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Brandee Ross

Help Diagnosing Glaze Imperfections (Pinholes, Blisters)

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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.

 

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Try knocking down cone 4 after a 15 min hold, and hold that lower temp for an hour or so to get to cone 6 to let things even out. Sometimes if you get to your target cone too quick, the glaze hasn't fully sorted itself out.

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What works well for me is a hold on the way down, 15 - 30 minutes at around 80 - 100F  less than your peak temp. Use cone packs to monitor the heatwork until you get it dialled in. It adds a little more to the cones but not much with an additional 15 min soak. If 15 min doesn’t do it then lower your top (temp after the soak) to when ^6 is at around 3:00 o'clock then when you get to the lower soak temp hold for longer but watch the cone until it’s tip touching shelf (for regular cones, not self supporting)

 

My guess is it’s your rutile that is off gassing and causing the pins in your green glaze recipe. Whiting does this too but you only have a titch of it in the glaze so probably not from this. 

 

Often pinholes and blisters are from gassing out of the claybody, not the glaze, but your bisque firing schedule is slow and you vent the kiln so I wouldn't change that. Have you tried these glazes on another claybody and if so did they blister / pin also? Also, pins can occur over the trimmed area of pots when using a grogged clay but your pins are also on untrimmed areas so this is less likely also. Gasses push the glaze up like a mini volcano then as the glaze heals over the top of the volcano smooths over and the glaze smooths out. Pins can be the remains of blisters that haven't completely healed. The other thing that can cause them is having too viscous a glaze so even with a soak the glaze isn't fluid enough to heal over the blister / pin, however as you are having this on multiple glazes then I would go back to looking at the firing schedule. Hope you get it sorted out, frustrating little things aren't they!

 

edited to add the last paragraph

Edited by Min
Tyler Miller likes this

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Bone dry:

post-1555-0-49318300-1503091629_thumb.jpg

 

I don't rush my drying. I would say most pieces take at least 4-5 days to reach bone dry, longer if I leave them under plastic (handles and other attachments). I spend 4-6 weeks making a new cycle of work so work could sit out for that long before being bisqued.

Edited by Brandee Ross

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Pinholes were fairly common for me also.

 

After the urging of GlazeNerd, I changed the glaze fire schedule to stop climbing at 400*F per hour when the kiln hit to 2050*F. Then climb at 108*F per hour to 2190*F and hold 15 minutes. Then I added Min's downfire hold at 2100*F for 10 minutes.

 

No pinholes.

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My current glaze program:

150°/hr to 250°F

400°/hr to 1676°F

120°/hr to 2167°F

18-20 minute hold, off

 

So my new program would be:

150°/hr to 250°F

400°/hr to 1676°F

108°/hr to 2167°F

18-20 minute hold

999/hr to 2100°F

10 minute hold

off

 

Is that correct? Maybe I should go a little cooler than 2100°F since I'm only going to 2167°F, maybe more like 2067°F?

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Meant to say the 2050*F, according to GlazeNerd, is the point at which the clay does some weird stuff. Going thru this temp slowly is supposed to greatly help. It did for me but I had to add Min's hold at 2100 to get rid of all the pinholes.

 

It had been expressed to about 125* from the 2050 till ^ temp. Your present schedule does this. Maybe just adding the 2067 hold for 10 minutes will get you pinhole free.

 

You certainly will get thru that magic number slowly with your proposed new schedule. With you stopping at 1676 then forward at 108* will add 4.5 hours to your schedule. 

 

Any reason for stopping at 1676*?

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If I am seeing it correctly; a grayish cast in your raw clay. If so, that means a higher % of higher sulfate/ carbon content ball clay was used as a plasticizer. While these types of clay add plasticity and sub micron particle sizes that help reduce absorption in a fired piece: they also off gas at much higher rates than other clay types.

Bisqing to higher temps will burn off carbons in clay, but not the sulfides. The feldspars reach their maximum effectiveness at 2190F, so that should be your peak temp.

150F to 250 F

400F to 2050F

108F to 2190F with 20 minute hold

9999 to 2100F with 10 minute hold

Kiln off.

 

A general rule of thumb: when you can see bare clay at the base of a pinhole, then it is highly likely it is the clay off- gassing. Glaze off- gassing generally presents itself as smaller, but more numerous pinholes.

 

Nerd

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Is it possible that my manufacturer's raw clay materials have just changed a bit in composition over time and that is why the problem seems more prevalent now?

 

I sometimes have both types of pinholes, but for the most part I can see down to the clay. I will try the new schedule when my bisque load completes. I cannot thank you enough for generously sharing your knowledge!

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Brandee:
I am seeing more and more incidents of large particles of feldspar in raw clay samples I test. Chemistry aside, it can take up to an hour to burn off some of the chunks I am seeing. I will edit this post later, to add pictures.

DH: at 2050F is when meta kaolin begins to convert to spinel formations. Clay is actually expanding up to the 2050F, then it begins to compact as the glassy matrix begins to form. 2050F to 2190F is the peak period of glass formation in a clay body: even more critical for stoneware bodies. Not only does it deal with pin hole issues from off-gassing, but also lowers absorption because of higher glass content. Min is also correct with her temp addition, because the glaze is still fluid enough to heal over, but not molten.

Nerd

 

50 35m

 
These are the large feldspar particles that I began seeing in raw clay material. If that trend continues, the incidents of pinholing and craters will increase.
 
Edit: Picture added.
Edited by glazenerd
Brandee Ross likes this

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Goldart clay has or had a high sulphur content. I used it in the 70s and stopped.

That may be your problem. Do you have any other clay to test? Could you borrow some from someone to see if the glaze does it on a different body?

 

Marcia

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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. 

 

attachicon.gif1.jpg

attachicon.gif2.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 is not quite in the food safe range, and whiting can cause outgassing.  If you want to bring the glaze into the food safe range and use wollastonite in place of whiting than  a better way to bring it into the food safe range is making a line blend, keeping the same Al/Si ratio.  A 7 tile blend :

 A                              B

Frit 3134  92.5        134.48

Custer      78.3         113.6

EPK          60.5          31.4

TALC         17.8          25.8

wollastonite 16.5        24.06

silica            84.4        20.65

 

Millilitres per sample  need 60 cc syringe pet store should have them

No  1    2   3   4   5   6   7

A   96  80  64 48 32 16  0

B    0   16  32  48 64 80 96

David

Edited by David Woodin

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sorry that i cannot help the discussion.  i am intrigued by the second photo with the concentric stripes.  are you saying there are problems with that glaze?  i ask because i have a similar glaze on a mug made by someone else that i have loved for years.   have always wondered how it was done, the consistently even spots are what i wonder about.  should they be there? or are they what you are trying to eliminate?

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Oldlady, the problem I want to correct shows more clearly in the first photo. There are some similar blemishes on the inside but they are less noticeable.

 

The spotting you refer to is not a defect. In fact, I really love that effect. It happens when I put my black glaze over the rutile. I think it's due to the high amount of iron oxide in the black glaze, sort of like what happens with oil spot glazes. The stripes are just wax resist.

 

Here's a commercial glaze series that produces the same result: https://www.coyoteclay.com/TwoStep.html

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thank you, i will try for this effect with my glazes.  i do not use commercial glazes but i will try Licorice over a couple of things i have on hand.  have been testing the licorice base for color and noticed that my white glaze over the colors produces something that i will explore.  i think my mug was done in licorice with a white on top.  i got it in 1985 while working in manhattan.  the potter was gravely ill at the time and i could not ask her for her technique.

 

 

post-2431-0-45808600-1503234968_thumb.jpg

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Brandee, I am looking at your glazes, and at what you had listed, looks like you are using the Van Gilder glaze series, or at least where they have been listed if not elsewhere. I also use the Rutile, and do not have this problem when used over the Cream Rust, or the Licorice. I stopped using the Waxy white because of the issues it had with durability, and the zinc which if you look into it, does have problems. I have changed it to some degree, but still not happy. Currently am using a liner glaze as my white. . . glossier than I like, but durable and not so temperamental. Are you using the Waxy underneath your first images? 

 

On another note, if I might pry a little further, are you sieving you glazes before use, I use an #80 mesh twice on all. Reason I asked, was the way the glaze had spots of green, that were speckled not dispersed. This glaze will hard pan also so if you may want to use a bit of bentonite, my percentage is 2 for the 100g base. 

 

If you read through my old posts, you will find that I am a dinosaur that fires up and down, or slow cools using only cone packs and kiln color for my firing process, as my L&L was purchased before controllers, and I wanted to have the easy option of firing my own way with it. Keeps me up at night, but glaze are more to my way of liking.

 

 

Hope this helps,

best,

Pres

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thank you, i will try for this effect with my glazes.  i do not use commercial glazes but i will try Licorice over a couple of things i have on hand.  have been testing the licorice base for color and noticed that my white glaze over the colors produces something that i will explore.  i think my mug was done in licorice with a white on top.  i got it in 1985 while working in manhattan.  the potter was gravely ill at the time and i could not ask her for her technique.

 

 

@ OLady, there are some recipes in the John Britt Mid Range Glazes book, pages 89-90. There are also some in Chappells Complete Book of Clays and Glazes, I believe he calls them boil through glazes. These recipes all have iron in the base glaze unlike the commercial ones in the link above. This is a combo that I’ve tried, it is very similar to one of the sets from the Britt book. Glaze thickness make a huge difference in the size of the spots. The base recipe in my screenshot is the second recipe, top glaze is the first. Base glaze in my recipe is dark brown, I tried it as is but add some black stain if you want it like your mug. The zirco can be increased if the white top glaze isn't white enough, you can also add colourants like cobalt to the top glaze. It does look just like your mug glaze. 

post-747-0-38928600-1503244567_thumb.png

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I also double sieve with an 80 mesh before every glaze session. I think what you are seeing are flecks of the black glaze. The bowl was dipped in the Rutile Green, then waxed, then MC6G Mod 4 Licorice applied. The second photo is the inside of the same bowl.

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Interesting discussion.

 

Let me ask this, if pinholes and blisters can be caused from the gassing out of the clay body I could assume that a thick wall in a pot can be more prone to pinholes than thin walls. Is this a valid assumption?

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Brandee, here's another theory you can try if the above suggestions don't solve the problem. I had a pinholing issue a few years ago when I switched claybodies. I tried slowing down my bisque program, and adding holds to the top of my glaze program. None of these things helped. The solution was to lower the final temp of my glaze program. Remember than pinholes/bubbles are also a sign of overfiring. I went from cone 6 to cone 5.75 and this solved the problem. My previous claybody was labelled cone 6. My new one is labelled cone 4-6. So the clay was melting at a lower temp and also affecting my liner glaze. It's possible that you have been slightly overfiring all along.

Pres likes this

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