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What Causes The Zit?


dhPotter

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This glaze looks like a rutile/iron comb glaze and if thats a yes than pitting can occur pretty easy. It can be the glaze -atmoshere-dust-clay impurities .

The cures are clean ware-more hold times -and for me the right amount of reduction-for you a little  more hold time in an electric.

If this glaze has rutile and iron as colorants this may always be an issue on and off.Stoneware clay can make it worse.

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Here ya go Neil...

The rim is 2 matte glazes. Strontium Crystal Matte Warm with a Matte Iron Red glaze over.

This is stoneware. Have seen this pitting when wiping the bisque with a damp sponge or just dusting with a dry rag.

 

Matte Iron Red

  Nepheline Syenite A270......    13.20  
  Silica......................    21.90  
  Calcium Carbonate...........    21.90  
  EP Kaolin...................    30.80  
  Talc........................     4.40  
  Strontium Carbonate.........     2.00  
  Tricalcium Phosphate........     5.20  
  Lithium Carbonate...........     0.50  
  Iron Oxide Red..............    12.20  
  Copper Carbonate............     0.90  
  Bentonite...................     2.00  
                              =========
                                 115.00

post-13363-0-63113400-1479226274_thumb.jpg

post-13363-0-63113400-1479226274_thumb.jpg

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Your glaze is prone to this as it has  Strontium Carbonate in it as well as Lithium Carbonate

​Glazes with barium or Strotium are problematic with pitting

Seen this as a woe for 40 years.

The matt is also a factor. For me its worse with stoneware but it happens on my porcelain.

I have used a few of these type of glazes since the 70s and pitting is part of the deal

If you ever cure this altogether we want to know about how you did it

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Mark you are a trip!!!

 

What cone are you glaze firing to?   6

What type of firing?   electric oxidation

What clay body?   stoneware Laguna wc 607, #55

What cone do you bisque to?  04

You said several different glaze combos are doing it? Is there a constant in any of them?  It may be the lithium, had another pitting on a mug in the same firing, but the lithium carb is only 1.2% and magnesium carb 2.4%. Was also thinking it could be the whiting, but this other mug that pitted uses wollastonite.

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I'll never comment on area I have no experience in trust me on that. And that is more and more as I get older.

One of my mentees  I worked with for two years used your Strontium Crystal Matte Warm in her production line for many years(cone6) .

I wish I had a dollar for every time these barium or Strontium glaze had flaws.

I just did kiln load last week in another cone range and pitting is the enemy of these glazes-They are also all matts.

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The fact that only one or two zits appear, makes me suspect of the cause. Secondly, a very large zit to boot: also somewhat of an abnormality of itself. So I will add this thread to my stoneware "watch" list. As stated prior: I am seeing large chunks of white debris in the samples I have been looking at (microscope). At first I thought they were soapstone, but now I am leaning to large particle sizes of feldspar minerals. If that holds to be true: it explains the small number of sites, and the size of the crater.

50 35m

 

As seen in the 200x close up. (The large dark/black specks are 35M, as a point of reference.)

 

Nerd

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When I first saw the small photo I was leaning toward it being something coming out of the clay. The big photo makes me think it's the glaze, though. Try firing a little bit cooler with a hold time. Maybe a little soak will fix the problem. Nerd, what's that program you use with the soak to cone 6?

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Neil... sure- do I get to throw in clay lessons to boot?

 

(your normal ramp) to 2050F     ( the temp when meta-kaolin forms spinel- the basis of mullite in a stoneware body). Fast ramp above this temp means less formation of mullite, and delays peak off gassing until the top of the ramp cycle. (less mullite also means less vitrification)

 

140F (from 2050) to 2190F with an extended hold. (or) 2230F with a short hold.

 

Potassium is normally the flux of choice in stoneware bodies, primarily because many of the fire clays, and ball clays used in stoneware have a fair percentage of Ka to begin with. 2050F to 2180F is when potassium is at its most effective melt temp, which also means its peak off-gassing temp.

 

Porcelain and stoneware are porous up to 2050F, when the effective melt of the potassium begins to close the pores as spinel is developing. Blowing past this key temp (2050) too quickly also means you are sealing the face of the clay directly exposed to the kiln temp, but the inner body is still ejecting gas. The key piece of evidence of creating this "trapped gas" scenario is much larger craters in the glaze: because the gas is trapped until enough pressure is built up to forcibly eject it through the clays surface, and through the glaze.

 

Potassium and sodium are in a gaseous state from 2050F,and up... (starts before that, but begins to hit its peak)

 

2190F with extended hold    ( 15-20 min. for smaller kilns, and up to 45 minutes for large kilns)

2230F with shorter hold  ( 5-10 minutes for smaller kilns and up to 30 minutes for large kilns) < the one I use. Not had any pinholes in over 2 years using this ramp cycle.

 

Nerd

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Excellent information! MarkC you are a good trip!

 

Thanks so much for your input!  Will adjust the firing schedule. Maybe can report results in about a week.

 

BTW I have another piece, 2 matte glazes overlapping, with a bigger crater. It is a green and blue matte. The biggest craters are on flat surfaces, this plate and a laid out platter.

 

Kiln - Olympic 2327 HE - about 7 cu ft

Glaze cone 6...

Segment        Rate F*/HR    Temp    Hold
   1            100         230     30
   2            400        1975      0
   3            100        2185^     15
   4           9999        1700      0
   5             50        1600     60
   6             50        1500      0

 

GlazeNerd are you saying when the kiln hits the 1975* mark to slow down to say 50* per hour till hit the 2185* and hold?

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DH:

 

>your normal ramp> to 2050F

<140F to 2190F with hold (pending kiln size)  or to 2230F with a much shorter hold.

 

2050F to 2180F is the most critical period for stoneware clay maturity. The pottery biz has gotten in the habit of  firing to mature glaze, which has the side of effect of not maturing the clay. If you fire to mature the clay, the glaze will automatically follow suit.

 

My firing schedule is :

400F to 2050F    140F to 2228F with 5 min hold.... bends a cone 6 perfectly.  Not had a pinhole in 2 years, except for some of my "extreme" experimental glazes.

 

Nerd

 

** pending kiln amperage (wattage) some adjustments can be made. Some kilns would struggle to 2050F @ 400F.

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  • 3 weeks later...

DH:

 

Add 10 minutes to your peak hold--and you should be home. OR ramp 10 degrees an hour slower on the high end cycle. It is the clay off gassing; so the particular blend you are using has a higher than normal amount of potassium flux. Which is actually a good thing ( pin holes aside) because the vitrification levels will be much higher.  You should also see a big difference in "weeping" issues. Now you are maturing the clay, not just the glaze.

 

Nerd

 

Try 130F from 2050 to your hold temp.

"the inny and outty" says you are right at the border.  One was healing ( the inny) and the "outty" says a bit more time.

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