keep stirring!
the author Sue McLeod has been a studio potter since 2010 and worked as the ceramics studio technician at a community studio in Victoria, British Columbia, Canada, for 6 years. She now works full time from home, researching glazes in her home studio and teaching three online glaze courses: Glaze Mixing Essentials, Mastering Glaze Consistency, and The Art of Glaze Chemistry. She also runs a large, free, glaze-support community on Facebook titled “Understanding Glazes with Sue.” Read more ceramics articles and register for her online glaze courses at suemcleodceramics.com.
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