Underfired or Overfired? Reading Your Cone 6 Glaze Results Correctly

You pop the kiln and your stomach drops. Again. The glaze looks nothing like the test tile. Chalky. Or maybe it's a puddle. Your first instinct? Blame the bucket. Maybe the glaze batch was bad. Maybe the studio elves swapped your jars. Relax. It's almost never the glaze's fault. It's the fire. Specifically, you're looking at either an underfired glaze or an overfired glaze, and the kiln controller is the biggest liar in the room. Those "cone 6 results" you expected? They mean exactly squat if the heatwork never actually hit cone 6. So take a breath. Grab a shelf post. Let's figure out what really happened.

When Your Glaze Quits Early

Underfired glaze looks tired. It has no shine. Feels like 120-grit sandpaper. Pinholes everywhere because the chemistry never got hot enough to smooth out. Colors that should be deep and glassy? Chalky and flat. You wanted satin, you got sidewalk. Here's the thing: if the witness cones are still mostly upright, your kiln wimped out. Maybe the elements are dying. Maybe the thermocouple is reading high. Doesn't matter. The cone 6 results you're staring at are actually cone 4 results wearing a disguise. Don't layer more glaze on top. Don't add silica. Just. Get. Hotter.

Too Hot to Handle

Overfired glaze is a disaster. It runs off rims and pools on your shelves like candle wax. Colors bleed into each other like a watercolor left in the rain. Sometimes it bubbles and pits because it literally boiled. Other times it goes super glossy and looks like cheap plastic. Gross. If you see rivers of glass stuck to your kiln wash, that's not a glaze feature. That's a cry for help. Your witness cones probably look like melted birthday candles. The kiln got carried away. Thermocouple offset is too low. Or the hold time is too long. Either way, you overshot. Pull back.

Read Witness Cones or Keep Guessing

Controllers have digits. Digits are not truth. Cones are truth. If you want real cone 6 results, you need to read witness cones. Real ones. Sitting on every shelf. A self-supporting cone bent so its tip touches the pad? That's maturity. A 90-degree bend? Also fine. But if the cone is barely nodding, you're under. If it's a puddle, you're over. Simple. Most people stick a cone in the sitter and call it a day. That's cute. The sitter cone only measures one tiny spot near the wall. The middle of your stack could be a whole cone off. Put witness cones where your pots actually live. Then look at them. With your eyes. Write it down. Stop trusting a beeping box.

Dialing In Your Actual Cone 6

So you know the problem. Fix it. Under? Add a 15-minute hold. Or bump the thermocouple offset down so the kiln works harder. Over? Drop the target temp by a cone. Check your elements for hot spots. Fire another test. Then another. Boring? Yes. Necessary? Absolutely. The goal isn't a number on a screen. It's the right heatwork. Once your underfired glaze starts melting and your overfired glaze stops crawling down the shelf, you'll know. Your cone 6 results will finally match your expectations. Clay isn't magic. It's just physics. And physics doesn't care if you're in a hurry.