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Glaze problem pinholes or blisters


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#21 neilestrick

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Posted 07 January 2014 - 03:58 PM

Your kiln has graded elements. The top and bottom elements run hotter to account for heat loss out the floor and lid. Glaze problems at the bottom could be from the bottom running hotter or colder. More likely colder. The thermocouple is only reading temp in the middle, so if the bottom is packed too tight, it can run cold. Put in cones bottom, middle and top to verify evenness.


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#22 Roberta12

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Posted 07 January 2014 - 08:42 PM

I just recently had a pinhole problem with a glaze that has been very stable.   But....I had two boxes of Bee Mix that had gravel in it.   I am thinking that there are some organic things that are unable to burn out all the way.  I do bisque to 04 and it is a glaze I mixed myself.   and have used successfully.  I only find the gravel chunks when I am trimming.  It's rather frustrating but I have been able to smooth over the holes left by the removal of the gravel, sooooo  I am wondering if my little pinhole is a result of the rough stuff in the clay rather than my glaze?

 

Roberta



#23 neilestrick

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Posted 07 January 2014 - 10:14 PM

It's possible for something organic to burn out and leave a pit in the glaze. But if it's something that's not organic, like a chunk of rock/mineral, I would expect it to melt out the surface or leave a raised area, not a void.


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#24 PranavDesai

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Posted 22 December 2016 - 01:45 AM

I am applying the glaze to the crucibles in the form of spray with spray gun. The glaze maturing temperature is around 1350 deg. celsius. I have glazed around 100 crucibles with the same technique. We preheat the crucible before glazing and the fire at the maturing temperature with a cycle of 4 hours and 20 minutes of holding time at maturing temperature. I also use three different fuel kilns (lets assume them as K1, K2 and K3). In K1 the cycle time is 2.5 hours.  One thing that I've observed is small crucibles in K1 are subjected to pinholes while large crucibles in K2 and K3 don't show any signs of pinholes. I have to fire small crucibles in K1 only. So, how can I solve this problem in K1 to eliminate pinholes?



#25 neilestrick

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Posted 22 December 2016 - 07:45 AM

I am applying the glaze to the crucibles in the form of spray with spray gun. The glaze maturing temperature is around 1350 deg. celsius. I have glazed around 100 crucibles with the same technique. We preheat the crucible before glazing and the fire at the maturing temperature with a cycle of 4 hours and 20 minutes of holding time at maturing temperature. I also use three different fuel kilns (lets assume them as K1, K2 and K3). In K1 the cycle time is 2.5 hours.  One thing that I've observed is small crucibles in K1 are subjected to pinholes while large crucibles in K2 and K3 don't show any signs of pinholes. I have to fire small crucibles in K1 only. So, how can I solve this problem in K1 to eliminate pinholes?

 

Slow down the firing or increase the hold.


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#26 glazenerd

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Posted 22 December 2016 - 09:31 AM

 

It blistered on Desert Buff clay but not on Little Loafers, which is a cleaner clay body

I will resist my temptation to post a ten paragraph explanation: with charts, graphs, and algorithms. (maybe)

 

High carbonaceous ball clay usually medium to darker browns and greys. (some exceptions). The more of this type of clay that is used in a clay recipe: the darker the final color will be. Bentonites (dark grey) are not used much, and if they are: in very low percentages: (so the deep grey color is not coming from bentonites.)  So the first clue that you are using a stoneware body with high carbon content is the color: med to deeper grey or browns. (some exceptions). The carbon problem is then compounded by four things: wall thickness, firing schedule, flux molarity, and the type of carbons: in the clay formula.

 

1. Wall thickness: the thicker the wall; the more time it takes for heat to reach the center, and the amount of time for carbons and gases to escape increase.

2. In bisq: the thicker the wall, or the darker the clay (carbons): the higher you should go above the benchmark 1800F (1000C) bisq temp. 1840-1860 with holds.

3. Flux molarity; as in higher percentages of flux additions= more off gassing; which requires a change in firing schedule as well.

4. Sulfur is the most common carbon found in clay; usually in the form of lignite (brown high sulfur coal), but can also be run of the mill yellow sulfur. Naturally occurring sulfur usually comes in the combined form of sulfates and sulfides. Sulfates will burn off easily, sulfides will not. In fact, sulfates will off gas, but sulfides will leave a residue; that can build up. If the carbon particles are large enough: 20-120 mesh: they will not completely burn off regardless in a normal glaze firing. In addition: very common to find large particles of soapstone in ball clay: which contain high alkali levels: and continue to off gas even at temperatures above 2190F.

 

50 35m

 

That big white chunk in the center is soapstone: the particle is larger than the 30 mesh chunks around it. Soapstone has very high levels of potassium (mainly). It will keep off-gassing long after your hold cycle ends.

 

Check one: if your darker clay has high levels of flux, coupled with high levels of soapstone: in conjunction with high levels of carbons (lignite): then the carbon residue will create a barrier at the face of the clay: and bloating will occur because higher levels of gas created by the higher levels of flux:  cannot penetrate No bloating- go to step 2.

 

Check two: excessive and large pin-hole problems have to be dealt with in the bisq cycle and the glaze fire cycle.  The bisq needs an extended hold, or better yet a slightly higher temp 1840-1860F

 

The glaze firing schedule can be found here: http://community.cer...reen-strength/ (bottom of page one)

 

**If off-gassing blisters are occurring at cone 10: large soapstone contaminants would top my list of possible suspects.

 

Nerd





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