Glaze results on the thin side?

As a studio technician at a busy pottery studio, it’s my job to mix and maintain 20 different studio glazes. I’m also the one studio users often go to for help when their glazes don’t work out 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 2 common reasons why the application could be too thin.

1. The glaze contains too much water, making it thin and watery in the bucket. (This means specific gravity needs to be adjusted.)
2. 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.

(If you want to learn how to measure specific gravity, you can download this free guide.)

When I measure the specific gravity and it’s where it should be, then I know the glaze was just 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 for too long. So how can it be too thin?

The #1 reason why a dipping glaze is applied too thinly is INADEQUATE MIXING.

Glaze particles will settle

Take a minute to think about what glazes are made 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 settle eventually.

An undisturbed bucket of glaze that’s been sitting long enough will have a layer of water on the top and solid particles sitting under the water. The heaviest particles will sink to the bottom and the lighter particles will layer themselves on top.

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. The top of the bucket is equal to the bottom of the bucket.

This way you get the correct proportion of solid glaze particles on your piece, and the correct proportion of solid particles to water so your application layer isn’t too thin.

Use a high speed mixer if you have one

The way to achieve this is to mix your glaze really well. A high speed mixer is ideal and 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.

This shows a small mixing attachment and a cordless drill that I use 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 will high speed mix each glaze once and then I’ll 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 hours, I will often high speed mix again partway through, just to be sure.

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 getting a high speed mixer if you don’t have one. Hand blenders are often abundant in 2nd hand shops so it should be fairly affordable.

Is it really necessary? Think about the difference between beating eggs with a fork vs with an electric egg beater. High speed is waaaaay faster and more effective.

But…

You might work in a studio where you don’t have access to high speed mixing devices and it wouldn’t be appropriate to bring your own. In this case, it’s up to you to use the mixing tools that are available to you and mix 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 witness a lot of glaze bucket stirring and I also timed myself to see when I would naturally stop.

I stopped at 20 seconds and I would consider myself on the thorough side, making sure to scrape the whole bottom and the sides and mix it all in.

Turns out, this isn’t quite long enough for our glaze batch size with our existing mixing sticks.

How long is enough time? Only 1 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 (click for recipe) that contains 4% rutile and appears very different when it’s thin vs thick.
• I used the stirring stick that is provided in our studio – a 1.5” dowel.
• The glaze bucket is a 30 gallon grey, plastic garbage can. The bottom isn’t flat, it has a hump in the centre with a “moat” around it (you know the ones?)
• The glaze batch size is around 20kg.

I stirred the glaze for 10 seconds, going back and forth, side to side, front to back, over the hump, in the “moat”, clockwise, 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 (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 3rd test tile for 6 seconds and called it a day.

How long should you stir for?

The above image shows the fired results and a great visual example of the effectiveness of mixing. Left = 10 secs, middle = 30 secs, right = 60 secs.

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 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.)

A fun thing to do is find a 60 second tune that you can sing or hum while you’re stirring. Hint: The intro to Queen’s Bohemian Rhapsody is pretty close. And so is the chorus of Dancing Queen by Abba.

If the glaze sits for more than a couple hours, I would give it a 60 second stir before using it again.

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!

I’d love to hear your thoughts and experiences, please comment below. If you found this article helpful, share with a fellow potter so they can improve their glaze results too.

 

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