Getting Clarity with Clear Glazes
There’s nothing worse than a cloudy clear glaze muddying up your beautiful slip design, screen printed images or coloured clay.
How can we make sure our clear glaze is always clear and prevent it from going cloudy? In this article, I’ll explain some of the factors that affect the transparency of a glaze, and an easy solution to a common cause of cloudy clears.
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 coloured 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.
|White/coloured glossy glazes||✔||✔||✔|
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.
- It could be under-fired
- It could be the chemistry of the recipe
- 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 #1 and #2 because reducing your glaze application thickness won’t make a difference if the issue is due to under-firing or chemistry.
1. It’s not clear because it’s under-fired
An under-fired 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 and dissolve 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’s under-firing 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 under-firing, you’ll want to increase your firing temperature. Under-firing isn’t good for your clay or glazes.
Be careful that you don’t increase your firing temperature too much so that it’s over-firing. Over-firing 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 temp.
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.
2. It’s not clear because of the chemistry
Glossy glazes without any colourant 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.
Here are a bunch of glaze tests that I did where the only differences between them are the silica and alumina levels.
For those of you who understand the UMF and Stull, these tiles are my recreation of the Stull chart at cone 6. SiO2 increases from left to right and Al2O3 increases from bottom to top. Flux ratio Na2O:CaO is 0.3:0.7 and B2O3 is 0.15 across the board.
These glaze tests don’t contain any colourants 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. And 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. If you’d like to read about how I created these tests and the chemistry behind them, check out my NCECA 2018 presentation – Understanding Cone 6.
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” vs “clearness” (meaning opacity vs transparency). Some are matte, some are glossy, some are running, some are underfired, some are crazed.
The clearest glazes fall on the 2nd row from the bottom (0.4 Al2O3) in the middle.
This is the 2nd row from the bottom. Only the 2 highlighted glazes are clear. The rest are white, even if only slightly.
But out of 50 different base recipes all fired to cone 6, there are only 2 truly clear glazes. The rest have some degree of whiteness to them.
My point is, 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 3 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 favour 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.
3. 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.
Here’s our studio clear glaze (click for recipe) applied very thickly over Plainsman M390 clay
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.
Here are 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 shown.
These pots are glazed with the studio clear glaze (click for recipe) we use where I work. The mugs on the left (by Margaret Hanson) have a thin layer of clear. The bowl on the right has a thick layer. Same glaze, different thicknesses, different results.
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.
I did some tests to figure out our 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 6 seconds. The difference between them is their specific gravity (SG), meaning their water content.
The tile on the left has the thinnest glaze layer. The SG is 1.4 g/mL (the most amount of water).
The centre 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. But it works pretty darn well and doesn’t craze on most of the clay bodies that we offer (Plainsman M340, M370, M390) so we are 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 6 seconds each, which is longer than I would normally dip my pots. Holding a piece in the glaze for less than 6 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.
How To Use Specific Gravity To Improve Your Clear Glaze Results
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? Click here to watch a video of me demonstrating how to measure 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:
1. 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 paint some underglaze on your tile so you can really see how clear your glaze is, especially if you’re using a white clay body.
2. 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.
IMPORTANT: 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.
3. If your SG is higher than 1.4 g/mL, add a 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.
4. Mix well and measure SG again. Dip another test tile and record the SG on the tile.
5. 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.
6. Fire all your test tiles and you’ll see how glaze thickness and specific gravity affect your clear glaze. Pick your favourite and then adjust your main bucket of clear glaze to match.
7. 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.
ALSO IMPORTANT: 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 Salts 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. For info and videos about flocculation, join my free Facebook group.
Once you’ve determined the best specific gravity for your clear glaze, write it on your glaze bucket and your glaze recipe and 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 (which isn’t all that often). 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.
Do you have a cloudy clear glaze that you’ve been struggling with? Tell me about it in the comments below. Or even better, post a photo and recipe in my Facebook group and I’ll help you troubleshoot it.
Good luck with your testing! If this post helped or inspired you, please share it with others.
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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!
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