Expert: Sam Kelly - 8/22/2009

Question
QUESTION: Hi I'm working mostly using southern ice porcelain.

I'm using plaster press molds that I have made. I'm finding that in some of the vessels that I make a split opens up in the rim of the unfired piece. Unfortunately sometimes it's not until it's final fire at cone 9 that it appears.

I'm wondering if I'm not working the clay enough before rolling it out.
Or is it more to do with the drying process. Any tips? Thoughts?

Also when you are using a press molds for a small bowl how do you avoid getting the join mark when you place the two pieces of clay inside?

What is the best way to place the rolled out clay in the press mold. Can you use the one sheet without cutting the circular shapes?

Thanks!


ANSWER: Hi Kerry, the only reason you should be using Southern Ice Porcelain is to attain translucency. If translucency is not a concern then use an ordinary porcelain that will probably be more receptive to manual handling. Porcelain is a difficult clay to use for handbuilding, a change in clay body will probably correct your cracking problems.

"Also when you are using a press molds for a small bowl how do you avoid getting the join mark when you place the two pieces of clay inside?"
Not sure what you mean here, why are you joining two pieces together?

Yes you can use one sheet, once pressed trim the outside waiste off.

Sam

---------- FOLLOW-UP ----------

QUESTION: I love the stark whiteness of the southern ice clay and I use it successfully in my smaller wearable pieces. I'm wanting that same whiteness in my vessels.

I use clear glaze and the colour of the clay body is a big part of my work. Is there a clay that you know of that at cone 9 fires to a similar stark white and not a buff or grey?

I do get success with hand building southern ice but am trying to cut down on the misses.


I read that if you place work that has a thickish wall to it 5mm then placing it in the kiln at a very low temp overnight before firing will help greatly. Any suggestions of what this low temp may be?  The idea being that it drives off all moisture beforehand in a gentle and slow way.  


The mold question was;

If I am using a smallish sugar bowl sized mold to make a vessel then I roll out 1 sheet of clay then do a half circle cut to place in one side of the mold then do the same with the other side. But I get a join mark as you can imagine.

If I use the one sheet  gets really squished together whilst being placed inside.  and doesn't find it's way to the bottom of the mold with out tearing.

 
Kerry

Answer
Hi kerry, I use Walkers Imperial Porcelain 4317
http://www.walkerceramics.com.au/porcelain%20bodies.htm
Its a ball milled porcelain that I wheel throw with and fires to translucency an Cone9, its the closest white to Souther Ice I have used.

I have a student that is using a porcelain paper clay, it is very white, she is making hand made leafs for a project and thay are firing translucent and not cracking. I am not sure of the manufacturer but I think it might be Blackwattle Gold Label Paper Clay
http://www.blackwattle.net.au/Stoneware%20Clay.html
Ring and talk to John, tell him Sam Kelly from the Coffs Harbour TAFE College told you to ring him.

Unfortunately you are not going to get away with the join marks, however the paper clay may be an improvement. I solved this problem for myself a few years ago by drying out the porcelain and using Walkers No.10 Stonware casting slip recipe, I substituted his clay for my dried out clay and sucessfully slip cast pots with it.The clay does not have to be in powder format, just in small lumps as it will slake down in the water/sodium silicate/dispex mixture.

The slow drying probably is only helping to get the clay really dry, water in molecular format is still present in all clays to 600degC. A lot of cracking occurs as the pot is cooling down, this is called dunting, an indication of dunting is the crack is clean and sharp, the glaze is sharp, which indicates the crack occured on the cooling down stage, after the glaze has set. You can try adding 10% molochite wedged into the clay body to strengthen it.

Sam