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Firing Cone 6 White Stoneware

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I received a grant that allowed me to buy a clay that I have never worked with. I am planning on doing some outdoor sculptures and purchased L&R Specialties Cone 6 White Stoneware because it was suggested to me buy a few people.

 

My first question is when firing the greenware am I supposed to fire at cone 06 and fire again at 06 for glaze.

 

Secondly, I recently fired a cylinder type object with this clay that was glazed in amaco underglaze. After the firing the underglaze glaze started to chip off. I put a clear coat over the under glaze hoping this would fix the problem and it too began to chip off as well.  

 

Thanks for your help in advance!

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That is what I was wondering. I thought maybe I put the wrong cone number in. So when I enter the number into the computer I type only the #6. I have a cone 10 kiln so it should be fine to fire that hot correct? 

 

Also, if I am firing this clay at cone 6 will underglaze that fires at 04/05 even work? Is this why the glaze is chipping off?

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your questions show that you really need a great deal of basic information, much more than you can get in a reasonable time here.

 

if you are on your own, learning this stuff takes years, a lifetime if you really get into it.  try getting some basic books from your local library.  they cover a lot of ground that may someday be only background for you but are important to a beginner.  the good ones have a list of cone temperatures and a glossary of terms you will see members of this forum use all the time.  it helps to have a background that allows you to uinderstand what is being said.  

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I have lots of books that I reference. The problem is all the books and websites start to intertwine strategies and techniques and it gets really confusing that is why I come here to get a straight answer to my questions. 

 

So can a ^ 04/05 underglaze work on a ^ 6 clay or does the cone for clay and glaze always have to match? 

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You would have to get an underglaze that says it will fire to ^6.  Amaco makes a velvet underglaze that will fire to ^6. And there are some others as well. I think one is Coyote. Just search on the net underglazes that fire to ^6.

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Most underglazes, that I have used, can be used from low fire temp (Cone 05/ 04) to at least mid-fire temps (Cone 6).  But, as others have stated, if the clay says Cone 6, you need to fire it to Cone 6.  Otherwise, it's weaker, and not vitrified.  The latter is the big issue, with an outdoor sculpture.  If the ceramic is not vitrified, it is porous and water will seep in.  It's even a bigger issue, if they are installed in an area that gets below freezing.  That water that seeped in freezes.  The frozen water expands, and the sculpture fall apart.

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the cone numbers matter because most of the commercial underglazes were developed to be used by people who go to places to "paint" pottery. they usually fire to cone 06 which is a much lower temperature than cone 6.  many colors used in underglazes, mostly reds, yellow and orange will burn out at higher temperatures.  you can use them at any temperature but if you want the color you see in the bottle, use the temperature it says to use on the bottle. underglaze is for that purpose, to add color under   a  glaze.   

 

fire your clay to the mature temperature that it says on the box.  whatever GLAZE you use should at least match that.  

 

your experience of being confused because you are looking at too many sources is common.  pick a good book and read a section at a time. do not go into deep water until you can swim.

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Hi

I am not an expert in practice but I can give you an overview. You will need an overview because when you run into issues , you'll need to figure them out.

Clay is similar to glass, it needs to vitrify in order to be stable and functional. Clays are sold according to what cone they vitrify at, low fire, med range and high fire.

You bisque the piece ( fire it so that the water that is chemically bonded is out, the piece is stable but still porous for the application of glaze)

The common practice is to fire a bisqued piece with a glaze that goes up to the temperature that will vitrify the clay and make it as porous as it is going to get.

The vitrification is the important part because that is what makes it a useable piece.

That being said people do anything at all once they know their medium, but if you start in the middle when things go wrong it will be hard to figure out cuz they could be failing on so many levels.

First off the thicker your piece , the more complications especially if the thickness is not uniform. It could not be dry enough, the clay could warp, crack.

Second the glaze has to fit the clay body, if one moves x amount and the other move y, if they are not within a certain percentage points of each other, when they come out of the kiln the glaze will crack or craze.

Glazes are a science unto them selves(as are clay bodies).

If you have a potter nearby, I would advise you to go to them.having someone steer you in the beginning stages of your project will save you a lot of grief later. If you don't , perhaps laying out the project here would help. There are nothing but experts here.

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Here is Amaco's color chart for their Velvet underglaze line at low-, mid-, and high-fire temps:

http://www.amaco.com/amaco-glazes-information/velvets-temperature-color-variation-chart/

 

Some of them turn a bit glossy at higher temps, some of them disappear or change color. I've never had problems with Velvets flaking off either before or after firing, it could be that you applied it too thick. It only takes one coat for full coverage in most cases.

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Bisque to 06, glaze fire to whatever the glaze needs and the clay will take (6, in this case).

 

Did you put the underglaze on before the bisque, or before the glaze firing?

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You can put the underglaze on before or after bisque. Certain clays may throw a wrench into underglazing greenware. It would be the same outgassing reason why most people bisque and then glaze, but it's not usually a problem with underglazes. I do both for the different painting effects that I can achive with the underglaze.

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