Throwing porcelain problems

[thiamant]

I have tried throwing porcelain other times, but not much. Recently I purchased a new batch and oof it was terrible. Fresh out of the bag and it was already cracking when trying to wedge... (Humidity looked ok) 

I tried to throw a couple of bowls off the hump, impossible. Once the bowl is finished I cut the bottom as usual. The clay sticks like hell and cannot be removed without deforming the bowl... Even if I let a generous amount of clay at the bottom (usually thats what I do to avoid problems when retrieving the finished piece from the hump). 

I dont know if these problems are related (cracking+ too sticky) and if this is because the porcelain quality is bad. I experienced the same issue before but it was from porcelain which was recycled a lot of times... I use a standard plaster table for wedging...

[liambesaw]

There are more plastic porcelains out there that are easier to deal with.  That sounds like a pretty temperamental one.

[oldlady]

why do you wedge clay of any kind straight out of the bag?   a machine much stronger than you has pressed out the air and excess water.   your wedging on a plaster surface removes more water and leads to cracks.  

try a different porcelain as liam suggests.

[thiamant]

I see. The stickiness problem is also a plasticity issue?

 

[Bill Kielb]

5 hours ago, thiamant said:

I see. The stickiness problem is also a plasticity issue?

I have never had a stickiness issue with porcelain. Bmix and other clays to me often make a mess and are super sticky compared to porcelain. Try another type of,porcelain.

[neilestrick]

If it was cracking during wedging then it was likely too dry. But if it was really sticky on the wheel it was likely too wet. That could be from the throwing process, though, since porcelain takes in water much more quickly than stoneware. So if you're taking a long time to center and using a lot of water it can easily become too wet, especially on thin, smaller pieces. Center quickly, pull quickly, then get the water off it and take your time shaping. In general I do not recommend throwing off the hump with porcelain because it is much more prone to S-cracks than stoneware when thrown that way. Porcelain is a different beast than stoneware, and it takes some getting used to. Once you do, it's fabulous.

There is a huge spectrum of good to bad porcelain out there, and it may take some trial and error to find one that works for you. If your clay supplier has several different porcelain bodies available, see if you can get samples of each to try out.

[liambesaw]

Drier porcelain also takes in throwing water much faster.

[thiamant]

On 4/4/2021 at 9:28 PM, oldlady said:

why do you wedge clay of any kind straight out of the bag?   a machine much stronger than you has pressed out the air and excess water.   your wedging on a plaster surface removes more water and leads to cracks.  

try a different porcelain as liam suggests.

The reason why I wedge the clay straight out is because when throwing off the hump I need a big cone shaped ball of clay. So I wedge it a little bit to shape it from rectangular to cone shape using spiral wedging. 

However considering it was probably on the dry side from the bag maybe I shouldve done it on wood instead of plaster. It didnt look that dry to me but porcelain is a new thing to me so...

[Morgan]

Not much I can add that others have not but I will simply say try a different one. In the past year I think I tried every possible one made (it feels like at least) to find which one worked best for me as well as glaze results. One of the reasons I love it so much is you simply cannot compare how glazes and colors pop on it in comparison.

And I will reiterate what Neil said...center and do your pulls quick. That said, I basically throw it dry after centering to help get some shapes I find near impossible on most porcelains.

[neilestrick]

I find that most porcelain bodies do need to be wedged at least a little bit out of the bag, unlike stoneware. If it's on the dry side, wedge on your wheel head. Porcelain has a much narrower range of acceptable moisture levels. Clay is what holds water, and porcelain is only about 50% clay, so it doesn't need as much water when it's mixed, and it's easy to add too little or too much. By comparison, stoneware bodies are 80-90% clay. When I worked for a clay company, we would adjust the moisture levels in an 1,800 pound batch of porcelain with 1 gallon of water at a time.

[Morgan]

Ya agreed on wedging. I have no science behind this but it almost seems to need to be slammed around too to get the particles to move around more. When I tried Helios I thought it was too dry and the clay in the bag was hard as a rock, truly. Once wedged VERY well it changed completely it was actually a pretty wild transformation.

speaking of that is a great porcelain, probably my second favorite to throw although it did not pass my glaze fitting tests.