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Hgardner

Firing green ware straight to cone6

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Hello! I’ve heard that you can fire greenware with glaze up to cone 6 in one go- ive never done it myself though. Is this something that actually safe for the pottery? Do I need to do it in a specific way? Fire in a specific way? 
 

Any input would be much appreciated!

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Yep, it's called once firing.  Should be able to find lots of information on it.  It does require a specific firing schedule, which is a slow bisque followed by a ramp up to glaze temps.  I don't do it, so that's about all I know.  I know some glazes react differently to this type of firing.

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Segments 2 & 3 are the difference between bisque ware and raw ware glaze firing. Segments 4 & 5 are to aid in letting pinholes heal themselves. Segments 6, 7 & 8 try to produce crystals in the glaze. Not the big flower looking crystals.

Glaze cone 6...

Segment        Rate F*/HR    Temp    Hold
   1            200         220     30-60

   2            100         500      0

   3            400        2050      0

   4            108        2185^     15

   5           9999        2085     20

   6           9999        1700      0

   7             50        1600     60

   8             50        1500      0

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The clay and glaze chemistry would change the firing rates.  Having the right chemistry between the clay and the glaze will lower the chance of flaking, but the other issue is moisture in the clay leading to cracking, bubbles, and explosions.

Steven Hill has some good articles on his website that are worth reading. http://www.stevenhillpottery.com/articles

I'm sure there are a lot of good older books at the library that would be great reads.

 

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FWIW, I have not altered glaze recipes because of raw glazing. My glazes mainly come from John Britt's ^6 glaze book and Mastering Cone 6 glazes. I don't use bentonite, instead using a  medium special mixed by another potter. This medium, I have been told, is similar to what Magma does to a glaze. As far as moisture, never had a problem. I pour glaze into cylinders, let dry overnight. After spraying the glazes on, the pieces sit overnight before being loaded into the kiln. The 1 main problem I have occurs in the late winter/ early spring. Our location is normally 50%+ humidity. But for a week or 2 humidity gets down to the 20-30% range. If pieces have already been bone dry when this occurs, when the humidity comes back the pieces will adsorb the humidity and create a hairline split from the rim straight down the piece. I have been raw glazing and single firing for 5 years now.

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8 hours ago, dhPotter said:

FWIW, I have not altered glaze recipes because of raw glazing. My glazes mainly come from John Britt's ^6 glaze book and Mastering Cone 6 glazes. I don't use bentonite, instead using a  medium special mixed by another potter. This medium, I have been told, is similar to what Magma does to a glaze. As far as moisture, never had a problem. I pour glaze into cylinders, let dry overnight. After spraying the glazes on, the pieces sit overnight before being loaded into the kiln. The 1 main problem I have occurs in the late winter/ early spring. Our location is normally 50%+ humidity. But for a week or 2 humidity gets down to the 20-30% range. If pieces have already been bone dry when this occurs, when the humidity comes back the pieces will adsorb the humidity and create a hairline split from the rim straight down the piece. I have been raw glazing and single firing for 5 years now.

So you are replacing the clay content usually required with a product which chemically does the same thing I guess.

Wonder if you'd get the splitting if ballclay or bentonite used instead. Just wondering here.

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thanks, john for answering the OP so well.   i have never poured glaze into greenware because i do not make anything deep enough to need pouring.   most of my work is flat or nearly flat so spraying works on the whole pot.     sometimes i do not spray enough on the entire piece and so i hold those pieces for a second spraying and a second firing later.  no problems there either.

hgardner, you might be surprised to know that you can fire anything if you go slowly.   i use the preprogrammed "slow glaze" firing in my L&L computerized kiln after the preprogrammed preheat.  never had a problem, one piece broke from my stupidity, nothing to do with the process.   that means greenware, previously bisqued or previously glaze fired pieces all in the same firing.    

because i make mostly flat things, i can use up to 9 shelves in my 27 inch deep kiln.  the last firing was using 7 shelves filled with work and it took just over 14 hours from switching it on.  lots of people want to fire glaze faster but they have had a firing to bisque already.  two firings seem counterproductive, and single firing works for me.

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Thank you.  I'm sorry to say this but, oldlady.

The OP's questions has to many variables to come out with a good straight forward answer.

Have you had any issues with bulk pre made clay and the organic content with the "slow glaze" setting?  I know at one point years (10+) ago it was an issue some pre made clay.

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6 minutes ago, Jhon said:

Thank you.  I'm sorry to say this but, oldlady.

The OP's questions has to many variables to come out with a good straight forward answer.

Have you had any issues with bulk pre made clay and the organic content with the "slow glaze" setting?  I know at one point years (10+) ago it was an issue some pre made clay.

I have issues with my red stoneware even with a nice long slow bisque and a slow glaze fire sometimes, I'd assume that you want a clay that doesn't gas off much for once firing.

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Perfect storm would be a dirty body gassing off combined with a glaze having early melting materials in it such as boron. If the organics haven't burned off before the glaze seals the surface there is going to be issues with pinholes and bloats. 

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oxygenlimbesaw

I never liked the idea adding oxides to clay or using clay that has a high oxide % and have only used red stoneware at other potteries.

I wounder if the oxide that's being used is synthetic and if it's causing an issue.

I think it has something to do with the way it reacts to oxygen at heat, but was unable to find any good information about the reaction and at what temp it happens at.

 

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23 minutes ago, Jhon said:

oxygenlimbesaw

I never liked the idea adding oxides to clay or using clay that has a high oxide % and have only used red stoneware at other potteries.

I wounder if the oxide that's being used is synthetic and if it's causing an issue.

I think it has something to do with the way it reacts to oxygen at heat, but was unable to find any good information about the reaction and at what temp it happens at.

 

Red clays are colored with iron disulfide.  Sulfides release sulfur vapor at temperature, so hard to avoid. Organics burn out from 1200-1700 degrees so I slow ramp through that range.  

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I used to do this a lot in my outdoor gas kiln. It is correct to use a glaze with a significant quantity of clay in the recipe. Just go by the book- slow warm up - (its called candling) even overnight then start the  regular firing in the morning. I used recipes specifically for once firing - some even went on as a slip right after trimming.  Dry things really well. Everything has to shrink and fit together at the same rate.

Edited by terrim8

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Babs, only replacing the 2% or so of bentonite with the "brushing medium". Still use EPK or Ball Clay as required by the recipe. The replacement more or less makes the glaze in the bucket thixotropic. I like that because I only glaze every 6 weeks or so. Glazes do not settle out in the bucket. This is the only difference in the recipes from the original printed recipes.

Several years ago I took a Steven Hill workshop. He had us bring "light or white" firing clay so we could spray glaze and then fire them that night. I only use light or white firing clay. OldLady uses a white firing clay, also.

Edited by dhPotter

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13 hours ago, Jhon said:

Iron disulfide is know to cause bloating and there is an article on this site that talks about it.

There is also a lot of synthetic iron that gets used in pottery.

I have never really heard this. Not saying it’s not out there. What would be a form of synthetic iron? Much of the planet contains iron, wouldn’t a synthetic be costly? Maybe a method to extract iron but greater purity?

Edited by Bill Kielb

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Synthetic just means synthesized, it is what we use as "high purity" and is made from steel mill waste.  But I would expect any clay body that uses synthetic iron oxide to stain everything it touches, which has been my experience wedging RIO into clay.

I highly doubt any clay body would use pure RIO when there are clays like redart and lizella, etc etc that are chock full of iron and much much cheaper and useful

Edited by liambesaw

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Most red bodies simply use a high iron clay as the source of iron. It's the best way to do it, as adding raw RIO tends to make the body more brittle. Black clay bodies are an example of where they do tend add RIO, since they can't get enough iron from clays. You can tell the difference, though. When they add RIO it's more red, and stains a lot worse.

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39 minutes ago, liambesaw said:

Synthetic just means synthesized, it is what we use as "high purity" and is made from steel mill waste.  But I would expect any clay body that uses synthetic iron oxide to stain everything it touches, which has been my experience wedging RIO into clay.

I highly doubt any clay body would use pure RIO when there are clays like redart and lizella, etc etc that are chock full of iron and much much cheaper and useful

Makes sense, iron is iron.

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22 minutes ago, Bill Kielb said:

Makes sense, iron is iron.

I really prefer the synthetic rio to the ceramics grade, it has a wayyy nicer and predictable result.  It's also like candy apple red in the bucket which is nice and dramatic.  But yeah, wear gloves because it doesn't come off of skin very easily.  I believe they make it by burning ferrous chloride or whatever from the mill waste, so not exactly environmentally friendly if you're into that.

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12 hours ago, liambesaw said:

I really prefer the synthetic rio to the ceramics grade, it has a wayyy nicer and predictable result.  It's also like candy apple red in the bucket which is nice and dramatic.  But yeah, wear gloves because it doesn't come off of skin very easily.  I believe they make it by burning ferrous chloride or whatever from the mill waste, so not exactly environmentally friendly if you're into that.

I never really thought about it actually so this is interesting  whether it’s RIO or BIO or YIO I was only interested in the concentration of Iron, mainly for glaze formulation.  Often it’s too much, so for fine adjustment  I would  pick a suitable source. I’ll have to do some research now. Thanks, interesting!

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fwiw, "Several thousand tons of iron oxide is recovered as a by-product each year and sold." from the steel mill where I worked; the incoming hot bands are washed and rinsed in a hot acid bath. The liquid (millions of gallons/year) goes through the "Acid Processor" to recover the hydrochloric acid, separate excess water, and remove the red powder...

Edited by Hulk
d

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7 minutes ago, Hulk said:

fwiw, "Several thousand tons of iron oxide is recovered as a by-product each year and sold." from the steel mill where I worked; the incoming hot bands are washed and rinsed in a hot acid bath. The liquid (millions of gallons/year) goes through the "Acid Processor" to recover the hydrochloric acid, separate excess water, and remove the red powder...

Glad to hear they're able to recover the HCl, do they then burn off the chlorides from the powder? Or do they ship it off to the lab? Recycle it into steel? A little of each?  I think industrial processes are so fascinating

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