Does any of this sound familiar?

You unload a piece from the kiln and the glaze turned out perfectly! So you glaze more pieces with the same glaze combination, fire them, and they turn out completely different…

OR

You mix a small test batch of a glaze and LOVE how it looks on the test tile. So you mix a larger bucket of the same glaze and it turns out completely different…

OR

You’ve got your glazing process down to a science but your results are still unpredictable.

OR

You have one of those glazes that’s one colour when it’s thick and a different colour when it’s thin but you can’t seem to control which result you get…

OR

You have a glaze that runs when it’s too thick and you hold your breath every firing, hoping you got the application right this time…

OR

You always do a 3-second (or insert any # of seconds) dip but your results aren’t always the same…

OR

Your glaze results are so inconsistent that you think you’ll never be good at this…

OR

You feel like you have no control over your glaze application and you’re tired of ruining your pots by glazing them…

OR MAYBE ALL OF THE ABOVE???

Good news!

All of these issues can be improved by controlling the thickness of your glaze application. Glaze application thickness has a huge impact on what the glaze will look like once it’s fired.

For example:

• Some glazes have a lot of depth when they’re thick, but appear flat when thin.
• Some glazes change colour when thick vs thin.
• Some glazes will run when they’re too thick.
• Some clear glazes are cloudy when they’re thick.

Since application thickness determines how our glazes will look once they’re fired, application thickness is what we need to control while we’re glazing.

When we have consistent application thickness, we have consistent fired results.

Consistent application thickness starts with consistent WATER content

The first step to controlling glaze application thickness is by controlling the WATER CONTENT of our glazes.

Water plays a big role in how glazes are applied to pottery. In any glaze bucket, you have solid particles suspended in water. The ratio of solid particles to water affects how thick your application will be.

When you dip a piece of bisque into a bucket of glaze, the bisque sucks water into its pores. This sucking action (called capillary suction) is what draws the solid particles to the surface of the bisque and holds them there.

The glaze appears to “dry” rather instantly, but the water is there, inside the pores of the bisque. The water then evaporates over time and the solid particles remain and become the glaze.

The more water you have in a glaze, the less solid particles there are (proportionately) to stick to the bisque. Less solid particles means a thinner glaze application.

More water = less solid particles = thinner application

Less water = more solid particles = thicker application

Each glaze will have a different “ideal” ratio of solid particles to water. You get to decide how you like each of your glazes best.

One glaze might look better when thicker, with less water and another glaze might look better when thinner, with more water.

The key here is once you’ve got your glaze to the thickness that looks the best to you, you can keep the ratio of solid particles to water the same every time you glaze.

This is how you get consistent glaze results from firing to firing. If the water content isn’t consistent, your application thickness will also be inconsistent and your results will be inconsistent.

But if your water content is always the same, then your application thickness will be consistent and your fired results will be consistent.

Examples of how water content affects fired results

In each of the photos below, the glaze on the left has less water and the glaze on the right has more water. Water content is the only difference between each pair of test tiles.

In every case, the tile on the left has a thicker application (more solid particles) than the tile on the right.

You may like the tile on the left better, or the tile on the right. There’s no right answer here. I just want you to see how water content affects the fired appearance of a glaze.

Sometimes it’s drastic, sometimes it’s subtle. A darker clay body will often enhance the differences.

All test tiles are made of Plainsman M340 clay. Click on glaze names for recipes.

Sue’s Woo Yellow – This is a glaze that’s dark where it’s very thin and on the edges. It’s lighter and you don’t get the dark edges when it’s thicker.

Sue’s Eggshell – Another glaze that’s darker where it’s thinner and on the edges. Lighter where it’s thick.

Raw Sienna – A subtle difference between these two, but it’s a deeper brown and glossier where it’s thicker.

Anna’s Green – The thicker glaze is a bit more blue and a bit runnier, as you can see by looking at the drips on the side.

Sapphire Blue – This glaze is drippy when it’s too thick and quite drastically dark and green when it’s too thin.

Blue Green Pink – A deeper blue and runnier when it’s thick. Much paler when thin.

Raspberry (Cedar Hill mystery glaze) – This glaze is yellow where it’s thin and pink where it’s thick. Could be a shocking difference if you’re expecting one and get the other.

White – This glaze looks exactly the same but it’s runnier when it’s too thick.

How to control the water content of a glaze?

If you always have consistent water content (meaning you always have the same proportion of solid particles to water) you have the best chance for consistent application thickness.

So how do we make sure we always have consistent water content in our glazes?

By measuring SPECIFIC GRAVITY.

Measuring specific gravity tells you the ratio of solid particles to water in your glaze bucket. It gives you a CALCULATED NUMBER that you can come back to every time you glaze.

All you need to measure specific gravity is a scale and a measuring container. I like using a graduated cylinder. I don’t recommend using a hydrometer to measure specific gravity and you can read all my reasons why in this blog post.

If you want to learn how to measure the specific gravity of your glazes, I’ve created a free pdf guide called The “Specifics of Consistent Glazes” It’ll walk you through all the steps (with pictures!). You can download and print it or keep it on your computer.

You can also watch a video of how I measure and adjust the specific gravity of a glaze here.

Learning to measure specific gravity has been a complete game-changer for me. Not only for improving my own glaze results but for all the glazes I manage at the community studio where I work.

We can now confidently teach beginners how to use our glazes because we measure specific gravity on a monthly basis to always keep them consistent.

It’s such a foundational glazing step that’s often overlooked or seen as unnecessary.

But if you’ve ever felt like you have no control over your glaze results and you think you have to leave it up to the “kiln gods” to determine whether your pieces will turn out or not, this is your chance to take your power back.

Through the use of a little math and science, you can be your own “kiln god” and know that your glaze results will always be consistent.

Let me know how it goes in the comments below! If this post helped or inspired you, please share it with someone.

Join my free community

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!

5 Ways You Can Make Your Glazes More Amazing (using glaze chemistry)

If you’ve ever wished your glaze could be just a little more…

• cost effective
• green/red/blue/purple
• compatible with your other glazes
• matte/glossy
• stable/fluid

… then you don’t want to miss this free class!

Date: Sep 21, 2023

Time: 10:00am Pacific / 1:00pm Eastern / 6:00pm UK time.

A replay will be available to watch for a limited time if you can't make it live.

The Art of Glaze Chemistry

Learn how glazes are formulated by using the UMF (unity molecular formula) and glaze calculation software to:

• create different glaze surfaces like matte, satin or glossy
• get glazes to melt at different temperatures or ensure your glaze is being fired to the right temp
• create durable, functional glazes
• fine tune the colours of your glazes
• fix crazing and other thermal expansion issues

This is a deep dive into all the chemistry that's happening inside the kiln.

Registration open Sep 21-29, 2023.

Learn to Mix Glazes from Scratch

If you’ve never mixed a glaze from scratch before and want to learn, I teach an online workshop called Glaze Mixing Essentials where I show you all the steps to mix a glaze and then test a base glaze with multiple colourants. Click the link for all the details.

September sale: Save $30 if you register by Sep 30, 2023.