clay thickness?

[Kabe]

Technical Question; I am making a 32 by 6 inch deep bowl for a friend of mine to use as a mold to melt glass into. It is for a church baptismal. I have already made the plaster mold for the clay and have shaped the clay into it. When I flipped it the wieght of the mold distorted the clay. ( 100+ lb of plaster and about 120 lbs of clay is a lot to flip) I remade to bowl,so now I can make another plaster cast to go over the top to hold everything in place when I flip the mold but I was wondering what anyones thought were on how thick I should go with the clay. All the work I have done so far is to get to where I can form the final bowl, I was going to go about 1 to 1 1/4 thick and reinforce the underside of the clay mold with clay spokes on the underside of the bowl. Really it will just be a big biscuit bowl. He will put a sheet of glass on top of it and melt if into the cavity. The clay will be fired to about 1800 (Biscuit) to start with and then I think he will fire the glass to around 1000 to slump it. I am not sure of the exact temp. The clay I am using is my heavily groged tile clay body. It matures at cone 6. It seldom warps. Cracking could be a problem when I first fire it and dry it. Hope not. I will let it dry and fire on a bed of grog. Question; how thick would you recommend the clay to be? I do not want to wait forever for it to dry and also do not want it to be so heavy that it takes a fork truck to move it. Happy firing Kabe

[Bobg]

I've never tried this before, but are you letting the clay dry to at least leather hard before flipping it? I would think that an inch would be too thick. I would go more along the lines of 5/8 inch and let it dry more before flipping. I would think that at an inch to inch and a quarter that you may have cracking. Like I said I've never tried anything that big so I may be way out of line.

 

Bobg

[Kabe]

Thank you, I just have to have it soft enough to add the pieces to the underside. I think I will go thinner like you suggested. Most sinks and stools are only about 1/2 inch thick but they are slip cast where mine will be a press mold. I'm not sure how that would effect the thinkness issue. But that's how we learn. The making of a mold on top will stop the distortion I think.That way all the clay is contained and has nowhere to go. I made the first form as the underside of the bowl, afterwards I thought it might be better to have shaped the mold so it was the inside of the bowl. Then I can run my clay through my slab roller and lay it over the hump, instead of trying to drop it into the cavity. The wight of that much clay presents challenges. I found that I can roll it on a 3 inch plastic pipe like you do pie-dough on a rolling pin and it is some what managable. by pouring the upper cast, I will have a cast of both. I suppose I could even use the two halfs as a slip cast mold, with a little modification. I do not have very much experience with slip casting. I hope to add it to my skill level someday. It would be a handy way to add detail to a fireplace, say you wanted to have some repeating figure or the like. Happy firing Kabe

[trina]

Thank you, I just have to have it soft enough to add the pieces to the underside. I think I will go thinner like you suggested. Most sinks and stools are only about 1/2 inch thick but they are slip cast where mine will be a press mold. I'm not sure how that would effect the thinkness issue. But that's how we learn. Happy firing Kabe

 

 

Hi there,

 

I would also agree you don t want to go too thick here, not that I have ever worked that big either. I would suggest though that you line the press mold with some canvass bands two or three inches wide. So that you can lift the bowl out of the mold without having to flip it. These lifting spots are easliy mended and you wont have to lift such a heavy mold. I would love to see a photo of how the whole thing looks in action... T

[Kabe]

Thank you, I just have to have it soft enough to add the pieces to the underside. I think I will go thinner like you suggested. Most sinks and stools are only about 1/2 inch thick but they are slip cast where mine will be a press mold. I'm not sure how that would effect the thinkness issue. But that's how we learn. Happy firing Kabe

 

 

Hi there,

 

I would also agree you don t want to go too thick here, not that I have ever worked that big either. I would suggest though that you line the press mold with some canvass bands two or three inches wide. So that you can lift the bowl out of the mold without having to flip it. These lifting spots are easliy mended and you wont have to lift such a heavy mold. I would love to see a photo of how the whole thing looks in action... T

 

 

 

The bands are a good idea. It may be that after I have made the top mold then the bowl will be upside down when I first form it and then I can add the reinforcement spokes without having to move it at all. As far as pictures go, the last picture I posted came out huge. I am not sure why my computer didn't read my mind and make the right corrections. All the other pictures I have posted have been ok . I have been taking picture of the process and when my computer and I get on the same wave link and I will try to post some. It is a really big bowl! It is the biggest One I have ever tryed to make. thanks for the help. kabe.

[perkolator]

in our studio we have 2 stainless steel industrial kitchen bowls that students use as a mold all the time. our bowls are 36" wide and about 12" deep. we are usually very successful with these and we use a relatively heavily grogged sculpture clay body that we mix in-house.

 

since our bowls are metal and non-absorbent, I normally advise students to put down a sheet of thin plastic (or newspaper) inside the bowl to keep clay from sticking. or course the bowl can only evap from the top-side, which is great when you need the outside to be slightly softer for attaching or carving. Thickness-wise, we have a general studio policy of building no thicker than 1". For most large-scale stuff such as a bowl this large, a 1/2-3/4" thickness is about where you'd want to be. Everyone likes to put clay in the mold with different techniques - some choose to use slabs (hand built or slab roller), but I prefer overlapping baseball-size chunks thrown hard into the bowl and then compacted with fist or sock full of grog, etc.

 

timing is key to pulling the clay from the mold and we have two methods that we like to use, but I'm not sure will work for you since I'm assuming your plaster mold will not allow this. one method is to simply put a sheet of plywood across the open form to sandwich the clay, then simply flip it over with 2-3 people, and you now have your nice bowl out (but upside down). now they can work on the bottom-side and do what they need to do. for flipping it back upright, we simply sandwich it again with boards and flip it back over. very seldom do we have issues doing this as long as timing is correct and the clay not too wet.

 

the second way we remove the bowl from our mold is a method to keep the bowl upright. for this, we sorta grab hold the clay positive and rotate the bowl out of the form. the clay stays relatively upright the whole time, it's the mold that mostly gets lifted/rotated away. i guess that's the best way to describe it.

 

 

if your friend is just using this bowl as a form to slump his glass in - why not just give him a plaster/hydrocal mold like many glass artisans use? i don't know much about glass slumping, but I always thought their slump molds were plaster and they just line it with a release agent.