Colorful Craters: A New Take on Foaming Glazes Chris Locke

What Is Needed 

Let’s take a step back. In order to make a crater glaze, you have to provide three things. First, the glaze must contain a material with a high Loss On Ignition (LOI). This means the material burns in the kiln and releases a gas. Some materials don’t lose any mass in the kiln, and some materials (like whiting or cryolite) release a lot of gas when fired. Second, the material has to do this after the glaze has started to melt. If the gas is released too early, it will escape, and you won’t get any bubbles. Third, the glaze must be viscous enough to trap the bubbles, and not heal over and flatten out if any of the bubbles pop. This perfect intersection of LOI, melting, and viscosity creates a glaze that can foam and swell, and give extra thickness and texture to your surfaces. 

Most of these glazes use silicon carbide as the high-LOI material. It’s readily available in various mesh sizes and is very reliable in its willingness to make a gas. Unfortunately, the gas it releases is carbon dioxide, and that excess carbon tends to make the glazes dull and gray. If you want your piece to look like real lava rock, this is fine. If you want it to look like a brand-new kitchen sponge, you might be dissatisfied with the muddy color. The solution is to find a high-LOI material that burns at the right temperature, but releases something other than carbon dioxide. 

Testing With Sulfur 

I’d previously had some difficulty with yellow stains causing bubbles in my gloop glaze, and with red, orange, and yellow underglazes causing pinholes or blistering in my clear glaze. The horrid stench of these underglazes was rumored to be caused by sulfur, and I thought sulfur gas might be less detrimental to my colors, so I started my experiments there. 

There are several readily available types of sulfur, and I went through a bunch of them. Elemental sulfur, aluminum sulfate, and ammonium sulfate are all used as fertilizers, so they are easy to obtain. There are a surprising number of food additives for use in very specific fields. Wine makers use things like potassium metabisulfite and potassium bicarbonate, which have the potential to release gas at the right temperature. I did some experiments with baking soda, plaster of Paris, cryolite (which releases toxic fluorine gas), cream of tartar, table salt, and every chemical we use to maintain the water quality in my hot tub. These tests were mostly unsuccessful. One of the hot tub chemicals (sodium bisulfate) worked surprisingly well, but I thought I could find something better. A friend recommended I try ‘dead sea salt,’ which, as it turns out, has some really corrosive stuff in it that caused the test tile to completely disintegrate over the following two weeks. The porcelain ended up looking like a flaky croissant and eventually turned completely to dust. My thermocouple failed shortly thereafter. I don’t even want to think about what it may have done to the bricks in my kiln. 

Discovering Epsom Salt 

Then, I realized I’d been sitting on another common source of sulfur all along. Epsom salt (magnesium sulfate or MgSO₄) is used to prevent the hard panning of glazes, and as it turns out, it releases sulfur dioxide somewhere just below cone 6. 

Not only does it release a good amount of gas at the right temperature, but it also puts out so much that it can be used to counteract the excess carbon from the silicon carbide, so they can be used together. As a bonus: it’s already a ubiquitous ceramic material, so you can get it from your preferred clay supplier (or your local pharmacy). 

Here’s the big drawback—it’s water soluble. When you mix it into a glaze, it dissolves in the water and then goes wherever the water goes. When you apply your glaze to a bisque-fired piece, the Epsom salt will soak into the clay body. When the water evaporates, it will carry the Epsom salt along with it and deposit fine crystals wherever the water was when it evaporated. This may not be ideal for some people, but if you’re adventurous enough to mix and use your own crater glazes, you might also be open to some of the weird effects this solubility may lead to. Either way, it’s definitely something to consider. 

Two other things to consider about Epsom salt: it’s a flocculant, so it will cause your glaze to thicken, and you may have to adjust water or add a deflocculant to account for this. I like my foam thick, so I use as little water as possible. Epsom salt is also mostly magnesium, which is a flux. In the photos, you’ll see that the presence of MgSO4 in the glaze contributes to a more fluid melt. If you want to try adding Epsom salt to your glaze, you may need to reduce some other flux to adjust for this. 

In ceramics, it seems as though the majority of what we know has been handed down from teacher to student over many generations. If we are content to take this information as fact and not question it, we may get stuck making all the same work that has already been made. In order to branch out and make something new, we must be willing to question “the way things have always been.” Just because crater glazes have always been kind of gray, it doesn’t mean they have to stay that way. 

NOTE: Sulfur dioxide (SO₂) is not good for you. As always, proper kiln ventilation is recommended. Do not stand next to the kiln and take deep breaths. It doesn’t smell like success; it smells like emphysema. That being said, sulfur dioxide is churned out in exponentially higher levels by pretty much anything that burns fossil fuels. Protecting oneself is always a good idea, but keep in mind “the dose makes the poison.” 

the author Chris Locke graduated from The George Washington University and moved to Austin, Texas, with his wife, Elizabeth. He’s the author of Draw Like This! and is a full-time ceramics teacher with the Round Rock Independent School District. To learn more, visit www.heartlessmachine.com.