Glaze and Firing Problem

[Briggs Shore]

I’m having a glaze issue I can’t figure out. I’m getting bubbling and blistering on the rims of some but not all of my pieces. It’s only the rims, not the walls or interior or bottom of pieces. The bubbling is definitely not just raising the glaze, it’s picking up the color underneath. Doesn’t seem to matter if it’s slip or underglaze, it happens on both.

Here are the details:

• Color is applied when clay is leather hard, so it’s bisqued onto the pot before glazing.
• The glaze recipe is 20% ea: EPK, Wooll, Custer, Silica, 3124
• I’m firing in an electric kiln to ^6
• The top of the kiln get’s about half a cone hotter than the bottom. The middle is a true ^6

• Clay body is Laguna’s ^5 frost porcelain.
• The color is either slip made with the clay body + mason stain OR Amoco Velvet Underglaze.

It’s much, much worse on wide low pieces like bowls, moderate on cups and mugs, and non-existent on vases. In this most recent firing, the vases were all on the top level, so I’m wondering if temp may have something to do with it.

I’d had this issue once before and thought I’d solved it by being making the rims a bit fatter and being careful to compress them really well with a chamois.

Any ideas? This happened to about half my most recent firing, and I'm loath to go into my next one without some idea how to avoid it. It has happened in previous firings on a small handfull of pieces, but this last one was just brutal. No idea what was different about it.

[oldlady]

just a question.   if the clay is ^5, why are you firing it to ^6?

[Briggs Shore]

2 minutes ago, oldlady said:

just a question.   if the clay is ^5, why are you firing it to ^6?

The clay's firing range includes ^6. It's just that the official name of the clay is "Laguna Cone Five Frost" and I didn't want any confusion.
Why am I firing it that way? Well, because a few years ago when I was doing extensive testing of clays and glazes, at a place that wouldn't let me fire my own kiln I was obligated to fire to ^6 and fell in love with this clay. I've been using it off and on for years since, and while I've occasionally fired it to ^5 in order to accommodate other glazes, I usually default to ^6 because there's a greater wealth of glaze/slip/etc formulas to pull from at that temp. I don't have a deep knowledge of glaze formulation, so I often use a try it and see approach when testing new glazes, and only use ones that have good results with little to no tinkering with after initial tests. This clay/glaze combo works beautifully 90% of the time, it's just this one firing that really sucked and I have no idea why.

[oldlady]

thanks for the very reasonable explanation.  i have tried frost, fired it at cone 6, and found that some glaze recipes do not work with it.   i know less about chemistry than the average infant  so i rely on the advice of several members here.  someone will know how to fix this problem.

[Babs]

Could be that your pots are a bit dry before applying the slip. Spritz pots all over before applying the slip esp. the rims which can be drier than the pot body. Slip can rise off pot if applied too dry... think that is what is going on on side of dish

Does the bubbling appear on unslipped ware?

Could be it is a bit high for this glaze on mug edges. Could try adding a bit of Silica . Or fire lower in load

The bloats could be the moisture point I expressed above or a body bloat but as not occurring on rest of pot, dry application of slip my bet

A soak may solve the mug problem.

Grind, smish bubbles. Apply a bit of glaze and refire lower in kiln with soak and see what that gives.

[liambesaw]

Surprised Hansen 20x5 isn't crazing on porcelain!  

Some mason stains and underglazes are refractory, could be that?  Does it ever happen on just the rims of a piece that has no underglaze/stain?

[Babs]

I use to get blistering on rim of pots mostly so I lowered temp and added soak which sorted that. Think I also added silica

The bigger " bloats" on side if dish. Dry pot on slip applucation. application 

Ywo thingsz going on here I think.

[GEP]

If the problem appeared very recently, and has appeared in the past, I’m wondering if this has to do with aging elements and thermocouples? When I’m getting close to needing new ones, my kilns tend to overfire. 

Are you using witness cones, which would tell you if there is an actual temperature change? Or, how many firings are on your current set of elements/TCs? 

[Bill Kielb]

11 hours ago, Briggs Shore said:

The clay's firing range includes ^6. It's just that the official name of the clay is "Laguna Cone Five Frost" and I didn't want any confusion.
Why am I firing it that way? Well, because a few years ago when I was doing extensive testing of clays and glazes, at a place that wouldn't let me fire my own kiln I was obligated to fire to ^6 and fell in love with this clay. I've been using it off and on for years since, and while I've occasionally fired it to ^5 in order to accommodate other glazes, I usually default to ^6 because there's a greater wealth of glaze/slip/etc formulas to pull from at that temp. I don't have a deep knowledge of glaze formulation, so I often use a try it and see approach when testing new glazes, and only use ones that have good results with little to no tinkering with after initial tests. This clay/glaze combo works beautifully 90% of the time, it's just this one firing that really sucked and I have no idea why.

Interesting glaze  chemistry seems reasonable, boron is spot on for cone 6. Another interesting observation of this glaze is lowering the silica towards 10% very likely makes it more glossy. With 20% silica this glaze needs more heat to melt fully than  moving it closer to 10%.  So interesting point to maybe try a line blend just to see if you find a gloss level you like more than the present.

I use frost and enjoy its qualities as well. Over firing can cause just as much trouble with glazes, so I am interested in what your actual cones say as well.  I think your  next step is to use cones first, maybe test with less silica and finally if it is influenced by something refractory underneath then upping you 3124 to make it melt better over refractory areas. I needed to increase the boron level in our glazes to get it to melt nicely over our frost porcelain with certain underglazes.

[Roberta12]

I am drooling over your handles, btw.  Does this happen with all colors or just the gray and the blue?  I ask because I have been using Amaco's Warm Gray underglaze and found it was crawling and bubbling once I put clear glaze over it and fired.  I talked to a potter who uses Amaco extensively and works for them, and she said the Warm Gray was problematic and many people mix the black and the white to get their own gray.  I have not had that happen with any other underglaze.  If I thin it down with water, it seems to be ok.  (just trying to use up the container that I have)  That's why I asked about color.  I also use porcelain.  Laguna #16

Roberta

[Briggs Shore]

18 hours ago, Babs said:

Could be that your pots are a bit dry before applying the slip. Spritz pots all over before applying the slip esp. the rims which can be drier than the pot body. Slip can rise off pot if applied too dry... think that is what is going on on side of dish

Does the bubbling appear on unslipped ware?

Could be it is a bit high for this glaze on mug edges. Could try adding a bit of Silica . Or fire lower in load

The bloats could be the moisture point I expressed above or a body bloat but as not occurring on rest of pot, dry application of slip my bet

A soak may solve the mug problem.

Grind, smish bubbles. Apply a bit of glaze and refire lower in kiln with soak and see what that gives.

So, I'm already pretty careful about making sure the pots are still moist enough to receive the slip because otherwise they crack. I often sponge them or set them overnight in a damp box to get them the right dampness. But I'm intrigued by this answer because it certainly does have something to do with the slip/underglaze on the rims. The pots seem to absorb the slip pretty well. The whole pot gets a bit soft after the application, which led me to believe it was adhering ok, but is there another way I might check for this?

I'm also intrigued by your second idea of adding silica and then firing cooler. This seems counter-intuitive to what's been happening so I'm really interested in your thinking. The bubbling is happening in the cooler part of the kiln, but your advice seems to be to *raise* the melting point by adding silica, and then re-firing cooler ... and my brain cannot wrap itself around why that might work? Especially when the hot spot in the kiln doesn't give me this problem, so if anything I would think firing hotter would be a solution?

I don't mean to sound ungrateful or like I'm second-guessing you, I just really don't understand and would love to know more.

[Briggs Shore]

18 hours ago, liambesaw said:

Surprised Hansen 20x5 isn't crazing on porcelain!  

Some mason stains and underglazes are refractory, could be that?  Does it ever happen on just the rims of a piece that has no underglaze/stain?

Yeah, I was also surprised it worked so well so effortlessly ... until it doesn't.  Could you please explain what you man by refractory? It's a term I keep hearing and I have no idea what it means. It almost never happens on un-colored pieces, and is worst on the grey (made with 1.5% MS6600) and the dark blue (Amaco Velvet Turquoise Blue V-327). Thanks so much for your help!

[Briggs Shore]

15 hours ago, GEP said:

If the problem appeared very recently, and has appeared in the past, I’m wondering if this has to do with aging elements and thermocouples? When I’m getting close to needing new ones, my kilns tend to overfire. 

Are you using witness cones, which would tell you if there is an actual temperature change? Or, how many firings are on your current set of elements/TCs? 

It's a good thought, and the elements and thermocouple are starting to show their age and should probably be replaced soon, but the last time the problem occurred was about a year ago when all equipment was much newer so unsure if that's the issue. Unfortunately I didn't have witness cones in this firing (I KNOW) but all other signs point to it reaching the usual temp over the usual timeframe.

[Briggs Shore]

18 minutes ago, Roberta12 said:

I am drooling over your handles, btw.  Does this happen with all colors or just the gray and the blue?  I ask because I have been using Amaco's Warm Gray underglaze and found it was crawling and bubbling once I put clear glaze over it and fired.  I talked to a potter who uses Amaco extensively and works for them, and she said the Warm Gray was problematic and many people mix the black and the white to get their own gray.  I have not had that happen with any other underglaze.  If I thin it down with water, it seems to be ok.  (just trying to use up the container that I have)  That's why I asked about color.  I also use porcelain.  Laguna #16

Roberta

Thanks so much! I've worked really hard on them, so this is such a meaningful compliment!

I explained above that it's worst on the grey (1.5% MS6600) and dark blue (Amaco Velvet Turquoise Blue V-327) so it makes sense that it's related to something specific about the colors but I don't know what.

[Briggs Shore]

14 hours ago, Bill Kielb said:

Interesting glaze  chemistry seems reasonable, boron is spot on for cone 6. Another interesting observation of this glaze is lowering the silica towards 10% very likely makes it more glossy. With 20% silica this glaze needs more heat to melt fully than  moving it closer to 10%.  So interesting point to maybe try a line blend just to see if you find a gloss level you like more than the present.

I use frost and enjoy its qualities as well. Over firing can cause just as much trouble with glazes, so I am interested in what your actual cones say as well.  I think your  next step is to use cones first, maybe test with less silica and finally if it is influenced by something refractory underneath then upping you 3124 to make it melt better over refractory areas. I needed to increase the boron level in our glazes to get it to melt nicely over our frost porcelain with certain underglazes.

Ok. Thank you so much for this, and yes, I do think some tinkering with the glaze formula is in order. It's just so frustrating because *I thought I had it down perfect* and really didn't want to have to go back to the drawing board.

I'm very interested that I seem to have gotten competing advice between you and Babs (the silica ammt) and will def do some tests when I can. My biggest hurdle here is that I've got a massive oval kiln, and it takes me a few weeks to fill it up ... so I'm going to have to find a small kiln to do some testing in before throw another huge kilnload of work onto the mercy of the kiln gods.

[GEP]

58 minutes ago, Briggs Shore said:

I didn't have witness cones in this firing (I KNOW)

I rarely use witness cones except when I’m trying to diagnose a problem. 

Even if you have a huge kiln, if you’re diagnosing a problem that might be temperature related, I think it’s worth the energy cost to fire the kiln empty once, with witness cones in the top, middle and bottom. This will give you a baseline knowledge of what your kiln is actually doing. So if adjusting the temp becomes the solution, you can do it accurately. 

[Min]

I agree with GEP that it looks like overfiring. It's hard to tell but it looks like the glaze has thinned out on the rims which would, if it is, be another indicator of this. 

The glaze recipe looks fine as it is, the Best Black 6600 stain you are using for gray I'm guessing is less than a couple percent. I've never found this stain to be refractory.

Before you start tinkering with the glaze recipe I would check the easiest thing first which would be firing the kiln with some witness cones on each shelf. Put some mass in the kiln, kiln posts or scrap unwanted pots and fire it the same way you've been firing. You want to replicate a normal firing.

@liambesaw, Frost has a published COE of 6.99 so it's one of the higher expansion porcelains so crazing shouldn't be an issue with this glaze recipe.

[Babs]

4 hours ago, Briggs Shore said:

So, I'm already pretty careful about making sure the pots are still moist enough to receive the slip because otherwise they crack. I often sponge them or set them overnight in a damp box to get them the right dampness. But I'm intrigued by this answer because it certainly does have something to do with the slip/underglaze on the rims. The pots seem to absorb the slip pretty well. The whole pot gets a bit soft after the application, which led me to believe it was adhering ok, but is there another way I might check for this?

I'm also intrigued by your second idea of adding silica and then firing cooler. This seems counter-intuitive to what's been happening so I'm really interested in your thinking. The bubbling is happening in the cooler part of the kiln, but your advice seems to be to *raise* the melting point by adding silica, and then re-firing cooler ... and my brain cannot wrap itself around why that might work? Especially when the hot spot in the kiln doesn't give me this problem, so if anything I would think firing hotter would be a solution?

I don't mean to sound ungrateful or like I'm second-guessing you, I just really don't understand and would love to know more.

Not a glaze chem guru but work on observation.

Thoughts are :

perhaps your glaze is overfired.

Does it do this on unslipped

so I lowered temp and soaked 15mins. with my glaze

the silica addition was to raise temp tolerance of glaze..

should have done just one but you know...

anyway it worked for me.

rims may have less glaze...thinner  so less absorption. 

Close also to shelf above possibly??

yes cooler spots can give this thus the soak at end of firing.

do you slow your firing ramp for last bit of firing?

anyway I can understand your ponder:-))