Techno File: Hard-Panning Glazes Sue McLeod

Appears in the October 2020 issue of Ceramics Monthly.

Have you ever had a glaze settle into a rock-hard layer on the bottom of your glaze bucket? It’s impossible to mix and even if you do get it mixed, it just settles out again. Here’s why hard-panning happens and how to fix it.

Defining the Terms

Deflocculate: To make a slip, glaze, or clay slurry into a thin liquid by altering the chemistry of the slurry to prevent the clay particles present from clumping together.

Flocculate: To make a slip, glaze, or clay slurry into a thick liquid by altering the  chemistry of the slurry and causing the clay particles to clump together.

Hard-Panning: When a mixed glaze with little to no clay content settles into a hard, nearly unusable mixture at the bottom of a container.

Saturated Solution: A chemical solution containing the maximum concentration of a solute dissolved in a solvent. Additional solute will not dissolve in a fully saturated solution.

Solubility: The degree to which a substance dissolves in a solvent to make a solution.

Specific Gravity: Is the density of a sample divided by the density of water. If the units are the same this becomes: weight of a sample divided by the weight of an equal volume of water.

Why Do Glazes Hard-Pan?

The phenomenon of hard-panning often happens to glazes that don’t have enough clay in them to keep the glaze in suspension. Clay is composed of very fine particles (which don’t settle fast if they settle at all in water) and is the main suspending agent in glaze recipes. Aside from clay, glazes are made up of rocks that have been ground into a fine powder. Naturally, they would sink (like rocks) to the bottom of the glaze bucket. Next time you make 10 kg of glaze, think about how hard it would be to stir a 10 kg bucket of rocks with a spatula. Due to particle size and charge, the addition of clay into a glaze (in the form of kaolin, ball clay, or bentonite) helps to keep the heavy particles in suspension, floating around in the bucket, and very easy to stir with a spatula. For this reason, it’s ideal to have at least 10% clay (kaolin or ball clay) in any glaze recipe.

For glaze recipes that contain almost no clay, 1–2% bentonite can be added with little to no effect on the fired results. Bentonite is also clay, but it’s much finer and has a smaller particle size and larger surface area than kaolin or ball clay. Because of this, it’s a much more powerful suspending agent. The addition of just 1% bentonite has approximately the equivalent suspending power as 5% EPK. So, if your glaze recipe has no other clay in it, add 2% bentonite. If your glaze has some clay but less than 10%, add 1% bentonite. This should be enough to keep your glaze suspended and prevent hard-panning. If you’re mixing a new glaze with little to no clay in it, you can add the bentonite to your recipe to start out with. It’s always better to prevent hard-panning than to have to fix it later.

If your glaze is already mixed and hard-panning, update your recipe notes now by adding 1–2% bentonite to it so you’ll remember to include it next time you mix a new batch. If you know the approximate dry weight of your already mixed glaze, you can still add bentonite after the fact. Just make an educated guess of the dry weight based on the last batch size you mixed and how much you’ve used of it. It doesn’t have to be exact, around 1–2% of the dry weight will be fine.

Troubleshooting Hard-Panning Glazes

We’ve established that glazes need at least 10% clay in them to stay suspended and if they don’t have any clay, we can add 1–2% bentonite. But, depending on a glaze’s composition, it may contain some ingredients that counteract the suspending properties of the clay. This means your glaze could have over 10% clay in it and still have trouble staying suspended. This usually happens in glazes that contain some form of soluble sodium. A lot of glazes contain sodium, which is one of our primary fluxes that help a glaze melt in the kiln. When the sodium remains as a solid particle suspended in the water, it has no effect on the glaze’s properties in the bucket, but if some of that sodium dissolves into the water, the glaze can become naturally deflocculated. Clay particles have positively and negatively charged sides and edges. They attract each other like magnets. This attraction is what helps the clay particles keep the rest of the glaze suspended. When positively charged sodium ions deflocculate the clay particles, their suspending properties are gone.

To sum up deflocculation, it’s when sodium ions are added to a glaze, whether intentionally or unintentionally, and they alter the chemistry of the slurry so that the clay particles repel each other (1).

Deflocculated glazes often have a thick layer of water on top before mixing and settle fast after mixing. They dry almost instantly after dipping a bisque-fired pot in the liquid glaze. In order to fix a naturally deflocculated glaze like this, we can add a flocculant and get the clay particles attracting each other again, which will help keep all the other particles suspended in the bucket.

The Epsom-Salt Solution

There are two main reasons why your glaze is hard-panning:

1. There’s not enough clay to keep the glaze suspended.
2. The glaze is naturally deflocculated.

Always deal with the clay issue first, if applicable, before moving on to the deflocculation issue. The key is, the glaze must have clay particles. If your glaze has clay in it then you can fix hard-panning by flocculating. Flocculation and deflocculation affect clay particles because they carry a charge. If your glaze doesn’t have any clay particles, it’s not naturally deflocculated, so flocculating it won’t fix anything.

Even a glaze with only 1% bentonite and no other clay can be flocculated/deflocculated. My favorite flocculant is a saturated Epsom-salt solution. Other flocculants exist but Epsom salt (magnesium sulfate) is easy to find and works really well.

Dissolve the Epsom salt in water, creating a saturated solution, to use as a flocculant (2). This breaks up the bond between the magnesium and the sulfate so the magnesium ions are free to attach themselves to the clay particles. A saturated Epsom-salt solution will have the highest possible number of magnesium ions per volume of water.

The Epsom-salt solution needs to be saturated to avoid diluting the glaze with more water. Before flocculating, be sure to measure the specific gravity as a way to calculate whether your glaze has the ideal amount of water for your application method. The specific gravity should be it should be ~1.5 (but higher for fritted glazes—exactly how much higher depends on the percentage of frit in the glaze). You must get the water content right before determining whether your glaze needs to be flocculated. If the glaze is too thin or won’t stay suspended because it has too much water, then flocculating isn’t going to help. In some cases, it will make glaze application worse. If the Epsom-salt solution is weak, it adds more water than necessary to the glaze. The water is just a medium for carrying the magnesium ions into the glaze bucket, so they can flocculate to the clay particles. We want maximum ions and minimum water added.

When using the saturated solution to flocculate a glaze, you use the liquid on top and leave the undissolved salt crystals on the bottom.

Every glaze composition is going to react differently to a flocculant. Some glazes will only take a few drops to become flocculated. The more clay in the recipe, the easier it’ll be to flocculate. Always start with a smaller amount than you think you need and work your way up. Start with these limits (3) and mix well with a high-speed mixer after every addition. If the amount you’re adding is having zero effect, keep adding small amounts of Epsom salt and mixing at high speed until you notice the glaze noticeably thicken, then stop.

A flocculated glaze will stick to your finger, rather than run off. And the movement in a bucket of flocculated glaze will come to an abrupt stop soon after the mixing stops. If you stop mixing and the glaze keeps swirling around in the bucket, it’s probably not flocculated yet.

Extreme Cases of Hard-Panning

Sometimes a glaze will be so hard-panned that you can’t get it to move at all. In this case, pour the water off the top of the glaze into a bucket. Save this bucket of water to add back into the glaze later. Take the remaining hard layer of glaze and add some Epsom-salt solution directly to it. Either use your hands (wear gloves if glaze materials are a concern) or a tool (such as a metal spoon or a spatula) to work the Epsom salt into the glaze layer. It may take some elbow grease to get it going, but eventually the glaze layer will soften and start moving. Then you can add the water you removed back in and proceed to use a high-speed mixer.

the author Sue McLeod is a potter and studio technician in Victoria, British Columbia, Canada. She teaches online glaze classes and writes articles about glazes and studio tips on her blog at www.suemcleodceramics.com.

If you love learning about and discussing glazes, check out Sue McLeod’s free social learning Facebook group, Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes, and fixing various glaze issues. McLeod also teaches an online class, Mastering Glaze Consistency. To learn more, visit https://suemcleodceramics.com/masteringglazeconsistency.