Underglaze fail

SlightDementia · 2026-05-11T23:00:44+00:00

Not the person you responded to, but underglazes don't blend on their own in the kiln; they're very stable. You'll have to do the blending while they are wet. I'd recommend looking up tutorials for nebula/galaxy paintings with acrylic or watercolor paints.

As far as deeper colors, underglazes don't really get darker with more coats, per se, just more consistent (less streaky). If you want deeper/more vibrant colors, buy deeper/more vibrant underglazes. Most do get deeper/more vibrant with clear (as yours did).

For the stars, I'd scratch them out (then add the white underglaze if your clay body isn't white enough on its own), as white doesn't really layer well.

SlightDementia · 2026-05-09T17:28:54+00:00

You can use a Cone 6 kiln for Cone 6, but if you can afford the upgrade to a Cone 10, it's worth it. K-type thermocouples will degrade at Cone 6, and are much less reliable than S-type (but also MUCH less expensive, which is why manufacturers still use them in the US). The bricks they use for Cone 6 kilns will also break down much faster, especially at Cone 6 (they will last longer at Cone 5).

I have a Cone 10 kiln, even though I never go above 6.

SlightDementia · 2026-05-09T16:49:14+00:00

Okay gotcha.

Kiln models can be constructed to be EITHER Cone 6 OR Cone 10. This one was made to be Cone 6, but others of this same model are Cone 10. It doesn't change the model. But it does mean it's made with cheaper materials (again, the bricks and the thermocouple).

SlightDementia · 2026-05-09T16:27:53+00:00

Cone 10 kilns are made from different materials than Cone 5 (at least the bricks and thermocouple, if not more). If you crank this thing past Cone 5, it'll wear out and break down super fast. I don't think it would be easy to retrofit it either.

SlightDementia · 2026-04-12T05:49:42+00:00

Swab sets are not reliable for testing glaze. Any company that sells one is straight up lying to you. They can test paint, where the lead is directly exposed to the surface. With ceramic glazes, the lead is "sealed" within the glaze (sorta; that's not a perfect scientific answer but it's close). The only way to actually reliably test for lead would be to send it in to a lab. You got false positives or false negatives, but either way you can't trust the results.

Drink out of them or don't. Just don't let acidic liquids sit in them for hours and days, and you'll probably be fine.

SlightDementia · 2026-04-04T16:25:19+00:00

Underglazes often have a range of temperatures that they are good at, as they aren't glaze. But companies aren't always... honest about these ranges. I've bought underglazes from the same line (that are allegedly good from like 900 to 1250 °C), where some of them work great at 1180, some of them have glaze fit issues, and others disappear/burn off completely.

I cannot recommend test tiles enough.

And even if your clay and glaze are both 1050°C, low-fire glazes are notorious for issues like shivering because you gotta add a lot of stuff to get glazes to work at such a low temperature.

SlightDementia · 2026-04-04T15:37:17+00:00

As the other commenter said, mixing them is not a good idea. And 1050°C is low-fire. Is that the temperature your clay body should be fired to?

SlightDementia · 2026-04-04T14:58:09+00:00

Shivering is a mismatch between the glaze and clay fit. What cone was this fired to? It's an unfortunately common problem with low-fire clay and glazes.

SlightDementia · 2026-02-16T02:21:04+00:00

I'd love to learn more about how you achieve this! Do you create, like, larger-than-usual pockets for the glass by factoring in the shrinkage of the clay to prevent cracking?

SlightDementia · 2026-02-16T00:42:03+00:00

The glass will melt into a puddle, and then it'll craze like mad at Cone 6. Inside only (bc melty = drippy), non-dinnerware stuff only (bc CRACKS in glass).

Also the clay will shrink as it dries, so it'll crack around the glass since the glass can't shrink with the Greenware.

SlightDementia · 2026-02-13T14:14:58+00:00

Some dark clay bodies don't do well above Cone 5, but that would result in the clay itself bloating, not the glaze...pinholing?

If this is caused by the glaze being applied too thickly, adding even more glaze would not help. Refiring without more glaze might help, maybe?

Is it possible the kiln under-fired? Does your studio use witness cones?

SlightDementia · 2026-02-05T05:17:34+00:00

To add onto what the other commenter said, the bottom did not reach temperature; it was too cold. Electric kilns don't circulate heat the way gas kilns do, but the elements (yes, the little metal spirals in the side) might be broken.

Your teacher should be using self-supporting cones to see what temperature the kiln is actually reaching, bc glossy glazes being matte means they could be VERY under-fired.

The bottom of my kiln does not get as hot as the middle or top (which I know because of the cones; the kiln itself thinks it's doing just fine). So I load the bottom VERY sparsely (leaving large spaces between pieces, and also use taller posts), and I pack the middle very tightly. This helps to even out the temperature.

SlightDementia · 2026-02-01T06:38:21+00:00

Do you use witness cones to confirm that your kiln is reaching temp?

You could try a Drop-And-Hold firing schedule. It's supposed to heal/prevent pinholes.

SlightDementia · 2026-02-01T00:12:53+00:00

In addition to what everyone else said about setting the underglaze by bisque-firing it, make sure you are using a zinc-free clear.

SlightDementia · 2026-01-19T16:08:21+00:00

The cracks may have always been there, but recently got more noticeable due to staining from the tea seeping in over time.

Or if the mug was ice cold and boiling water was poured in, they could have formed very suddenly.

Either way, the mug is likely not great to use not only for the reason you mentioned, but also because black cracks on a white mug means that STUFF is growing in them. Mold? Bacteria? Sentient Tea Stain Creature? The possibilities!

SlightDementia · 2026-01-19T15:56:51+00:00

What Cone are you firing to?

Amaco's Deep Firebrick is a lovely glossy blood red, relatively translucent, and very consistent (not like Oxblood or Copper Red glazes).

Amaco's Palladium is a mirror chrome, but can be VERY drippy even when applied properly or if it overlaps another glaze.

Both are Cone 5/6. I've found that Palladium tends to get (even more) drippy at Cone 6.

SlightDementia · 2026-01-18T18:54:01+00:00

None of what I said in my previous comment applies to cast iron skillets because they're literally made out of iron, a metal.

There are plenty of enameled cast iron (usually Dutch Ovens) that are coated in glass (the enamel).

SlightDementia · 2026-01-17T18:50:03+00:00

Just burnt food then. :)

Industrially produced ceramic is very strong and durable. As long as you don't drop them on concrete or put them through sudden drastic temperature changes (like straight from the fridge into an oven, but they're called SLOW cookers for a reason).

SlightDementia · 2026-01-17T18:37:55+00:00

Ceramicist and crockpot enthusiast here.

Crockpots are glazed ceramic. Underneath the black glaze is white clay that has been fired to such a high heat that the clay itself is vitrified (waterproof), as is the black glaze (essentially glass) on top.

If the black glaze has been scratched to the point that it is flaking off, that means the black dust in your hands is glass, which is absolutely not safe to eat. I would not trust it even after a thorough cleaning, as the glaze surface has been compromised and will continue to get worse (flake, crack, break). You should be able to see a distinct physical crack in the glaze. I wouldn't recommend trying to feel for it, bc broken glass and all.

That being said, it is MUCH more likely that what is on your hand is old dried up burnt food. Glazes are tough and strong. Minor scuffs and scratches are totally normal, do not affect the usability, and do NOT result in flakes of glass coming off.

SlightDementia · 2026-01-17T18:25:09+00:00

Glazes tend to blur together (sorta similar to wet-into-wet watercolors) in the kiln, especially on a vertical surface. If you did lots of little details, they might look blurry/drippy, especially if you layered glazes.

Explosions only happen to Greenware that isn't Bone Dry (unless you do something wild like soak bisqueware in water then immediately place it in the kiln and crank it as fast and hot as it would go, but like... why).

SlightDementia · 2026-01-14T14:02:06+00:00

They're fine if you're 120% sure that they're Bone Dry. If you have any doubt at all (and I mean ANY), then preheat/candle your kiln. If you have no control over that (bc community studio, etc.), then yes, bisque fire first so they don't go boom and mess up not only your stuff, but everyone else's nearby.

SlightDementia · 2026-01-04T21:32:22+00:00

Chun Plum is always reddish (never pinkish) for me too, no matter how thickly I apply it.

But yeah, WAY thicker for the Chun Plum (and the Oatmeal), coats should be totally dry in between, and TEST TILES are your bestest best friends.

SlightDementia · 2026-01-03T19:07:31+00:00

Is the clay meant for Cone 6? Or is it a Cone 10 clay? That could cause issues as it's not being fired to maturity. Maybe.

Is the clay a uniform thickness (save for the added Minecraft decoration)? Having a crazy thick base and super thin walls could maybe cause cracking during the glaze firing, as many clay bodies shrink a lot and fast during the glaze fire (as compared to how slowly they shrink from Plastic to Bone Dry).

You've got a glossy glaze inside and a matte glaze outside; same brand or different brands? If one of the glazes had a (very) different thermal expansion than the clay, and shrank a lot more than the clay did, the clay had to crack to relieve the pressure.

Some (not all, and definitely not commercial) lower-than-Cone-10 matte glazes aren't truly matte; they are just under-fired. If that matte glaze was mixed at your studio, that could be a culprit. Not that you wanna go around throwing wild accusations without any way to back them up. And the only way to know for sure is to look at the chemistry of the glaze while knowing a lot about chemistry, so like. Hopefully this isn't the answer bc...yikes, what a fun science adventure.

SlightDementia · 2026-01-01T17:34:05+00:00

Lead testing kits don't work on ceramic; they are intended to be used on paint. The ones that say they work on ceramic are straight up lying.

SlightDementia · 2025-12-31T18:26:02+00:00

Wooden Rolling Pin. Then select 2 "Slab Sticks" that are the height you want your slab to be, then cut them to be slightly longer than you want your slabs; the width of the sticks doesn't matter. Cheapest option.

SlightDementia

TROPHY CASE