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What causes this? I fired 3 of these, this was the only one to do it.  They were all 3  painted with different colors.  I used this yellow on a different tray, and it didn't do it to that one.  ugh ...

 

IMGP1991_zpsbf6e7b27.jpg

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could be several things

could be not fired enough or not soaked

could be the bisque was too fast or not hot enough,

could be the bisque soaked it up top fast which could be resolved by smoothing it when the glaze was dry.

 

Those are my guesses.

 

Marcia

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It was on top with another platter. The other platter came out great.  They were both on top during the bisque firing also.

Fired to Cone 5.

The rate part I don't understand. In the book it has different degrees for different segments?  I did the slow firing with a hold of 10 minutes.

 

cone5firing_zpsa675ebf6.jpg

 

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Soak means a hold at top temperature, but anything over 10 minutes is going to change the heat work by a significant amount and the longer a soak will eventually get you into a cone 6 firing.  You need to be using witness cones to keep track of what is going on.  This is why I would write my own program, because your last segment was at 120 deg /hour.  Unless your kiln has new elements it probably can't go at 120 deg per hour.  The computer controller recalculates the firing based on how fast your kiln can fire within certain limits. My last segment in my user program is anywhere from 30 to 80 deg per hour.  This then means you have to make changes to the top temp because you are firing slower and can get the same heat work. It sounds complicated but isn't.  Are you using a commercial glaze? If you need more information about making a user program than contact me directly.

David

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Soak means a hold at top temperature, but anything over 10 minutes is going to change the heat work by a significant amount and the longer a soak will eventually get you into a cone 6 firing.  You need to be using witness cones to keep track of what is going on.  This is why I would write my own program, because your last segment was at 120 deg /hour.  Unless your kiln has new elements it probably can't go at 120 deg per hour.  The computer controller recalculates the firing based on how fast your kiln can fire within certain limits. My last segment in my user program is anywhere from 30 to 80 deg per hour.  This then means you have to make changes to the top temp because you are firing slower and can get the same heat work. It sounds complicated but isn't.  Are you using a commercial glaze? If you need more information about making a user program than contact me directly.

David

 

My kiln is new. It's only been fired a few times. And yes, I use commercial glazes.  It does sound complicated. I also made a hat looking thing, it also pitted. The rest of the stuff looks good. The hat was on top also. I left the top peephole out during the entire firing. Would that of caused a problem?

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It is probably a combination of a few things. Was this piece stacked with another piece during the bisque firing? If so it could have not gotten proper burn off of the agents inside the clay.  You could have put an extra coat on this one by mistake and thus it was too thick. If this one is near the top peep hole and the other wasn't then the burn off could have settled on this one. If this piece was near the peephole, the glaze could have cooled to fast before it had time to melt and smooth over properly, since it would have had air flow coming in that was much cooler than the kilns environment. These are just a few ideas off the top of my head.

 

The first thing to do to help with pinholes is a vent kit. I know the hard way. Took me like 15 firings without a vent kit to finally get one, then once I did it took 5-6 firings to get it right, now I don't have pinholes anymore. 

 

I have a few questions:

 

1. Are the pin holes on the entire platter or just that one side?

2. If so was that side near the peep hole?

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The peep being open could be a problem.  Some more questions, is there a top shelf over the pieces?  Is the kiln stacked so that there are at least 2 element between shelves?  I would say the firing is too fast and another way to get a slower firing than the slow glass is to fire at cone 4 and hold until 5 goes down.  If you might be interested I can give you a hold time that may do this.

David

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The pinholes are on the whole platter but not the cup that sits on it (it's a chip & dip tray).   It's painted the same yellow inside. And the hat, only the part painted black has pinholes, the brown didn't.  Do you think it might be the paint?  The yellow and black are Laguna, all the other colors are Amaco.  They both are Cone 5 glazes.  That platter was not near the peephole. The other platter that turned out fine was.  There was no shelf on top.  And yes there were 2 or 3 elements from that shelf to the lid.  This is the hat with pinholes on the black but not the brown.

 

IMGP1997_zps697f11c6.jpg

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In most kilns  a top shelf over everything helps.  If you want to use a longer soak try firing to cone 4 and hold 45 minutes.  But have a self standing witness cone where you can see it during the firing, so you can stop the firing if it is too long a soak.  This certainly will tell you if a longer  and slower firing is needed.

David

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In most kilns  a top shelf over everything helps.  If you want to use a longer soak try firing to cone 4 and hold 24 minutes.  But have a self standing witness cone where you can see it during the firing, so you can stop the firing if it is too long a soak.  This certainly will tell you if a longer  and slower firing is needed.

David

 

I have some cones that came with the kiln. I will use them the next time I fire just to see what's going on in there. If it needs adjusted, I will get a hold of you. When you say stop the firing, do you mean hitting the stop button?

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The only way to really tell is going to be, you guessed it, testing! I haven't even fired real pots in the last month because I have been testing so much. I know it sounds awful, but it prevents headaches and questions like this. I kept making beautiful pots, then would open my kiln and get really depressed at how terrible everything went. Now that I am testing, I am a lot happier when I open my kiln. 

 

 

Just curious, what was your bisque like, how heavy was it loaded, did you stack? How did you places theses pieces which had pin holes?

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I had all my small stuff on bottom. On top I had that hat and 2 chip & dip trays.  I put it all back in the same for both bisque and glaze firings.  Just 2 shelves full.  And I did put the bowl on that pitted tray when bisque firing. I made it all one piece, than the dog decided he wanted to eat the bowl. So I had to remake it and just fired it on top of the platter.

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 And I did put the bowl on that pitted tray when bisque firing. I made it all one piece, than the dog decided he wanted to eat the bowl. So I had to remake it and just fired it on top of the platter.

 

Can you explain this more in detail? What do you mean by, "I had to remake it and just fired it on top of the platter?"

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I made the tray, than the bowl and attached the bowl to the tray to make it one piece. Dried it out than the dog chewed the bowl off the tray.  So, I scraped the rest of the bowl off and made another bowl. When I fired the bisque firing, I just put the bowl on the tray. When I glazed fired it, I put glaze on the bottom of the bowl and set it on the tray and fired it so the glaze would make it stick together.

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It was on top with another platter. The other platter came out great.  They were both on top during the bisque firing also.

Fired to Cone 5.

The rate part I don't understand. In the book it has different degrees for different segments?  I did the slow firing with a hold of 10 minutes.

 

cone5firing_zpsa675ebf6.jpg

The rate column is the temperature at which the kiln is firing per hour for the number of hours in the last column to achieve the temperature in the third column.  Your first segments, not listed, are for delayed start and preheating.  So, for your slow glaze, the kilns starts out heating at 150 degrees an hour until it reaches 250F -- or about 1.20 hours (depends on the temperature inside the kiln at the start).  This allows for physical water from the glaze and steam to be released before the glaze materials start to soften and close off the water from escaping.  At 250F, the rate of heating increases to 400 degrees an hour until it reaches 1915F -- or a bit over 4 hours.  At the point, the rate of heating is slowed to 120 degrees an hour until you reach 2165F -- or a tad over 2 hours.  Slowing down at the end, along with a soak, allows the glaze to become more viscous and homogenized and have time to thoroughly melt before cooling starts.  If you heat too fast at the end and  then go into cooling after hitting peak, your bubbling glaze is more likely to have blisters/craters.  So you want a nice smooth transition at the end and start of cooling. 

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As for pitting, the first fix is in your bisque firing. Add a short hold to your bisque -- 10 minutes is more than enough. That allows any impurities and junk in your clay body to burn out. If that doesn't happen at bisque, then it happens at glaze firing when the temperature is higher. (And which is why earthenware potters often bisque one cone higher than glazing.) As others mentioned, pitting can also come from dust on the bisque surface (wipe items with a damp sponge before glazing, etc. and clean your glaze area before glazing, as well as after) or glaze application thickness (maybe too thin; if too thick, the glaze was more likely to crawl). And, if you are brushing on glazes, make sure to use a cross pattern for the various coats to get even coverage and allow sufficient time for drying between coats (lest you move the bottom coat around while putting on the next one).

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When I bisque, I do a slow bisque cone 04 with a preheat of 2 hours and a 2 minute hold.  I also wipe everything with a damp cloth before glazing.  What do you think of refiring this piece at a cone lower?  Do you think this will even out the glaze?

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Refiring to fix glaze defects (like pitting) is generally a crapshoot. It may work, it may not . . . I've seen it go both ways (though more often in the not category). Just approach it as a test . . . if it works, then it is a good test, if it does not, it is still a good test because you learned how your glazes will look refired.

 

You may need to go hotter (longer soak at cone 5, maybe go to cone 6), not cooler (cone lower) to even out the glaze. Firing lower, the glaze may not melt enough to become viscous enough to heal over the pitting. Plus, you don't want to risk the rest of the kiln with underfired work at a lower cone and your glazes will not look the same at that temperature.

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You could try re-firing it/them in with a bisque or ^06 glaze firing.  I just got a mug out of the kiln today that was pitted after a ^6 firing, and it has smoothed off amazingly in a lower firing.  The colours have also changed slightly, but that's a risk you take.  Re-firing could improve a bad piece or worsen an indifferent piece.  

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