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Sad first firing - amaco PC glazes - help please!


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Hi all! I’m new here :)

I’ve thrown lots of ceramics over the years, as a student in a classroom setting (so, lots of ceramics classes where someone else loaded & fired the kiln, haha), but am brand new to firing my own kiln.

I recently set up a studio off the side of our garage, and acquired a used Duncan EA 1020-2 manual kiln. I bisque fired about 25 pieces to cone 06 with no obvious issues; firing took about 10.5 hours with top peep hole open (I don’t have a vent, so needed oxygen to enter the kiln in some way - figured leaving a peep hole open was the best option?).

I bought several pints of Amaco Potters Choice glazes (blue rutile, oatmeal, chun plum, lustrous jade, and 3 or 4 others), and did lots of reading/searching on these forums and other sites, and gleaned that in order to get bright colors with these glazes, you must brush on many layers, quite thick; so, that’s what I did. 

Glaze fired to cone 6, with a small cone in the kiln sitter. I did not have witness cones (I very, very much regretted this almost instantly, but as I’m brand new to firing, I wasn’t aware I needed them when I was ordering all my initial supplies, so I had none), but around hour 12 I began to worry that something...wasn’t right, because of the time elapsed.

The cone 6 glaze fire took 16 hours before the kiln sitter triggered and only reached temp when I finally plugged the top peep hole. I let it cool overnight and opened this morning, to...major disappointment.

Very dark glazes, browns and dark greys, some purples but those were all very dark and almost matte looking. Tons and tons of pinholes, as well, which are new to me - I’ve read that many things can cause them? 

Without guidance cones I know there isn’t much to go by, but I suspect the kiln overfired, due to the very dark, less than stellar glaze results and the time it took to finish - could overfiring lead to these results? Or does anyone have any insights, helpful recommendations, encouragement? 

I know it happens, especially in the beginning, but I’m still kind of bummed. Thank you all for your help!

 

-Kati

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Hi Kati!

I started out trying for a bent over cone 6, where cone 7 starts to move a bit on the hotter shelves, now I'm shooting for solid cone 5 on the hot shelves, where a 5 cone shows movement on the cool shelves. From peak, I'm dropping 100F, holding there for an hour, then slow cool (all zones set to low) to 1850F before shutting down - for bubbles and pinholes. The potter I bought the kiln from fired to cone 7 - too hot for the red and buff clays I'm using. The red turns purple, and the buff weeps little pimply droplets.

Witness cones on each shelf, a pyrometer (real time feedback), and detailed notes, including times, temps, settings, then results from each level with reference to the clays, glazes, thickness o' the wares, etc. ...I just don't remember well enough, so, notes!

I don't use premade glazes, perhaps other contributors can help you there. In the local JC ceramic lab I thoroughly mixed the communal glazes, ladled out enough to a clean bucket to work with, sieved it, then glazed my pieces - mixing and sieving can definitely make a difference!

Lots to read on this forum 'bout pinholes; see also Tony Hansen's site (a treasure, truly), e.g. Pinholing (digitalfire.com)

Keep at it! Expect a few setbacks here and there.

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It sounds like you kiln needs a check up,  maybe some new elements.    If I have a load take over 12 hours I start checking my elements,  all of your elements can turn red but still need to be replaced.   They are worn out and not producing enough heat,  every time I buy a used kiln I plan on replacing the wires.    Denice

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I mean, I do a 12 hour firing on a glaze, but that’s on my kiln’s slowest setting. If an electric is struggling to reach temperature, usually that means the elements are dying or dead. There is a different quality to the kinds of pinholes you get in overfired ware than there is in underfired. Is there any way you can post an image? Cell pictures work best as they’re already low res. Most newer model phones will prompt you to select an image size before you post, but if yours is an older model, email it to yourself and shrink it through that system. 

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Thank you all so much for the replies & help! I’ve ordered some large cones so at least I’ll be able to tell what’s happening next time instead of just relying on the kiln sitter (first mistake, haha). I will definitely read some of the forum posts on pinholes, as well! Great idea! 

All of the elements are at least glowing, but it’s absolutely possible that some of them are on their last leg, thus not working to their full potential. We just purchased a multimeter volt tester, so I’ll check each element this evening and see what’s happening. From everything I’ve read I assumed something was wrong simply due to the length of time the firings were taking, so it makes sense that something might be going badly there. 
 

Also, my kiln only has knobs that read “low, medium, high, and hi-fire” - I’ve been doing two hours on low, two hours on high, and then the rest on hi-fire. I’d read that that was correct to “ramp” up, but if you guys know better, please tell me! Haha

Will try to attach some pictures! Thank you again!

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A53AE264-1E1C-46FC-97BA-380E7903B376.jpeg

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8675A035-8EA9-442F-9346-0B72CDF03056.jpeg

92B650A6-E192-478A-AAD2-DF961E281B21.jpeg

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18 minutes ago, Callie Beller Diesel said:

I mean, I do a 12 hour firing on a glaze, but that’s on my kiln’s slowest setting. If an electric is struggling to reach temperature, usually that means the elements are dying or dead. There is a different quality to the kinds of pinholes you get in overfired ware than there is in underfired. Is there any way you can post an image? Cell pictures work best as they’re already low res. Most newer model phones will prompt you to select an image size before you post, but if yours is an older model, email it to yourself and shrink it through that system. 

Just posted some images! I hope they worked :lol:

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I had similar pinhole woes with PC glazes in an electric kiln with limited controls and no pyro.

I believe it is due to not being able to hold well at any bisque temps to fully gas it off.

Messing with the limited controls while reading a many zone pyro is the only way to relieve all headaches IMO.

Sorce

 

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I use the glazes you listed, but I have a fairly new programmable L&L. I do slow glaze and slow cool for cone 5. I single fire greenware (not bisque) on which I have brushed on 3 decent coats of the PC. Not too thin but also not too thick. The controlled cool takes forever, but the slow glaze fire only about 7-8 hours.  Never any problems, so far. This is a detail of a wall piece/a fish...Sapphire Float & a touch of Ancient Copper.

20210110_200728--.jpg

20210110_201101--.jpg

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My guess on the pics (op's problem pics; yours look good-oh Lee), looks like craters where the clay is blowing gas - bare clay at the bottom. Are the craters mostly where the clay is thinner/thinnest? That would be the part that got hotter, methinks.

If the bisque was plenty absorbent enough for you, perhaps go a bit hotter on bisque next go-round, and plenty of time at ~1500F, with ample oxygen, to burn out stuff. From there, perhaps a cooler glaze fire - if, indeed, the last load was over fired. Third, per Mr. Hansen's suggestion in the article linked above, drop and hold after peak glaze temp, and a bit of slow cool as well.

If you plan to watch the cone packs during firing, careful placement is helpful, be sure to wear adequate eye protection against the harmful rays (and the small but real chance of flying bits), and consider painting a little line of iron oxide on the crucial cone (likely the 5) - helps make it visible. I just watch the pyrometer, having compared my notes against what the cones tell me (keep good notes!).

If you've used that same clay and glaze without issue, your confidence in eliminating the problem is likely high, eh? On t'other hand, if that's new clay/glaze, perhaps pick up a bag of something you're familiar with, and include some o' each in your next firing round - learn more, faster, maybe? fwiw, I've tried some clays that I probably won't use again, same for a few glaze recipes - a clear that clears bubbles on red clay, and clear that doesn't craze on low expansion clays...

Good luck, post back how it goes!

Oh, see also Hansen's article on blisters (craters): Blisters (digitalfire.com)

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Try bisquing Cone 04. Slow bisque .

I think overfired and glazes too thick.

What state was the sitter cone in?

Is the rod in sitter cone in good nick or does it looked tapered?

Check you are positioning cone properly

Would be worth checking out sitter mechanism  ,it may need adjusting.

Can you post a pic of sitter cone after firing? 

Start recording a log of all firings and results of firings.

Invest in a thermocouple and pyrometer.

Install a timer to shut off kiln about 1/2 hr after estimated time of firing.

Never rely on sitter to turn kiln off.

Whew!

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I can't offer any solutions, but feel sad for you. :( 

I sometimes use Amaco PC glazes. I bisque to 06, fire to 6, happy with results for the most part. Yes, need to put on thick coats, but also test to see which ones are really runny.

Good luck!

Betty

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On 1/10/2021 at 7:06 AM, skatischmidt said:

am brand new to firing my own kiln.

I recently set up a studio off the side of our garage, and acquired a used Duncan EA 1020-2 manual kiln. I bisque fired about 25 pieces to cone 06 with no obvious issues; firing took about 10.5 hours with top peep hole open (I don’t have a vent, so needed oxygen to enter the kiln in some way - figured leaving a peep hole open was the best option?).

Me too, Kati! I'm curious how you did your bisque, since I had to make three attempts on my kiln to work out a good program that would reach ^06 and give me a 10 hour burn. My kiln is a Duncan EA-820 kiln (Teacher Plus, manual with dial only, no pyrometer).   My elements are brand new, so the first  attempt had the sitter reaching ^06  and shutting down after 4.5 hours just using the "ceramic" setting (which on a clock face would be at about the 2:30 position. I had my kiln set up like yours--top peephole open, lid down). Second try I started with the dial at 9:00 position for an hour, moved up to 5:30 or Overglaze for two hours, then up to Ceramic (2:30) and it fired only 5 hours total. On attempt number 3, I started the dial at 10:00, moved it up to 8:00 two hours later, then to up to 5:30--where my kiln is marked "Overglaze" for another couple hours and the rest at 2:30 or Ceramic). This firing had the witness cones looking good and the sitter dropping at nearly 10 hours.  Facing my first glaze firing, and I'm afraid of making a mess of it. I have about forty pieces waiting to be glazed and I'm gonna have to push through and let myself fail and learn, I guess, experimenting with dial shifts to control the temperature rise. Unless the government decides on another stimulus check so I can invest in a good pyrometer or an electronic controller!! 

I'm linking a discussion from this forum that was really helpful to me with our kind of manual kiln: 

 

Its a sad fact that we beginners can rarely afford all the fancy whistles and bells, and wind up buying old equipment from the more advanced potters who are trading up. I've noticed (ironically) that the most common advice I've seen from advanced potters is to buy more and better tools.  Manual kilns thus become like a gateway drug.... 

Edited by Catatonic
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On 1/10/2021 at 5:25 PM, LeeU said:

I have a fairly new programmable L&L. I do slow glaze and slow cool for cone 5.

Any ideas how you would do a slow glaze on a manual kiln? I've heard others talk about this--is it about letting the time do the job and not necessarily the temperature? Want to understand the principles at work so I can see how to achieve it with or without an automatic controller (which I can't yet afford.)  I imagine a slow cool can be achieved by keeping the kid down until the outside of the kiln has cooled? 

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Slow firing simply means to turn the power up slowly so the firing takes a certain amount of time to complete. Sort of like bringing something to a slow boil. The power starts low and you gradually turn it up. Often you will find schedules like 2 hours on low, then 2 hours medium, then on high till finish. 

Slow cool usually entails turning the power back on at some point in the cooling cycle and adding just enough heat to maintain a lower temp for some time. Most kilns cool very fast at top temperature so to combat this the elements in the kiln are turned on for enough time to maintain a lower temperature. Generally this works best with an automatic controller, else the operator has to cycle things on and off manually.

Edited by Bill Kielb
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With patience and attention, prescribed firing schedule may be achieved with a manual kiln, however, a pyrometer is almost required - I say "almost" because there are those who can judge temperature by color and possess lots of experience with their kiln/process/ware.

I watch my pyrometer, flip switches, and take notes. Let me add here that I have adequate venting and safety gear.

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2 hours ago, Catatonic said:

OOPS did it twice. (How do you remove links????)

Each post has 3 little dots on the right side, click on those then click "edit" then you can delete the duplicate link you posted. You have the option of adding a reason why you did the edit in a little box at the bottom of the edited post. If you have problems with this just pm any of the mods for help if you need it. 

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1 hour ago, Bill Kielb said:

Slow cool usually entails turning the power back on at some point in the cooling cycle and adding just enough heat to maintain a lower temp for some time. Most kilns cool very fast at top temperature so to combat this the elements in the kiln are turned on for enough time to maintain a lower temperature. Generally this works best with an automatic controller, else the operator has to cycle things on and off manually.

With a kiln sitter, turning it back on after the pyro bar has melted would be a challenge--I imagine I would have to hold the switch up by hand to keep the elements on, yes? After first being present when the thing initially dropped. That is some serious kiln sitting I'm gonna be doing!

 

Edited by Catatonic
Cuz.
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2 hours ago, Catatonic said:

With a kiln sitter, turning it back on after the pyro bar has melted would be a challenge--I imagine I would have to hold the switch up by hand to keep the elements on, yes? After first being present when the thing initially dropped

No you don’t need to hold the trip arm up by hand. After the kiln trips off lift the trip arm up to about 45degrees then press the white button and gently release the trip arm. It will rest without tripping the power switch off.  Kiln will keep firing until you either press the trip arm down to (re)depress the trip arm to trip the power switch  or if your kiln has a timer it runs down and shuts off the power. It will take a fair bit of experimenting to figure out how long to hold it without a pyrometer, you’ll need to figure out a schedule for turning the elements down so you don’t overfire  the kiln.

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6 hours ago, Catatonic said:

Any ideas how you would do a slow glaze on a manual kiln? I've heard others talk about this--is it about letting the time do the job and not necessarily the temperature? Want to understand the principles at work so I can see how to achieve it with or without an automatic controller (which I can't yet afford.)  I imagine a slow cool can be achieved by keeping the kid down until the outside of the kiln has cooled? 

Log everything you do and see how you go. Each kiln Different.

Not sure why you want to glaze fire fast.

Learn to use colour through peep hole to gauge temp.

Fired manually for decades. Fancy gear makes it easier but Not foolproof by anymeans.

The obvious answer would be to turn the dials up minimally till 600degC  (about 6hours.)

Increase heat till about 80degC below target temp then slow it down again .

Witness cones on each shelfand infront of peepholes.

Thermocouple and pyrometer relatively inexpensive and well worth the cost.

I fired 100degC/hr  to 600degC. So 6hrs. one peep on top out TILL about 200dsegC

150degC /hr to 100degC below target

80deg/hr to target. Soak 10mins,  if want.

Log the temp and temp range on the hour and record colour in kiln.

Log time and time kiln on for.

Place a bank of cones near peephole somevfolk puta blackline through themto make them stand out. Use approp. Goggles.

Fancy stuff not essential. Observing and noting is.

Check out  Pres. He has posteda colour chart here.

Not boring eh?

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On 1/17/2021 at 11:34 PM, Babs said:

Not sure why you want to glaze fire fast.

I'm not trying to do a fast glaze--trying to figure out how NOT to do one. ;0)

I've never done a glaze fire on my Duncan EA-820 before, so its a brave new world. I've done four bisque fires, and I'm still dialing in how to get my kiln to climb at the right rates such that it doesn't hit my target too quickly. So the idea of doing this with glazed work is intimidating. I'm reading John Britt's book on Mid Fire Glazes, and the firing routines he shows tend to assume you have an automatic kiln. So I am looking for any wisdom I can find that will show me if, and how, I can do some of those routines with my type of kiln.  

 

On 1/17/2021 at 11:34 PM, Babs said:

Fired manually for decades. Fancy gear makes it easier but Not foolproof by any means.

I am SO GLAD to hear from someone who is well versed in "old school" firing.  Like I've said before--we newbies are most often working with equipment that the masters have long since discarded for more modern stuff, so we need to hear what they learned before they traded up! Thanks for your helpful advice!

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8 hours ago, Catatonic said:

we newbies are most often working with equipment that the masters have long since discarded for more modern stuff, so we need to hear what they learned before they traded up!

Not many folks can fire by color very accurately anymore.

Buy a cheap pyrometer, writing down readings and power levels and temperature can improve your feel a whole bunch. Fairly cheap pyrometers on Amazon.

I have Even built spreadsheets for IPad, Android, Windows, google sheets ..... that allow folks to log and see their average firing rates. Of course pen, paper, and  a little math always works and is always available.

This type of experienced knowledge for you kiln is Impossible to get .............................without the experience part actually.

Have fun!

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5 hours ago, Bill Kielb said:

This type of experienced knowledge for you kiln is Impossible to get .............................without the experience part actually.

Okay, Bill, you nailed me!  ;0)  

Yes, I finally accepted the need to invest in a pyrometer, and ordered a Skutt pyro a few days ago after reading all these posts.  So in the meantime I've kiln-washed my shelves, and will be making a bunch of test tiles so when it arrives I can observe a bisque operation and prepare for a glaze test-firing. 

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3 hours ago, Catatonic said:

 

Okay, Bill, you nailed me!  ;0)  

Yes, I finally accepted the need to invest in a pyrometer, and ordered a Skutt pyro a few days ago after reading all these posts.  So in the meantime I've kiln-washed my shelves, and will be making a bunch of test tiles so when it arrives I can observe a bisque operation and prepare for a glaze test-firing. 

Kiln wash on post ends and the cone holder end of kiln sitter, inside of kiln,  helps too.

Check out how to place kilnsitter cone correctly.

Great about the thermocouple and pyrometer purchase.

All the best.

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I have been using a dual Skutt pyro for 15 years, it is easy to use and makes holding and down firing a breeze.    My kilns are old enough I have a metal rod that I use to push the metal button back in, then I gently lower the trip arm.   Most holds are no longer than a half hour so you don't have to babysit the kiln very long.   I go out to my studio when it is time for it to click,  the temperature drops quickly so you need to be close by and get it turned back on.  Many glaze formula's will give you the temperature and length of hold you will need for that particular glaze.  Denice

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