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Brandee Ross

Help Diagnosing Glaze Imperfections (Pinholes, Blisters)

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Mea, I think this may be my issue. I tried GlazeNerd's schedule and it actually worsened the problem.  With the new program, I got over 50% loss but bent a perfect cone 6 on all shelves.

I suspect my flaws were isolated bubbles/blistering that was exasperated when I switched from a downdraft kiln vent to the overhead style (a little more heat inside the kiln). My earlier firings were more like cone 5.5-5.75. The clay is rated to cone 8 but I guess that doesn't mean it has to like it.

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6826eefd53e7bb9f179e28e0861986d9ed410673-1.jpg

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If it got worse at a higher temp, then chances are it's over fired. To me it looks like that glaze is unstable, not that it's a clay problem. Especially with a slow bisque and a clay that's rated to cone 8, I doubt that the clay is the problem. That glaze is out of limits in boron, and maxed out in everything else. Add 4% copper to that and it's really over-fluxed. To deal with the issue, you either need to fire lower to cone 5, which would mean your cone 8 clay is 3 cones under fired, or increase the stabilizers in your glaze. I would try increasing the silica and EPK in equal amounts, in 3% increments, until it stabilizes. Chances are it's going to start affecting the glaze color, though. A.R.T. Clay used to have a similar glaze in their catalog, and it had to be somewhat unstable in order to get the green color. Any attempt to stiffen the glaze or stabilize it ruined the color.

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I posted images with the green glaze, but I am having the same defect on all my glazes and some of them use a different base:

Licrorice Mod 4 (MC6G)
Ferro Frit 3134    26.00
Custer Feldspar    18.00
Talc        5.00
Whiting        4.00
Ball Clay    25.00
Silica        22.00
ADD    
Red Iron Oxide    9.00%
Cobalt Carb    2.00%

Bright Sky Blue (MC6G Glossy Base 1)
G-200 Feldspar    20.00
Ferro Frit 3134    20.00
Wollastonite    10.00
EPK Kaolin    20.00
Silica        18.50
Talc        11.50
ADD    
Cobalt Carb    1.00%
Red Iron Oxide    0.50%
Rutile        6.00%

French Blue (MC6G Glossy Base 2, same base as Rutile Green)
Custer Feldspar    22.00
Whiting        4.00
Talc        5.00
EPK Kaolin    17.00
Silica        26.00
Ferro Frit 3134    26.00
ADD    
Cobalt Carb    1.00%
Rutile        4.00%

Raspberry (Van Gilder)
Nepheline Sye    18.00
Ferro Frit 3134    14.00
Whiting        20.00
Ball Clay    18.00
Silica        30.00
ADD    
Chrome Oxide    0.20%
Tin Oxide    3.80%

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I could be wrong here, but if the problem only started occurring when you went from changed vent styles, then its obviously something to do with the vent? Are you sure your vent is pulling enough oxygen during the firings? Cause it sounds like you had it going really strong before when you said that :

Quote

I suspect my flaws were isolated bubbles/blistering that was exasperated when I switched from a downdraft kiln vent to the overhead style (a little more heat inside the kiln). My earlier firings were more like cone 5.5-5.75.

So I am going to go out on a limb here and say that I think your clay needs to be bisque with a stronger vent pull. If you were firing at ^5.5 compared to ^6 with only a change in the vent. That was some mega draft going on. So either pull a plug out during the bisque firing, or turn up your overhead. I am not sure how those work though for actually pulling oxygen into a kiln. Never used one. I thought the overhead ones were mostly for fumes, where the downdrafts are actually to increase the oxygen environment. Which can be a visible difference in results for some clays and glazes.

Edited by Joseph F

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I’ve used two of those glazes in the past in a kiln with no vent, no problem with craters whatsoever. Your bisque and glaze schedule both look okay to me which leaves 1 other variable, that being the clay. I would beg, borrow or buy a different clay and give it a shot. If you don’t mind white clay I would try a ^6 smooth white stoneware (like B-Mix or one of it's knockoffs). Could be the manufacturer of your current clay has some contaminated materials. 

The cost of a new bag of clay is nothing  compared to loosing so many pots.

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@Joseph F the change in cone was from trying the new schedule. When I fired my usual schedule there was only a minor difference in heat work in the two vent systems. 

I asked Vent-a-Kiln about the oxygen and they assured me I wouldn't notice a difference... but they were trying to sell me a vent, ha ha! I ordered a downdraft vent yesterday so  I'll hold off doing any more firings until it's installed. The problem started before I replaced the vent but perhaps it was due to wear and tear; the old downdraft was pretty beat up when I replaced it.

That top peep is always open during bisque but maybe the overhead isn't getting enough fresh oxygen into the kiln. 

@Min I will also try another body; the clay I'm using is Rovin's most popular: https://rovinceramics.com/collections/white-moist-clay/products/ro-14-light-stoneware They also carry a few Laguna varieties so I'll give that a try.

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I began having the same issue with a new batch of laguna soldate 60, something changed in the mix. I ordered a ton because a friend needed some for his school, the texture and color of the claybody is slightly different than what they were selling and I am also having similar issues with some cone 6 glazes with pin holes and craters, I still have 1000#'s of the old clay it fires fine. The problem I'm having is only in the mid fire range. I also use several different cone 6-10 clays and the problem is isolated to the new batch of soldate 60 for me, perhaps some similar element has changed recently in your clay?

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1 hour ago, Brandee Ross said:

That top peep is always open during bisque but maybe the overhead isn't getting enough fresh oxygen into the kiln. 

I am not sure about the overhead stuff I could be way off, and maybe the top peep open with an overhead is too much vent, with downdraft you don't open any peeps.. I really don't know for sure. I misunderstood your post. I thought you had said when you changed the vents you dropped a half a cone; which seemed really odd. 

The best thing to do would just to be fire some other clay bodies side by side with the trouble body and look at it. That will be the quick way to eliminate a lot of these questions.

It eliminates: 

1. Bisque Schedule

2. Oxygen in Kiln

3. Glaze Issue or Clay issue

4. Firing Schedule.

If you pull out all the samples with pinholes, then you know its not clay. Then it has to be glaze, bisque, glaze schedule, or application.

If you pull out all the samples and only your clay has pinholes, then its clay. Then it is has to be a batch issue if nothing else has changed.

 

Edited by Joseph F

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The Vent A Kiln hood, which sits over the kiln, is designed to remove fumes, odors, and excess heat.  Basically, like the hood over a range vents odors and smoke out of the kitchen.  A downdraft vent that is attached to the bottom of the kiln draws oxygen from the top through the kiln, along with removing fumes, odors, and heat. 

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@Joseph F With an overhead vent hood, the top peep is supposed to be left open all the time.

@Brandee Ross I use an overhead vent hood, which is 15 years old and coughing on its deathbed now. I'm going to replace it with another overhead vent, because of the possibility that an undermounted vent will change my firings. Leaving me to figure out how to fire my pots again. I can't afford to spend that kind of time troubleshooting! So I don't blame you for wanting to go back to the same vent you had before. 

I still think the issue is related to temperature, rather than oxygen or impurities in your clay. But only you can say for sure by eliminating the possible causes in your own situation.

I'm dying to know the final answer. 

Joseph F likes this

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@GEP I use a lot of wax resist so I'm actually thinking of using both vents, but with the overhead raised up at the max 4" (or slightly more). The overhead vent definitely makes the room more comfortable to be in than the downdraft alone, both in terms of smell and temperature. 

I'm dying to know the answer too, because this is my busiest month of shows and it's extremely stressful not having enough inventory to make those fees worthwhile!

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On 8/18/2017 at 10:30 AM, Brandee Ross said:

This is a problem I have had intermittently since I started making pottery at home in 2010, and lately it seems to be happening to more pieces each firing. 

 

post-1555-0-97946800-1503075775_thumb.jpg

post-1555-0-68140300-1503075778_thumb.jpg

 

1. Are these pinholes or blisters? Sometimes they are sharp on the edges.

2. How can I correct this?

 

Helpful (?) details:

  • I am doing a slow bisque firing to cone 04. I hold for 10-15 minutes at peak depending how tight I've loaded the kiln.
  • Bisque firing profile: 80/hr to 250F, 200/hr to 1000F, 100/hr to 1100F, 180/hr to 1676F, 80/hr to 1945.
  • Glaze firing to cone 6. I do a programmed "slow" firing on Bartlett controller to 2167F and hold for 15-20 minutes depending on load. Witness cones show cone 6 achieved.
  • Stoneware clay, made by a local manufacturer (all-purpose Goldart-based body rated cone 6 to 8. Contains 4.5% fine grog).
  • 10 cubic foot kiln, electric
  • Kiln is vented with a Vent-a-Kiln hood that is only 2 months old, replaced broken downdraft vent. Problem occurred with both vents.
  • Trouble occurs sporadically with all my glazes, which I mix. The green glaze recipe in example photos:
    Rutile green:
    Talc 5,
    Custer Feldspar 22
    Whiting 4
    Silica 26
    EPK Kaolin 17
    Ferro Frit 3134 26
    ADD Rutile (light) 6%, Copper Carb 4%
  • Happens on all types of work: mugs, bowls, etc. 

I've read on the forum about correcting pinholes with a slower bisque but I feel my firing is pretty slow already. Do I need to slow it more? Why does it only happen to certain pieces? Sometimes two identical pieces glazed and fired at the same time in the same way result in one unblemished piece and one as shown above.

Working from home on my own and really feeling out of my depth... and incredibly frustrated.

 

Your glaze firing program doesn't agree with the Barlett slow glaze program, the temperature of 2167 F even with a 20 min hold woudln't be cone 6 , some where around cone 5 and cone 5.5.  You are getting cone 6 past 90 degree so your kiln must be going very slow.  How many hours is the firing? Two of the glazes on the next page are within cone 6 food safe limits. Bright sky blue and Rasberry.  So probably your clay is suspect.

David

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Brandee:

When a clay body is over-fluxed, or improperly mixed (lacks homogenous blending), or has large particle feldspar debris: you get serious off- gassing that looks like this unglazed sample. 

gallery_73441_1183_1065772.jpg

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Bruce: been awhile since I took this one. From memory-20X.

Here is a close-up of a pin- hole at 200x. It is roughly the same size as shown on Brandee's green sample above.

That help Bruce?

gallery_73441_1250_13454.jpg

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On 9/2/2017 at 0:57 PM, Brandee Ross said:

I posted images with the green glaze, but I am having the same defect on all my glazes and some of them use a different base:

Licrorice Mod 4 (MC6G)
Ferro Frit 3134    26.00
Custer Feldspar    18.00
Talc        5.00
Whiting        4.00
Ball Clay    25.00
Silica        22.00
ADD    
Red Iron Oxide    9.00%
Cobalt Carb    2.00%

Bright Sky Blue (MC6G Glossy Base 1)
G-200 Feldspar    20.00
Ferro Frit 3134    20.00
Wollastonite    10.00
EPK Kaolin    20.00
Silica        18.50
Talc        11.50
ADD    
Cobalt Carb    1.00%
Red Iron Oxide    0.50%
Rutile        6.00%

French Blue (MC6G Glossy Base 2, same base as Rutile Green)
Custer Feldspar    22.00
Whiting        4.00
Talc        5.00
EPK Kaolin    17.00
Silica        26.00
Ferro Frit 3134    26.00
ADD    
Cobalt Carb    1.00%
Rutile        4.00%

Raspberry (Van Gilder)
Nepheline Sye    18.00
Ferro Frit 3134    14.00
Whiting        20.00
Ball Clay    18.00
Silica        30.00
ADD    
Chrome Oxide    0.20%
Tin Oxide    3.80%

I know it is a bad time to think about your problem, but you are mixing different base glazes together when  you double dip.  You could probably get the same effect using the same base but changing the colorants.  there may be times that you have a Custer base and than a G200  or Nephline Sye base on top of it, for me this would invite trouble.  There is no reason why you are stuck with different bases to get what you want.  It will require some color testing but both glazes would be compatable.

David

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testing time even though you are in your busiest period.

I'd start with the temp, fire to a lower cone and go slowly up to end of firing and hold for 10-15mins.

(I have a copper rich glaze that hates to be rushed at the end, or it blisters, likes to be fired slowly over the last ramp and even though listed as a Cone 6 really needs to reach C 5  from 1080D centigrade at about 60Deg. C /hr  and be held to allow to settle and smooth over.)  I used to  refire the flawed ones after crushing the blistered areas and slapping glaze on them to a lower cone before I worked it out, or fixed the prob at my basic level.

and then if not fixed, follow the advice, one variant at a time.

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I had a successful glaze firing. I added the downdraft vent (still using the overhead vent also, helps with wax fumes and ambient room temperature) and went back to my original firing schedule. I changed the hold from 20 to 16 minutes with better results, and then finally to 10 minutes with the best results. Thank you for everyone's thoughtful advice and help. I appreciate all of it!

Joseph F likes this

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