There’s nothing worse than a cloudy clear glaze muddying up your surfaces. Learn some of the factors that affect the transparency of a glaze, and an easy solution to a common cause of cloudy clears.
Defining the Terms
Opacity: The degree to which something reduces the passage of light.
Translucency: The quality of allowing light, but not detailed shapes, to pass through.
Transparency: The quality of being easily seen through.
Transparency, Translucency, and Opacity
Glazes can have varying levels of transparency, translucency, and opacity. Transparency and opacity are opposites. Translucency falls in between. Transparent means all light can pass through, and we can clearly see what’s on the other side (like a clear glass window). Translucent means some light can pass through, but we can’t clearly see what’s on the other side (like sheer curtains or a frosted glass window). Opaque means no light can pass through, and we can’t see what’s on the other side (like heavy drapes or a boarded-up window). Clay can be translucent or opaque. Clay can’t be transparent. Glazes can be transparent, translucent, or opaque. They can also have varying degrees of transparency and opacity. Clear glazes can be transparent and translucent. Clear glazes can’t be opaque, by definition. White and colored glossy glazes can be transparent, translucent, or opaque. Matte glazes can only be translucent or opaque. Matte glazes can’t be transparent, by definition. Here’s a chart to lay out what I just explained above.
The best clear glazes are completely transparent and clearly show the clay and designs underneath. The transparency of a glaze can be reduced when it contains particles that block light from passing through. The more light-blocking particles in a glaze, the less transparent it will be.
Reasons Why Your Clear Glaze Isn’t Completely Clear
If your clear glaze isn’t all that clear, here are a few common reasons why:
A It could be underfired.
B It could be the chemistry of the recipe.
C It could be that your application is too thick.
Glaze application thickness is the easiest of these issues to test and solve and I’ll explain how in this article. But first I’ll talk about A and B because reducing your glaze application thickness won’t make a difference if the issue is due to underfiring or chemistry.
A. It’s not clear because it’s underfired.
An underfired glaze is a glaze that’s not completely melted yet. Glazes are made of powdered materials that melt in the kiln. The powdered materials start out completely opaque in their powdered form. As the temperature increases during the firing, those powders melt, dissolve, and cool into glass. If your glaze isn’t completely melted, those unmelted particles are going to block the amount of light that can pass through. Small differences in firing temperature can impact glaze results. If the clarity of your clear glaze varies depending on placement in the kiln, it’s possible that your kiln has hot and cool spots that are affecting your glaze.
The first step is to figure out if your kiln is underfiring by using witness cones. I recommend putting them in multiple places throughout your kiln so you can see if there’s any variation of temperature throughout. If your kiln’s underfiring, you’ll want to increase your firing temperature.
Underfiring isn’t good for your clay or glazes. Be careful that you don’t increase your firing temperature too much so that it’s overfiring. Overfiring can have a negative impact on your other glazes (causing them to run) or on your clay body (causing bloating). I always fire to the temperature that’s best for my clay body and then make sure I’m using glazes that also work at that temperature.
You don’t want to change your whole firing for just one glaze because it will affect so many other things. It’s much simpler to change the glaze. If your witness cones tell you that your kiln’s firing to the right temperature for your clay and glazes, then you can rule out temperature as your issue and focus on the glaze itself.
B. It’s not clear because of the chemistry.
Glossy glazes without any colorant or opacifier in them can range from clear to white. In other words, they range in opacity and transparency. The appearance of any particular glaze is based on its chemistry.
In figure 1, you can see a bunch of glaze tests that I fired where the only differences between them are the silica and alumina levels. These glaze tests don’t contain any colorants or opacifiers, they’re all made from the same ingredients and they’re all chemically identical in every way except for two factors: silica (SiO2) and alumina (Al2O3).
These test tiles are arranged in a specific way. Moving from left to right across each row, each tile has small additions of silica. The first glaze on the left of each row has slightly more EPK than the first glaze in the row below it. So moving from left to right, silica is increased. Moving from bottom to top, EPK is increased. I won’t get into too much chemistry in this article because there’s a lot that would need to be explained to fully understand the differences between all these test tiles.
The reason I’m showing this to you is so you can see the small window of really clear glazes that exist within all of these glaze possibilities. As you can see, there’s a wide range of whiteness versus clearness (meaning opacity vs. transparency). Some are matte, some are glossy, some are running, some are underfired, and some are crazed. The clearest glazes fall on the 2nd row from the bottom (0.4 Al2O3) in the middle (2). But out of 50 different base recipes (all fired to cone 6) there are only these two truly clear glazes. The rest have some degree of whiteness to them. My point is that formulating a truly clear glaze isn’t all that easy because a clear glaze can be just a few grams of material away from being white and milky. Because of this, it’s really important that you’re weighing out materials accurately when mixing your clear glaze. Make sure you’re using a good scale and weigh each material carefully.
If you do find a great clear glaze recipe that fits your clay body, hang onto it and use the steps in part C below to make sure your results are always as clear as can be.
If your clear glaze isn’t quite clear and you determine the issue isn’t temperature or glaze thickness (that I’ll describe below), and you’re weighing your materials accurately, then your issue could be chemistry. It’s possible that it’s just not a very clear glaze to begin with. It may have been “clear enough” for the person who created it. Maybe they were glazing porcelain and couldn’t see the milkiness or cloudiness. If it’s not enhancing your pieces then you’d be doing yourself a favor to move on and find a new one. Chemistry is chemistry and unless you know how to manipulate the chemistry to fix it, it’s probably not going to get much clearer.
C. It’s not clear because it’s too thick.
Some glazes are very clear if they’re thin enough, but they go cloudy when thick (3). A super transparent glaze can still contain tiny, light-blocking bubbles or particles throughout the glass but when you have a thin layer, there are so few of them that they aren’t visible to the eye, and the glaze can appear completely transparent. If the thickness of the glaze increases, those tiny particles also increase and eventually start clouding up the glaze. Figure 4 shows some squiggles to illustrate what I mean.
Imagine that these squiggles are tiny light-blocking bubbles or particles. In a thin layer, they’re spaced out and don’t really impede the ability of light to pass through. But as you add more layers, more particles fill in the spaces and the glaze layer becomes hazy, milky, or cloudy as you can see in the bowl.
When I see good, clear results sometimes but not all the time, I know that the issue is not with the clear glaze itself. The issue is with the application thickness. And the best way to control application thickness is by measuring and maintaining the glaze’s specific gravity (SG).
I did some tests to figure out our studio’s clear glaze’s optimal specific gravity. I used a dark stoneware so the clouding is more obvious (Plainsman M390). All of these tiles were dipped for six seconds. The difference between them is their specific gravity (SG), meaning their water content. Figure 5 shows the thinnest glaze layer on the left, the SG is 1.4 g/mL (the most amount of water). The center tile SG is 1.5 g/mL (more water). The tile on the right has the thickest glaze layer. It has a specific gravity of 1.6 g/mL (and the least amount of water). See how the glaze clouds up with an increase in specific gravity (and decrease in water)?
I admit that this isn’t the clearest glaze out there, and a dark clay body will always enhance a clear glaze’s imperfections (6). But it works pretty darn well and doesn’t craze on most of the clay bodies that I’ve tested (Plainsman M340, M370, M390) so I’m happy with it. The way to get the best results with this particular clear glaze is to keep the specific gravity around 1.4 g/mL. And remember that these tiles were dipped for six seconds each (7), which is longer than I would normally dip my pots. Holding a piece in the glaze for less than six seconds will give an even thinner layer.
The reason I hold test tiles in for longer than my pots is because small test tiles will generally absorb less glaze than a large piece of bisque. I would rather see a thicker glaze layer on a test tile, knowing that it’s going to be a closer representation of the results on a large piece. And since this is a studio glaze where lots of beginners are glazing, the glaze results tend to naturally be on the thicker side. When you’re first learning to glaze, it’s harder to dip really fast. There’s definitely an art to it.
If your clear glaze goes cloudy when it’s thick, follow the steps below to determine the best water content for your glaze and keep it at its optimal level.
Use Specific Gravity To Improve Clear Glazes
Measuring and maintaining specific gravity is the easiest way to make sure your glaze results are always consistent. When your glaze always has the same ratio of water to solids, then your application will be consistent as well.
So how do you measure and adjust specific gravity? Use a graduated cylinder or syringe to weigh any volume of glaze. Divide the grams by the milliliters to get the specific gravity. You can also download my free step-by-step guide to keep for reference. Once you’re familiar with the steps for measuring specific gravity, here’s what to do with your clear glaze:
a. Measure the specific gravity as it is right now, without changing anything. Dip a test tile and record the SG on the tile. You may want to brush some underglaze on your tile so you can really see how clear your glaze is, especially if you’re using a white clay body.
b. What’s your SG? If it’s higher than 1.4 g/mL then continue with the steps below. If it’s at 1.4 g/mL or less, then your glaze already has plenty of water in it and I wouldn’t add any more. 1.4 g/mL is the lowest I keep the SG for regular glazes. Some special effect glazes need a lower SG.
Note: If it’s your first time adjusting specific gravity, I recommend mixing a small test batch or removing a small sample of your glaze
from the main bucket to experiment with. I don’t want you to mess with a large bucket of glaze in case you don’t like the results.
c. If your SG is higher than 1.4 g/mL, add a very small amount of water to your glaze. Always start small until you’re comfortable with how much water it takes to reduce the SG by a certain amount.
d. Mix well and measure SG again. Dip another test tile and record the SG on the tile.
e. If it’s still above 1.4 g/mL repeat the steps above, dipping a tile with each adjustment as you bring your SG down to 1.4 g/mL.
f. Fire all your test tiles and you’ll see how glaze thickness and SG affect your clear glaze. Pick your favorite and then adjust your main bucket of clear glaze to match.
g. I generally use the highest SG that gives me the effect I want. So if your glaze is crystal clear at 1.45 g/mL and 1.4 g/mL, I would go with 1.45 g/mL as my target SG for that glaze. Too much water in a glaze can lead to application issues.
Note: Your glaze sample might become really thin and watery while performing these tests. If that’s the case, you may need to flocculate your glaze with a saturated Epsom salt solution. This will thicken your glaze back up again for better application. A thin glaze in the bucket doesn’t always mean it has too much water. Once you’ve determined the best specific gravity for your clear glaze, write it on your glaze bucket, on your test tile (7), and on your glaze recipe, then be sure to keep your SG at that number.
I recommend measuring specific gravity on a regular basis so you’re always going to have the best results. Since water naturally evaporates, it’s common for your SG to increase if you haven’t glazed in a while. I measure SG every time I glaze my pots (less than once a month). If you’re glazing all the time, I would measure SG every couple of weeks and especially every time you mix a new batch of glaze.
If none of your test results are an improvement, then you may need to find a new clear glaze. You’re welcome to try my clear glaze recipe if you’re working at cone 6 or search the many glazes on glazy.org. Good luck with your testing!
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, 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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There’s nothing worse than a cloudy clear glaze muddying up your surfaces. Learn some of the factors that affect the transparency of a glaze, and an easy solution to a common cause of cloudy clears.
Defining the Terms
Opacity: The degree to which something reduces the passage of light.
Translucency: The quality of allowing light, but not detailed shapes, to pass through.
Transparency: The quality of being easily seen through.
Transparency, Translucency, and Opacity
Glazes can have varying levels of transparency, translucency, and opacity. Transparency and opacity are opposites. Translucency falls in between. Transparent means all light can pass through, and we can clearly see what’s on the other side (like a clear glass window). Translucent means some light can pass through, but we can’t clearly see what’s on the other side (like sheer curtains or a frosted glass window). Opaque means no light can pass through, and we can’t see what’s on the other side (like heavy drapes or a boarded-up window). Clay can be translucent or opaque. Clay can’t be transparent. Glazes can be transparent, translucent, or opaque. They can also have varying degrees of transparency and opacity. Clear glazes can be transparent and translucent. Clear glazes can’t be opaque, by definition. White and colored glossy glazes can be transparent, translucent, or opaque. Matte glazes can only be translucent or opaque. Matte glazes can’t be transparent, by definition. Here’s a chart to lay out what I just explained above.
The best clear glazes are completely transparent and clearly show the clay and designs underneath. The transparency of a glaze can be reduced when it contains particles that block light from passing through. The more light-blocking particles in a glaze, the less transparent it will be.
Reasons Why Your Clear Glaze Isn’t Completely Clear
If your clear glaze isn’t all that clear, here are a few common reasons why:
A It could be underfired.
B It could be the chemistry of the recipe.
C It could be that your application is too thick.
Glaze application thickness is the easiest of these issues to test and solve and I’ll explain how in this article. But first I’ll talk about A and B because reducing your glaze application thickness won’t make a difference if the issue is due to underfiring or chemistry.
A. It’s not clear because it’s underfired.
An underfired glaze is a glaze that’s not completely melted yet. Glazes are made of powdered materials that melt in the kiln. The powdered materials start out completely opaque in their powdered form. As the temperature increases during the firing, those powders melt, dissolve, and cool into glass. If your glaze isn’t completely melted, those unmelted particles are going to block the amount of light that can pass through. Small differences in firing temperature can impact glaze results. If the clarity of your clear glaze varies depending on placement in the kiln, it’s possible that your kiln has hot and cool spots that are affecting your glaze.
The first step is to figure out if your kiln is underfiring by using witness cones. I recommend putting them in multiple places throughout your kiln so you can see if there’s any variation of temperature throughout. If your kiln’s underfiring, you’ll want to increase your firing temperature.
Underfiring isn’t good for your clay or glazes. Be careful that you don’t increase your firing temperature too much so that it’s overfiring. Overfiring can have a negative impact on your other glazes (causing them to run) or on your clay body (causing bloating). I always fire to the temperature that’s best for my clay body and then make sure I’m using glazes that also work at that temperature.
You don’t want to change your whole firing for just one glaze because it will affect so many other things. It’s much simpler to change the glaze. If your witness cones tell you that your kiln’s firing to the right temperature for your clay and glazes, then you can rule out temperature as your issue and focus on the glaze itself.
B. It’s not clear because of the chemistry.
Glossy glazes without any colorant or opacifier in them can range from clear to white. In other words, they range in opacity and transparency. The appearance of any particular glaze is based on its chemistry.
In figure 1, you can see a bunch of glaze tests that I fired where the only differences between them are the silica and alumina levels. These glaze tests don’t contain any colorants or opacifiers, they’re all made from the same ingredients and they’re all chemically identical in every way except for two factors: silica (SiO2) and alumina (Al2O3).
These test tiles are arranged in a specific way. Moving from left to right across each row, each tile has small additions of silica. The first glaze on the left of each row has slightly more EPK than the first glaze in the row below it. So moving from left to right, silica is increased. Moving from bottom to top, EPK is increased. I won’t get into too much chemistry in this article because there’s a lot that would need to be explained to fully understand the differences between all these test tiles.
The reason I’m showing this to you is so you can see the small window of really clear glazes that exist within all of these glaze possibilities. As you can see, there’s a wide range of whiteness versus clearness (meaning opacity vs. transparency). Some are matte, some are glossy, some are running, some are underfired, and some are crazed. The clearest glazes fall on the 2nd row from the bottom (0.4 Al2O3) in the middle (2). But out of 50 different base recipes (all fired to cone 6) there are only these two truly clear glazes. The rest have some degree of whiteness to them. My point is that formulating a truly clear glaze isn’t all that easy because a clear glaze can be just a few grams of material away from being white and milky. Because of this, it’s really important that you’re weighing out materials accurately when mixing your clear glaze. Make sure you’re using a good scale and weigh each material carefully.
If you do find a great clear glaze recipe that fits your clay body, hang onto it and use the steps in part C below to make sure your results are always as clear as can be.
If your clear glaze isn’t quite clear and you determine the issue isn’t temperature or glaze thickness (that I’ll describe below), and you’re weighing your materials accurately, then your issue could be chemistry. It’s possible that it’s just not a very clear glaze to begin with. It may have been “clear enough” for the person who created it. Maybe they were glazing porcelain and couldn’t see the milkiness or cloudiness. If it’s not enhancing your pieces then you’d be doing yourself a favor to move on and find a new one. Chemistry is chemistry and unless you know how to manipulate the chemistry to fix it, it’s probably not going to get much clearer.
C. It’s not clear because it’s too thick.
Some glazes are very clear if they’re thin enough, but they go cloudy when thick (3). A super transparent glaze can still contain tiny, light-blocking bubbles or particles throughout the glass but when you have a thin layer, there are so few of them that they aren’t visible to the eye, and the glaze can appear completely transparent. If the thickness of the glaze increases, those tiny particles also increase and eventually start clouding up the glaze. Figure 4 shows some squiggles to illustrate what I mean.
Imagine that these squiggles are tiny light-blocking bubbles or particles. In a thin layer, they’re spaced out and don’t really impede the ability of light to pass through. But as you add more layers, more particles fill in the spaces and the glaze layer becomes hazy, milky, or cloudy as you can see in the bowl.
When I see good, clear results sometimes but not all the time, I know that the issue is not with the clear glaze itself. The issue is with the application thickness. And the best way to control application thickness is by measuring and maintaining the glaze’s specific gravity (SG).
I did some tests to figure out our studio’s clear glaze’s optimal specific gravity. I used a dark stoneware so the clouding is more obvious (Plainsman M390). All of these tiles were dipped for six seconds. The difference between them is their specific gravity (SG), meaning their water content. Figure 5 shows the thinnest glaze layer on the left, the SG is 1.4 g/mL (the most amount of water). The center tile SG is 1.5 g/mL (more water). The tile on the right has the thickest glaze layer. It has a specific gravity of 1.6 g/mL (and the least amount of water). See how the glaze clouds up with an increase in specific gravity (and decrease in water)?
I admit that this isn’t the clearest glaze out there, and a dark clay body will always enhance a clear glaze’s imperfections (6). But it works pretty darn well and doesn’t craze on most of the clay bodies that I’ve tested (Plainsman M340, M370, M390) so I’m happy with it. The way to get the best results with this particular clear glaze is to keep the specific gravity around 1.4 g/mL. And remember that these tiles were dipped for six seconds each (7), which is longer than I would normally dip my pots. Holding a piece in the glaze for less than six seconds will give an even thinner layer.
The reason I hold test tiles in for longer than my pots is because small test tiles will generally absorb less glaze than a large piece of bisque. I would rather see a thicker glaze layer on a test tile, knowing that it’s going to be a closer representation of the results on a large piece. And since this is a studio glaze where lots of beginners are glazing, the glaze results tend to naturally be on the thicker side. When you’re first learning to glaze, it’s harder to dip really fast. There’s definitely an art to it.
If your clear glaze goes cloudy when it’s thick, follow the steps below to determine the best water content for your glaze and keep it at its optimal level.
Use Specific Gravity To Improve Clear Glazes
Measuring and maintaining specific gravity is the easiest way to make sure your glaze results are always consistent. When your glaze always has the same ratio of water to solids, then your application will be consistent as well.
So how do you measure and adjust specific gravity? Use a graduated cylinder or syringe to weigh any volume of glaze. Divide the grams by the milliliters to get the specific gravity. You can also download my free step-by-step guide to keep for reference. Once you’re familiar with the steps for measuring specific gravity, here’s what to do with your clear glaze:
a. Measure the specific gravity as it is right now, without changing anything. Dip a test tile and record the SG on the tile. You may want to brush some underglaze on your tile so you can really see how clear your glaze is, especially if you’re using a white clay body.
b. What’s your SG? If it’s higher than 1.4 g/mL then continue with the steps below. If it’s at 1.4 g/mL or less, then your glaze already has plenty of water in it and I wouldn’t add any more. 1.4 g/mL is the lowest I keep the SG for regular glazes. Some special effect glazes need a lower SG.
Note: If it’s your first time adjusting specific gravity, I recommend mixing a small test batch or removing a small sample of your glaze
from the main bucket to experiment with. I don’t want you to mess with a large bucket of glaze in case you don’t like the results.
c. If your SG is higher than 1.4 g/mL, add a very small amount of water to your glaze. Always start small until you’re comfortable with how much water it takes to reduce the SG by a certain amount.
d. Mix well and measure SG again. Dip another test tile and record the SG on the tile.
e. If it’s still above 1.4 g/mL repeat the steps above, dipping a tile with each adjustment as you bring your SG down to 1.4 g/mL.
f. Fire all your test tiles and you’ll see how glaze thickness and SG affect your clear glaze. Pick your favorite and then adjust your main bucket of clear glaze to match.
g. I generally use the highest SG that gives me the effect I want. So if your glaze is crystal clear at 1.45 g/mL and 1.4 g/mL, I would go with 1.45 g/mL as my target SG for that glaze. Too much water in a glaze can lead to application issues.
Note: Your glaze sample might become really thin and watery while performing these tests. If that’s the case, you may need to flocculate your glaze with a saturated Epsom salt solution. This will thicken your glaze back up again for better application. A thin glaze in the bucket doesn’t always mean it has too much water. Once you’ve determined the best specific gravity for your clear glaze, write it on your glaze bucket, on your test tile (7), and on your glaze recipe, then be sure to keep your SG at that number.
I recommend measuring specific gravity on a regular basis so you’re always going to have the best results. Since water naturally evaporates, it’s common for your SG to increase if you haven’t glazed in a while. I measure SG every time I glaze my pots (less than once a month). If you’re glazing all the time, I would measure SG every couple of weeks and especially every time you mix a new batch of glaze.
If none of your test results are an improvement, then you may need to find a new clear glaze. You’re welcome to try my clear glaze recipe if you’re working at cone 6 or search the many glazes on glazy.org. Good luck with your testing!
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, 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.
Unfamiliar with any terms in this article? Browse our glossary of pottery terms!
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