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RobS

Glaze Blistering Help

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Hi all,

 

Just unloaded a glaze firing and have multiple pots with blisters and really am searching for why.

 

Here is a picture of a pot and test tile fired in the same load (actually about 2" away from each other).  It is the same commercial glaze double dipped on both pieces.  I have other pots and test tiles with the same issue so I really don't think it is an application problem.  Zero of the 15 tiles blistered while 75% of the pots did.

 

I'm assuming I'm screwing up something with the firing.  Here's what I did:

 

temps in ^F, cone 6 oxidation, electric 

 

hold 250 few hours

350/hr to 2000

150/hr to 2200

hold at 2200 for 15 mins

350/hr to 1900

125/hr to 1350

off and naturally cool

 

Witness cones on every shelf were about textbook.

 

My thoughts are either I should be starting the slow cool at a hotter temp or holding at ^6 longer.

 

Any input or advice is greatly appreciated.  Thank you.

 

Rob

 

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post-64856-0-90410300-1413128105_thumb.jpg

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Hi RobS and welcome to the forums,

 

How did you bisque fire the pots, were they stacked inside each other, do you run a vent or have the plugs out for part of the firing and to what cone did you bisque? Another thought is how thick the glaze application is on the pots versus the tiles?

 

Have you used these glazes in a slightly hotter firing or with a longer soak? I have one glaze that likes to blister and pinhole, I slow the firing down at 1900F to 100F an hour up to 2200F and hold for 20, seems to help for this glaze. 

 

There is a good article on blisters here: http://digitalfire.com/4sight/troubleshooting/ceramic_troubleshooting_glaze_blisters.html

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Min,

 

Thanks for the quick reply.  Bisqued to ^04 with "standard" schedule, no stacking of pots, no vent except for a fan in a door exhausting the room, bottom plug in around 750 and top plug in 1500 ish.  both plugs left in until cool.  I did this with both the bisque and glaze firing.

 

The glaze application is definitely suspect in the little bowl in the picture with the cones, but many of the other ones in that load are glazed what I thought was properly and still have pinholes and blisters.  My glazes were at 1.45 checked and mixed an hour or so before using.  They are all commercial (Amaco potters choice and a local company that mixes their own) glazes so I don't suspect the recipes.

 

I think I will try creeping up on 2200, this little kiln can go way too fast but I installed an infinite switch and sit there and babysit it through the top end and initial cooling phases.  I have lots of testing to do I guess.   

 

Thanks for the advice.

 

Rob

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I would just leave the top plug out the entire firing(both bisque and glaze) and plug it when its done, if your glazes benefit for a slower cooling. Also what is the upper-range on that clay body? I know I have some clay bodies that are cone 6, but blister like crazy at cone 6.5/7. If your holding your your kiln for 10 minutes at cone 6 you could be getting the clay to hot. 

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How big of a kiln is it?   Maybe a bit lower peak temp, like 2195, and double the hold time. the cones could look the same, but the glazes could mature better??

Could the glaze be thicker on the pieces than on the tiles? Looks like runs where the tiles don't sem to have any movement.

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Rob -- This might be a combination of the glazes (Amaco's Potter's Choice are Cone 5, although they will fire to Cone 6), and your firing program. Taking the glazes to cone 6 and then holding may be pushing them past their limits. Try firing to Cone 5 and then holding for 15 minutes (which should give you a soft Cone 6). Regarding your firing schedule, why are you holding at 250F for a few hours? You may want to hold at 200F for a bisque firing to get any physical water out of the wares, but there is no need to hold a glaze firing at that temperature. You might want to slow down your approach to top temperature from 150F/hour to 80F/hour. That will give your glazes more time to melt together. Then add your 15 minute hold.

 

From the pictures, it appears your blisters are on the outside of the wares, but the insides look okay. Or, did the insides also blister? The interiors will be somewhat cooler/not as hot as the exteriors due to wall thickness.

 

You didn't tell much about the clay body. Does it have any ingredients, such as a high iron oxide concentration or manganese in it? Those are ingredients you would want to work on at bisque -- maybe adding a 10 to 15 minute soak at top temperature in bisque to burn out all the nasties.

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Is this a new clay?  I had the exact same type of blistering using a commercial glaze.  It only happened on a brown clay or if over fired.  The glaze was cone 5.  I couldn't even go to 5 1/2.    The glaze worked fine on a white clay though.

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Thank you all for the excellent advice and information.  

 

The clay body is a Laguna WC-608 which is a cone 6 clay with specks, it says it's compounded to look like iron spotting.  

 

The kiln I'm currently using is a small Duncan EA-716 (8 sided, 15.5" x 15.5" inside).  I have an old Reward kiln (Model 822, 18.5" x 22") that I'm refurbishing that I will be using most of the time in the future.  I think I'll have better luck with a larger kiln as it should be more thermally stable than the little one, and it will offer me more control as it has 2 banks of elements.

 

Regarding the firing, the 250 hold was mostly accidental as I had it set to low and got busy doing things around the house.  My intent was to just hold it there for a short time to eliminate any water left from glazing.  

 

Of the pots that did blister, a majority (nice precise mathematical measure) of the blistering was on the outside but there were blisters on the inner surfaces of the affected ware as well.

 

Based on all the great advice from you generous folks, I think I will slow down as much as I can from 1900 to barely 2200 and hold there for 15 mins.  Of course, I will pay better attention to the bisque firings to ensure proper firing and soaks, and take care when dip glazing those large surface area pieces (I can say for sure that at least 1 piece was heavy on the glaze thus blisters).

 

After a 16 year hiatus, I'm finally getting back to the art I love and I'm sure I'll have more questions as I go along.  I lived in Colorado for many years and started doing pottery through the city of Ft Collins and continued for a dozen years.  I moved across the country, had some life events and just lost touch with pottery.  I now have time for it and have a house with a large poured concrete basement.  I'm working on converting a 20' x 40' chunk of it to a pottery studio, which is another topic altogether :).  

 

Unrelated question for those out west.  When I was in Colorado, there was a clay body that I really liked and I'm wondering if anyone knows of it.  I've searched online and had no luck.  I used to purchase it from Mile Hi Ceramics in Denver and it was a reddish iron spotted stoneware, fired to cone 6 and called Mountain Stone.  Does anyone know if it is still available and from whom?

 

Thank you all for the advice and time.  I hope to get to know the folks here as I continue learning and growing.

 

Rob

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I am no where near as technical as I should be when it comes to firing in my electric kiln but I have noticed that when I rush things in my old manual L&L, (I.e., turning to high too soon) I get lots of problems in my glazes. I have even fired cone 05 glazes to cone 6 with no problems (except for the occasional run or drip) when I fire nice and slow.

 

Lots of luck to you and have fun!

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My bet is it is the spots in the clay, either iron or magnesium.  So make some small bowls to use to test with.  Use a white clay and your spotted clay.  Place one of each clay on the different shelves in your kiln.  I even mark mine, T = top shelf, M =  middle shelf, B =  bottom.  My kilns fires cones 5, 5 1/2, 6, bottom, middle, top.  Till I get a vet I just deal with it.

 

 

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