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docweathers

Porcelain vs white stoneware coated w/ porcelain

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I have only used white stoneware at ^6 but I am thinking about trying porcelain. From what I understand porcelain is like throwing cream cheese. ... which sounds hard. However, from what little I know about such things, I would think that a thin coat of porcelain over white stoneware would give the same look but would be much easier to large pots with.

What am I missing here???

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From what I've heard,  shrinking rates between the porcelain and stoneware are usually very different,, leading to warping and cracking. Subject to correction from more experienced members. I've also seen potter's covering stones are with casting slip instead of slip from slaking porcelain clay. My thoughts are make the move to throwing porcelain, easier in the long run. If I've run out of porcelain then I throw with drones are and underglaze with white giving the colour change you want, I've been using a ketchup squeeze bottle running in front of a fan brush to give a stroke free appearance, (saw this tip on YouTube..

Practise and find the method that works best for you. 

In S.A. we don't have a supply of the really white porcelain. Ours fire to more of a cream colour, so I throw with porcelain and if I want a white look I use white underglaze.

Edited by AndreaB

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Yes, you can put a layer of porcelain slip over stoneware to make a whiter base so  glazes pop more. Find 2 bodies with similar shrinkage and coe or make up a white porcelain slip or engobe. The huge difference though is the pot isn't going to have the wonderful luscious, glassy, translucency of straight porcelain. 

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You could probably achieve the look you are going for using a grolleg based terra sigillata.  I've played around with some,  the bisque pots were chalk white prior to final firing.  the COE is more forgiving since it is a much thinner layer.  I didn't use glazes, so I cannot speak to if there was any effect on the glaze.  The fumed pieces did have more of a porcelain appearance.  The were also much more susceptible to fingerprints in the leather hard state.

 

 

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Throwing porcelain isn't as hard as people make it out to be. It really isn't. You either throw with less water or you throw with the same water but get the pots done faster. That is the only difference I have noticed throwing with it. Get a good porcelain like Standard 365 and its a dream to work with. I think it will be easier to learn to throw porcelain than to find a porcelain and a stoneware that have the same shrinking rates and it will be much less work than coating all your pots with slip.

That being said I do a lot of slip work, almost all of my work has a slip of some kind on it now. It is a beautiful feeling slipping pots. So if you do decide to go this way then have fun! 

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I find a good porcelain to be easier to throw than most white stoneware bodies. White stonewares usually have poor particle size distribution- everything is super fine- and they're overly plastic. I've also found white stonewares to be just as likely (if not more so) to crack as porcelain. They key to working with porcelain is to work quickly, because it will absorb water faster than stoneware. As long as your pieces are even in thickness, you won't have any problems.

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I agree with Min and Nerd, if you want to put porcelain slip over stoneware. I've done it to get the best glaze fit/colour balance for a favourite celadon of mine, and it works a treat.

That said, try throwing some porcelain. It might not be as bad as you think.  If you buy a box of porcelain and don't like throwing with it, you're not really out much, and you can say you tried it. If you do like it, well, you have your next clay rabbit hole to go down. 

Edited by Callie Beller Diesel
Added info

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Adding to what Callie said, if you don't like it, just make it into a slip, so either way you have to buy some porcelain. I personally learned on b-mix which was a white stoneware and I went to my pottery supply and they were out so I bought some porcelain instead, I ended up being confused as for why anyone would use a white stoneware.

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 "I ended up being confused as for why anyone would use a white stoneware.".... please expand on this.

On test tiles tiles I have gotten porcelain clay to stick very well to white stoneware bisque and through  firing. The porcelain coating makes both bisque and glaze fire clay slightly whiter. However,  there is no color difference between favorite glazes like Selsor Oribe and Copper Red w/Silicon Carbide between that applied directly to the stoneware vs over the porcelain layer. 

From what I understand, I should get brighter colors over the porcelain.

I don't get it???

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You might not always get brighter colors and it depends on how thick your porcelain slip is. You have to remember that a lot of the beauty from porcelain comes from the way it forms a glassy surface and is transparent in nature, as Min stated earlier. That allows for glazes to look brighter than most stonewares. If you just using a slip there is a good chance you might not be getting the full effect. It also depends on what porcelain you are using. There are big differences in porcelains. I have used some that fire almost greyish. I have used some that looked lightly glazed as they came out of the kiln with a slight sheen. So you are going to get different bang for your buck on different porcelains. My favorite for glazing and color was Laguna's Frost. That body was the most beautiful porcelain at cone 6 I ever found, but it wasn't the easiest to work at the time for my skill level so I stopped using it. Standard 365, that Neil recommends all the time, is my favorite all-purpose porcelain. I use it for my white slip as well.

16 minutes ago, docweathers said:

"I ended up being confused as for why anyone would use a white stoneware.".... please expand on this.

This is all my opinion:

So dark stoneware, in general, gives very rich surfaces, a warm undertone and drastically changes the way glazes look sometimes.  However, you don't get any of those benefits from a white stoneware. I have also found that the white stonewares I tried were sometimes harder to use than porcelain anyways. They had cracking problems and didn't really bring anything to the table that was better than porcelain. So I started to think about it and realized, why even work with a white stoneware? What is the purpose of a white stoneware body? It doesn't bring brighter glazes or translucency, and it doesn't add rich undertones like dark stoneware. That is why I don't understand why anyone uses it. The only thing I can think of is that people are afraid to try porcelain because everyone says it is hard to use, in reality, it might be 10% harder to use, maybe... But I think it is one of those random potters tales that go around. I was afraid to try it too, I bought some and was like. I don't get the problem here. I actually like throwing porcelain a lot, I would use it every day if it wasn't for my desire to have a dark grey/brown/black body for my work.

Edited by Joseph F

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If you like both porcelain and black clay, US pigment has a black porcelain  that is supposed to be pretty good. 

My throwing skills are descent. I guess I need a porcelain clay sample kit to try out several to see what works best :). 

I have used white stoneware almost exclusively because it seemed like a bad for a beginner to add the extra variable of different clays to the complexity of different glazes.  I did try some white porcelain that Glazenerd sent me.  It did seem to throw pretty easily. 

And yes, I have also had cracking problems with white stoneware. I did not realize that white stoneware was more prone to that.

Since I throw completely dry, the extra water absorption of porcelain would not be a problem.

.... I think the above is taking to myself to talk myself into switching to porcelain.... 

Thanks all for providing me with the grist to chew on.

 

 

 

 

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I use the black ice all the time. I bought 300# of it, had my supplier order it from their Savannah warehouse. I am not a huge fan of the black ice by itself. It is way to wet out of the bag... I actually mix it with my stoneware body to turn a red body with too many speckles into a dark reddish black body with just the right amount of speckles. 

If you want a white porcelain and you have access to standard clays get 365. 

You will be surprised how easy it is to use and your cracking problems will go bye bye.

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I love throwing porcelain. I throw many types. My favorite is Coleman for Obvara because I think it makes the best patterns for crackled surfaces. 

I like ^6 Frost for translucent porcelain at ^6. You need to control water use on porcelain as it sucks it right up. I throw dry and have no problems. The cylinder is with about 8 pounds of porcelain. It was pushed out to make a crackled orb. This porcelain I bought from Matt Fiske before he left for Denmark.

It is from Laguna, but I don't know the name.

 

Marcicylinder#2.jpg

MarciCrackle2018#2,jpg.JPG

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Standard 365 porcelain is a fabulous throwing body, and goes translucent at cone 6. I agree with Joseph that coated stoneware doesn't affect glazes the same as full porcelain. Not sure why, though. There's something to that glassiness going all the way through the body.

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Just now, neilestrick said:

Standard 365 porcelain is a fabulous throwing body, and goes translucent at cone 6. I agree with Joseph that coated stoneware doesn't affect glazes the same as full porcelain. Not sure why, though. There's something to that glassiness going all the way through the body.

Because a thin layer of porcelain can still leach the iron, magnesium, and titanium from the stoneware body.  The higher the metal oxide content of the stoneware, the greater the degree of change in the glaze. Then comes the change in refractive index. Etc.etc. blah blah and blah.( short version of lengthy chemical reactions) 

Nerd

quick clay chemistry lesson:

translucent porcelain has less than 0.25% molar iron, magnesium. And less than 0.50% titanium.

regular porcelain has less than 0.50% iron and magnesium! and less than 1.00% titanium.

As the flux content goes up, so does the translucency- via glass content.

stoneware can have anywhere from ..75 to 7% iron! higher magnesium! and titanium.

as all glaze makers know??iron and magnesium are colorant oxides. As the content goes up in the clay, it leaches into the glaze and changes it's color. Does the same thing to porcelain slip if too thin, and if the stoneware has high oxides content.

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Glazenerd does mix and sell a fancy porcelain. I have tried a little of it. From a beginners point of view, it seems to throw easily. However, I , as a beginner have no comparison to any other porcelain . 

 

I would think that white stoneware is white because it has very little iron, magnesium and titanium..... to leach???

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This is mostly redundant to the above post since this forum will not let me edit the above post

------

Hey dude, I want the full chemistry lesson. No shortcuts allowed.

Glazenerd does mix and sell a fancy porcelain. I have tried a little of it. From a beginners point of view, it seems to throw easily. However, I , as a beginner have no comparison to any other porcelain . 

 

A beginner would think that white stoneware is white because it has very little iron to leach???

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Stoneware:     Buff to light tan.     0.75 to  1.25% iron )  very common to have up to 20% kaolin. ( no iron)

deep tan to light brown.  1.50 to  2.25% iron

dark brown / slight reddish   2.25 up 3.00 iron. However, at this point carbons are also increasing, and bloating issues possible.

red stoneware.. 3.00  iron and up.  All red clays are high iron- 5-7% by weight.  

The darker the clay; the higher the iron  AND higher the carbon content. 

Most all iron in clay comes from iron sulfide FeS. It is the off gassing sulfu (ides) that cause bloating, not carbons.

doc.. Never ask a nerd for simple answers.

 

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Doc: picture is worth a thousand words.  The iron is migrating / melting at 2050F, as the temp climbs the other large particles of iron will join in. The GPA inside the clay body will push it towards the surface. I called Van der Walls, his wife said he is  resting...

large.5a898d0dd7f28_SunFeb1808-20-52.jpg.e37bd6c28c7ba1510c7d7fee8335a69f.jpg

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Doc 

You have it right about stoneware being easier to throw . Porcelain is denser tighter particle size. Its stronger but harder to work with. Coating with porcelain slip is a compromise as stated by others as the glaze is not exactly the same as a pure porcelain body (most likely offgassing).

Porcelain has no iron in it .

If you want large pots go ahead and coat them,I'm sure they will work good enough.

 

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I'm not saying you cannot throw porcelain big its just takes more skill at throwing.

I just threw some stoneware after only working with Porcelain for many years and it practically threw itself. Those stoneware pots are for a salt fire next month.

 

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