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Bubbles in Glaze


Narelle

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I fired some mugs yesterday and got a heap of bubbles in the glaze. I'm only new to ceramics so I'm not sure what went wrong.

The mugs were fire to cone 6 (999c). The fire was done in stages, first to 250c for 2 hours, 500c for 2 hours, 750c for 2 hours and the to 999c and held for 20 minutes at that temp.  After the 20 I turned off the kiln and left to cool naturally overnight.

Any feedback would be great, thanks

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1 hour ago, Narelle said:

The mugs were fire to cone 6 (999c).

I think you might have meant cone 06? Cone 6 would be approx 1220C

What cone is your glaze meant for?

Would you clarify your schedule? Can't quite figure out what you mean. What is the rate per hour you are increasing at in each segment?

Are you using witness cones to verify your final cone?

Hi, and welcome to the forum :)

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Hi Min.  Yes, the glaze is meant for 06.

Only have 06 cones, still waiting for others to come in the mail, as I said in the original post, still very new to this.

The schedule is one that I found on a website and copied.  Have not worked out how to program the temperature controller on the kiln yet, so have been doing it manually.

The ramp up to 250C took 20 minutes, held at that temp for 1hr40mins.

The ramp up from 250 - 500C took another 20 minutes, held at 500C for 1hr40mins. Closed the front peephole at 500C

The ramp up from 500 - 750C took around 30 minutes, held at 750C for 1hr30mins. Closed the top peephole at 750C.

The final ramp from 750 - 999C took 45 minutes, held at 999C for 20 minutes.

Then shut the kiln off, left the peepholes closed. Left cool overnight before opening the door. The mugs were warm in the morning but not hot.

I have the rate of cool down for the first hour or so, written down if you would like to know how quickly it cooled down. I will have to go and get my notes to remember.

 

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Try to figure out the controller because the ramp is what is supposed to take the time, not a fast ramp and then a hold at each.  So the first ramp to 250c, instead of taking 20 minutes with a hold, should take 2 hours.  The second ramp to 500 should again take 2 hours to reach, no hold. And on and so forth.  Fast temperature changes aren't good. 

Ramp should look like a ramp, yours are more like a staircase.  Hope that analogy helps

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OK, thanks Liam, that makes sense. We  have just started another 2 mugs today, as we have not figured out the controller yet, will try smaller steps of 100C each time, or maybe even 50C, but will still have to do a hold at each step, maybe 1 hour instead of 2. 

Will be doing a slower cool down for the first couple of hundred C as well.

Hopefully we will get this worked out.

Forgot to mention in the first post, this glaze is called Northern Lights, and has the crystals in it.

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+1 to what Liam said about learning to use the controller.

You don't need to do a hold at each step. 

A cone simple 06 glaze schedule could look like this:

ramp 1   65C / up to 120C / 0 hold

ramp 2   200C / up to 850C / 0 hold

ramp 3   40C / up to 975 / hold until the 06 cone drops, tip  just about touching kiln shelf, probably about 20 minutes plus or minus 10 minutes or so. You would have to test with cones and your actual firing conditions.

At this point you could either shut the kiln off or do a drop and hold or just do a slow cooling.  A drop and hold is good for glazes that have pinholes. After ramp 3 you would add a 4th ramp: 9999C / 920C / hold for approx 15 minutes. This is just a suggested drop and hold rate, you would have to test it with your kiln and glazes.  At this point you could either shut the kiln off which would work for most gloss glazes or if you need a slow cool you could drop by 90C and hour to approx 800C, dependant on the type of glaze you are using. 

 

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3 minutes ago, Min said:

ramp 1   65C / up to 120C / 0 hold

ramp 2   200C / up to 850C / 0 hold

ramp 3   40C / up to 975 /

 

Thats helpful Min, thanks.

I am not clear on these ramp steps though, should they be a slow increase like Liam said in his post ? How long should these ramps take ?

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The 65c to 120c means the rate of heating is 65c per hour, so to get to 120c it would take about an hour and a half accounting for a room temperature start. Thats ramp 1. Then once the temperature hits 120c, the rate of heating goes up to 200c an hour. So to go from 120c to 850c will take about 3 and a half hours.  That's ramp 2, now third ramp is 40c per hour up to 975, so from 850c to 975 at 40c per hour it'll take about another 3 hours.

So in total that's about 8 hours total where the temperature is gradually growing the entire time

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4 minutes ago, liambesaw said:

The 65c to 120c means the rate of heating is 65c per hour, so to get to 120c it would take about an hour and a half accounting for a room temperature start. Thats ramp 1. Then once the temperature hits 120c, the rate of heating goes up to 200c an hour. So to go from 120c to 850c will take about 3 and a half hours.  That's ramp 2, now third ramp is 40c per hour up to 975, so from 850c to 975 at 40c per hour it'll take about another 3 hours.

So in total that's about 8 hours total where the temperature is gradually growing the entire time

Thanks Liam, now I understand it a lot better. 

I had a play with the controller, and have figured out how to get the temp to ramp up a lot slower. 

I will program those ramps into it tomorrow morning, and try another batch, hopefully with a much better result than that first lot.

This is a great forum, glad I found it.

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There is a lot more to this than just entering a number. A pyrometric cone measures the result of both heat plus time. If, for example, we just programmed the kiln to go as fast as it could to 999C then the temperature would be 999C but the clay and glaze would almost certainly be underfired because the heat didn't have time to mature the clay and glaze all the way. 

In the ramp I posted above I really slowed the last segment down, at 975c there is then a hold (soak) added. The hold will add heat work to the cone (and clay/glaze). How long you hold for will be determined by the cone. You need to wear eye protection (welders or kiln goggles) and actually look through a peephole at the cone. When the tip is just about touching the shelf you will have reached a mature cone 06. You could probably go faster than the 40C and hour I posted but it's a conservative slow rise and if your glaze is prone to pinholes or blisters it could help with these problems. Like you are finding out, it takes some trial and error to get a firing schedule sorted out. If your kiln has some pre-programmed schedules built in you could also try one of those. Your kiln manual should explain about this. If your kiln didn't come with one they are usually available online.

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Thanks Min. Things are a lot clearer to me now after having read the responses to my question from yourself and Liam.

My kiln is an old one, it has been fitted with an aftermarket temperature controller. There is a manual that came with it, but it has been translated from Chinese to English very poorly, so its not easy to understand it.

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So getting back to the the mugs with the bubbles, pictured in post #1. These bubbles are about the size of a pin-head, the photo was taken with a macro lens.

Can these mugs be re-fired to get rid of the defects in the glaze, or are they a bin job ?

I dont mind if they cant be saved, as its a learning experience, but if they can be fixed I might give it a try .

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Here is a typical slow glaze program, you might try. This should end in about 7 hours when your 06 witness cone falls.

Rate        (C)                       Temperature     (C)             Approximate time         

65 deg per hour.          To 120 degrees                        1.2 hours                             

200 deg. per hour.      To 900 degrees                         3.9 hours

50 deg.  per hour         To 1032 degrees.                    2.6 hours

 

This is a fairly standard way to enter a ramp schedule in many controllers. Notice, no hold times necessary at this point until we have a real reason to add one. Right now you need a good firing under your belt to see if you have a pinhole issue.

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Hi @Narelle.  Northern Lights def does NOT like to be fired fast.  You have nothing to lose by re-firing those mugs.  They are not useable as they are, so give it a try.

All the info above from Min and Liam is spot on.

Your set-up sounds like mine, except I was the owner that added the controller to my kiln.

Can you post some pictures or tell us the make and model of your controller, I might just have the same one.  I have my own kiln, and use one at the centre I go to.

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Hi Chilly, thanks for the reply. If re-firing the mugs, would I put more glaze on to help fill the pinholes, or just stick them back in as they are. And would I just fire to the same temperature again ?

My husband fitted the new controller to my kiln. This is the link to the manual http://www.sah.rs/Termoregulatori/Manual/YUYAO/XMT-808(2007).pdf

If you have a similar one in yours, it would be good, as you might be able to answer a couple of questions we have regarding the programming of it.

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Went through this manual and the translation is truly bad. Unfortunately I did not see anything that would allow a ramping schedule to be entered. It really seems this control is a limit controller with  one set value at a time. Hope I am wrong about this. If true one would need to setup all the base parameters, auto tune the controller so it matches the kiln then manually enter the desired setpoint values at each ramp stage of the firing 

hope I got this wrong but paging through each page again it seems to be a limit controller.

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Hi Bill, yes that manual is something else. Most of it just doesnt make sense. 

It does have a 30 step ramp and soak, I should have posted this 2nd manual as well.

http://att.finglai.com/comp/fl-en/instrument/digital-temperature-controller/XMT-8-ramp-soak-description.pdf

 

Following the instructions as best we could, the auto tune function didnt seem to go through its process properly, how long should it take do you think ?

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200deg C per hr is very fast. And goes through crucial points which should be taken slowly.

I'd go for 100C/ hr to 600 for ramp one then 150/hr to 900 then 60 /hr to 999 .

Slowing down the last ramp lets glaze settle.

Those bubbles look similar to what I get when I fire a specific glaze too quickly towards end . I'd try that then if still bubbling cut out the long soak as this may be overfiring this glaze.

An old poster told me a tip.

Grind those bubbles, dab glaze on and refire at the slower end rate.

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How long.

1st ramp 6 hours

Second ramp 2hrs

Last ramp say you were to go 90dec/hr for simple maths  1hr.

Plus soak of 20 minutes during test 1 gives you total of 9hrs 20 min.

If bubbles still happening. Cut the soak time in half. Then still not good. Remove soak.

Let's know what happens

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Hi Babs. Thanks for that info, so much to learn.

When you say grind the bubbles, do you mean to take them down until smooth with the surrounding glaze ?

I have 2 more mugs firing at the moment, same glaze, but using the firing schedule suggested by Min and Liam. Will see how it goes this time.

Its just starting its ramp down after a 20 minute soak at 999C. Almost midnight here, will wait until the kiln turns off, then wait until morning to have a look. 

Will let you all know how it turns out tomorrow.

Thanks all for the help.

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1 hour ago, Babs said:

200deg C per hr is very fast. And goes through crucial points which should be taken slowly.

I'd go for 100C/ hr to 600 for ramp one then 150/hr to 900 then 60 /hr to 999 .

Slowing down the last ramp lets glaze settle.

Those bubbles look similar to what I get when I fire a specific glaze too quickly towards end . I'd try that then if still bubbling cut out the long soak as this may be overfiring this glaze.

An old poster told me a tip.

Grind those bubbles, dab glaze on and refire at the slower end rate.

Every pot that has ever been fired using a Bartlett V6cf controller which includes many many manufactures rebranded under the name dynatrol  etc has been fired at 400 degrees F or 200C per hour in a slow glaze firing so this speed has proven aweful safe over many years and probably hundreds of thousands of pots. So this speed does not seem excessive and has proven fine over time. As to the end of firing  most automatic controllers default to 120 F per hour in the last 250 F of firing so slowing this down further may or may not help the bubbles but may also affect your final temperature when the cone bends.

your schedule is the one that works for you but as far as slowing down some special way for quartz inversion, I just spent a year of  research saying much (not all) of that is mainly myth.

in the end whatever works for your wares and glazes is the correct schedule, just that 400 F (200C) per hour should not be a concern for cracking typical wares.

this speed results in a 7-8 hour firing for cone six.

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2 hours ago, Narelle said:

Hi Bill, yes that manual is something else. Most of it just doesnt make sense. 

It does have a 30 step ramp and soak, I should have posted this 2nd manual as well.

http://att.finglai.com/comp/fl-en/instrument/digital-temperature-controller/XMT-8-ramp-soak-description.pdf

 

Following the instructions as best we could, the auto tune function didnt seem to go through its process properly, how long should it take do you think ?

Thank goodness, I could not  find it in the previous no matter how hard I looked! To answer your question decent pid controllers will take up to 1/2 hour to find a cycle time then begin calibration from there. If you feel it is in auto tune.I would let it go for quite some time as long as the high limit was previously set to a safe value and I monitored it regularly. Doing this by hand usually takes hours, some graph paper and a bunch of math.

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