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Clear glaze is often applied over underglaze decoration to protect pottery, make it non-porous and help underglaze colors pop. It can also be used as a way of bringing out the natural beauty of a particular clay body. However, even a little bit of cloudiness can really detract from the overall look of your pottery. So, what does turn clear glaze cloudy, and what can you do about it?
The main factors that turn a clear glaze cloudy are under firing and applying glaze too thickly. Glaze can also be milky if its chemical balance is not quite correct. Clear glaze is transparent if it is free from particles and bubbles that prevent light from passing through it.
A clear glaze that has been fired correctly will protect the piece from discoloration and moisture. It will also enhance the look of the colors if they have been used.
Think about the last time that it rained heavily and you went outside. The trees, the roads, and other things that are soaking wet all seemed more alive and saturated, didn’t they?
This is because the water enhances the colors and patterns underneath it. Dark colors look even more dark and bright, cheerful colors look even more bright and cheerful.
In short, this is what a clear glaze actually does: it protects the piece of pottery or ceramic and enhances the colors and patterns.
What are the Different Kinds of Glazes?
Not all glazes are clear. Here’s what you need to know:
Opaque Glazes
An opaque glaze is the opposite of a transparent glaze. It cannot be seen, though, since it is a solid color. This kind of glaze is useful if you want to add dramatic color to a piece of pottery.
Translucent Glazes
A translucent glaze is in between an opaque glaze and a transparent glaze. Light can pass through it and one can see through it but it is not completely clear. Matte glazes are translucent by nature because they contain small particles that refract light and cause the matte look.
Clear Glazes
The clearest glazes are completely transparent. They allow the most light to pass through and reveal and enhance all of the detail underneath.
Particles and bubbles in a glaze prevent the light from passing through easily. And this will make a clear glaze cloudy. There are processes that contribute to the presence of these impurities, so let’s take a look at those now…
What Makes Clear Glaze Cloudy or Milky?
You might have noticed that your clear glaze sometimes doesn’t look great after being fired. It might be cloudy or milky and this can obscure the colors and designs underneath. In fact, a cloudy glaze can really ruin a piece. This is the last thing you want after spending all of that time and effort! So, what makes a clear glaze cloudy or milky?
There are normally three reasons why your glaze turns out milky or cloudy:
Under Firing Can Make Clear Glaze Cloudy
A glaze is actually made up of particles that melt in the kiln when being fired. The problem is that if they are not completely melted, it can cause the glaze to look milky or cloudy in appearance. In this scenario, the unmelted bits of glaze are blocking the transmission of light.
To make things more complex, you can also have hot and cold spots in your kiln. This difference in firing temperature can affect how your glaze melts and how long it takes.
Applying Glaze Too Thick Can Make Clear Glaze Cloudy
Realistically, no clear glaze is going to be free of little bubbles and particles. But if the glaze is thin enough, you won’t even notice these areas and light will be transmitted just fine.
The problem is that if you use the same clear glaze and apply it too thickly, these little bubbles and particles will add up and create a slightly milky or cloudy look.
This can cause an overall milky hue, or cloudy drips and ripples.
Is Chemistry Making Your Clear Glaze Cloudy?
Though there are three types of glaze, opaque, translucent, and clear, the fact is that chemistry between all three of them can vary by only small degrees. For example, many glazes that are considered to be clear may have materials such as silica and alumina in them that cause them to be slightly less transparent when fired.
In fact, you could take a dozen clear glazes and fire them on different pieces of test pottery. Even with ideal firing conditions, the glazes could all turn out completely different depending on the chemical composition of the glaze.
Is Your Clear Glaze Cloudy? How to Prevent Milky Glaze
If you’re consistently getting milky or cloudy-looking glazes, the good news is that you can do something about it. Here’s some advice:
Fix the Under Firing
One of the biggest causes of under-firing is cool spots in a kiln. The best way to deal with these is to identify where they are and if they exist. Place witness cones throughout your kiln to test out the temperature.
If you find that there’s too much temperature variation, you can increase the temperature of your kiln. Just be careful not to increase it too much or you’ll find that your pieces are being over-fired and that’s a whole other set of problems to fix.
If it turns out that there isn’t enough variation in temperature to cause the glaze issues, it could actually be the glaze itself. Make sure that you’re using a glaze that will melt at the temperature you’re using.
Fix the Glaze Thickness
A mixture that’s too thick can also make a clear glaze cloudy. Glazes are described as having a specific gravity1. This measurement refers to how many particles of glaze material there are per unit of glaze.
The thinner a glaze is, the more water it contains per measuring unit. The thicker it is, the less water it contains. Thick glazes have an increase in specific gravity.
Specific gravity is important because if a glaze is too thin, its coverage will be inadequate. But if it is too thick, it won’t bond properly to your ceramics.
What’s more, during firing, gasses are created and released from clay and glaze. If glaze is too thick or dense, the gasses are unable to escape. This can make a clear glaze cloudy or milky. You can read more about what happens if you apply too much glaze here.
There’s some amount of trial and error in this, depending on how long you dip your pieces in the glaze, but if you increase the water content of your glaze to about 1.4 g/ml, you should get a consistently thin glaze that will be more transparent.
Manage the Chemistry
If you make your own glazes, the best way to manage the chemistry is to carefully weigh each ingredient. This will ensure better glaze recipe management.
Given that there may only be a few grams of difference between a milky glaze and a clear one because of the materials added, this may also be a process of trial and error until you find a glaze that works for you.
Final Thoughts
The use of clear glazes is important to the look and aesthetic qualities of both pottery and ceramics. Unfortunately, cloudy or milky glazes are all too common due to several common reasons, including chemistry, thickness, and firing conditions.
In some ways, making the perfect glaze and ensuring that the remaining processes are optimal is a matter of extensive trial and error and testing. This is especially the case for beginners and even those who are intermediate in such arts.
If you find that there is something in your process that has made your clear glaze cloudy, don’t lose hope! Use this to improve your glaze technique, your glaze recipe, and your firing. After all, we never truly learn anything unless we’re willing to also fail. And even the most experienced potters and ceramicists get it wrong from time to time.
References:
- Specific Gravities and Fluidity Factors in Glaze Slips. Journal of the Arkansas Academy of Science. E.S. Amos. 1947