Ceramic Human Posted February 21, 2019 Report Share Posted February 21, 2019 We're firing in an electric kiln to cone5. Been getting green spots like this on several mugs/week. It's usually only 3-4 out of 500 but that starts to add up over years of business. Do you guys have any experience with this defect? Link to comment Share on other sites More sharing options...
Hulk Posted February 21, 2019 Report Share Posted February 21, 2019 Hi again! Th' larger spot looks like an aggregation o' the smaller spots - looks like some o' the smaller spots are black, some green-ish; that true? Any road, suggest that you post recipes for glaze and clay, fired how, and how (and how often) glaze is sieved. Link to comment Share on other sites More sharing options...
Ceramic Human Posted February 21, 2019 Author Report Share Posted February 21, 2019 This particular glaze is the eggshell from Coyote. We mix a 25lb bags at a time and then sieve using 100 mesh. Not sure on the recipe for the dry mix, but I've attached a photo of the suggested water amount and specific gravity of eggshell. We only sieve after the initial mix. How frequently SHOULD a glaze be sieved? Link to comment Share on other sites More sharing options...
neilestrick Posted February 21, 2019 Report Share Posted February 21, 2019 Crack one open right at the spot and see if it's coming up from the clay or if it's in the glaze. Link to comment Share on other sites More sharing options...
JohnnyK Posted February 21, 2019 Report Share Posted February 21, 2019 What tools do you use to mix the glaze? Is there a possibility that you might be using a tool that was used to mix the teal or green glazes that might have a miniscule amount of residue that sloughs off when you mix the eggshell. It sounds too simple , but when you're talking about .008% defects is it really something worth worrying about...unless you're like my wife who will spend 3 hours to find a one cent error in her checkbook balance? Link to comment Share on other sites More sharing options...
Ceramic Human Posted February 21, 2019 Author Report Share Posted February 21, 2019 5 minutes ago, JohnnyK said: What tools do you use to mix the glaze? Is there a possibility that you might be using a tool that was used to mix the teal or green glazes that might have a miniscule amount of residue that sloughs off when you mix the eggshell. It sounds too simple , but when you're talking about .008% defects is it really something worth worrying about...unless you're like my wife who will spend 3 hours to find a one cent error in her checkbook balance? I should have supplied more information. I didn't really give you guys a lot to go on. My bad. I'll look into the tools and possible contamination. A rate of 5/500 is 1% defect rate. I thought if it were a really simple fix then it would be worth reducing defect rates by 1%. Heck, even if it's not a simple fix. You fix two or three problems like this, and reduce your defect rate by half or more. I figure that's worth the time. Link to comment Share on other sites More sharing options...
Ceramic Human Posted February 21, 2019 Author Report Share Posted February 21, 2019 1 hour ago, neilestrick said: Crack one open right at the spot and see if it's coming up from the clay or if it's in the glaze. I'll crack open the next few that come out. Thanks. Link to comment Share on other sites More sharing options...
liambesaw Posted February 21, 2019 Report Share Posted February 21, 2019 You've been having issues with clay contamination, might be related? Link to comment Share on other sites More sharing options...
Ceramic Human Posted February 21, 2019 Author Report Share Posted February 21, 2019 1 hour ago, liambesaw said: You've been having issues with clay contamination, might be related? Yeah, I thought about that and it's definitely possible, but I thought I'd ask you guys just to see if it was easy to rule out. There are so many factors, but I'm grateful to have this forum as a resource to help me get acquainted with the process. Sometimes it makes my head spin. I don't want to be an out of touch manager. The more information the better. Link to comment Share on other sites More sharing options...
BFF Posted December 6, 2019 Report Share Posted December 6, 2019 @Ceramic Human Did you ever get to the bottom of what was causing this? I’ve been getting identical spots so very interested Link to comment Share on other sites More sharing options...
AshleyP Posted September 24, 2021 Report Share Posted September 24, 2021 Older post however we are experiencing this issue too! Have you discovered the cause? Link to comment Share on other sites More sharing options...
Russ Posted September 24, 2021 Report Share Posted September 24, 2021 Id say its Chrome. sometimes it will blister. Iron in a reduction atmosphere will also make green. I dont suspect iron. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted September 24, 2021 Report Share Posted September 24, 2021 @AshleyP the OP hasn’t been back to the forum since March of last year, according to their profile. If you PM them, they should receive an email that someone has messaged them on the forum here. A lot of things do wind up being dependant on materials used and other situational things. If you give us your particulars, we can also help you trouble shoot. It could be a contamination issue of either clay or glaze. It looks more like chrome than copper to me too. If you’re working in a communal studio, or if there are other people’s pots in the firing, maybe that’s a source? The suggestion to break one apart at the site of the green speck to see if it’s bleeding through from the clay or not is a good place to start. Link to comment Share on other sites More sharing options...
JessCullen Posted March 31, 2022 Report Share Posted March 31, 2022 Not even sure if this is possible, but I am wondering if those green spots are cast off from aging thermocouples. I struggle with this issue as well. Link to comment Share on other sites More sharing options...
Hulk Posted March 31, 2022 Report Share Posted March 31, 2022 Interesting! I'm seeing little brown to black bits from the tip of the thermocouple; if they are small enough, they incorporate into the glaze melt, else, there's a bump. I've been gently tapping the probe, tip down, afore replacing it back, which seems to help some? The rain of little particles goes in the trash. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted March 31, 2022 Report Share Posted March 31, 2022 If the thermocouple is spalling, they’re affordable, and easy to replace. There is some chrome content in a type k thermocouple, so green bits in the glaze from one is plausible. I don’t think bits would travel through the kiln or anything, but flakes could fall down and land on pots directly underneath. There’s lots of other possible points for clay or glaze contamination, but we’d have to know specifics of materials and working situation to help troubleshoot better. Link to comment Share on other sites More sharing options...
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