Issue related to slumping top surface. Any ideas?

[George S]

Hi, I've been trying to make a side table in the shape of a "liquorice" , after the high fire, the top surface slumps considerably. this piece is about 60 cm tall and  slip casted, I've seen ceramic side tables of a similar size and shape with tops that don't suffer from this amount of slumping. Does anyone know what changes I can make to avoid this slumping? Also, I've already  made it so the top surface is somewhat covex prior to firing, this helps slightly but not much.

 

 

 

Edited July 22, 2022 by George S

[Min]

When you say high fire, you are firing to cone 10? (approx 1300C) Are you spraying the glaze? Any chance you could fire them unglazed upside down on a waster slab then warm it up and spray a lowfire glaze on them and refire right side up? Nice work, looks like a really fun piece.

Welcome to the forum.

[Jeff Longtin]

Is it safe to assume the piece is about 30 cm wide? That's a large diameter for a slip cast slab of clay.

I would suggest a smaller form within the piece supporting the top. Ideally the bottom surface, of the top slab, would be unglazed. 

I do this on several pieces. I apply kiln wash to the top of the inner support and the clay piece comes off easily.

A bit of a pain but it almost entirely eliminates slumping of wide, and flat, slip cast forms.

ps. after the inner support form has been high fired, cone 6, I keep it on the shelf and re-use it when necessary. The kiln wash prevents it from sticking to the piece of the kiln shelf.

Edited July 6, 2022 by Jeff Longtin
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[George S]

17 hours ago, Min said:

When you say high fire, you are firing to cone 10? (approx 1300C) Are you spraying the glaze? Any chance you could fire them unglazed upside down on a waster slab then warm it up and spray a lowfire glaze on them and refire right side up? Nice work, looks like a really fun piece.

Welcome to the forum.

Hi, thanks for the response and welcoming! Actually I fired these works at 1200c, originally at 1300c but I decreased the temperature in a attempt to decrease the slumping. As for firing them unglazed and upside down, do you suspect this is how many commercially made ceramic "side tables" would be made? Worth a try, thanks!

[George S]

On 7/6/2022 at 8:42 PM, Jeff Longtin said:

Is it safe to assume the piece is about 30 cm wide? That's a large diameter for a slip cast slab of clay.

I would suggest a smaller form within the piece supporting the top. Ideally the bottom surface, of the top slab, would be unglazed. 

I do this on several pieces. I apply kiln wash to the top of the inner support and the clay piece comes off easily.

A bit of a pain but it almost entirely eliminates slumping of wide, and flat, slip cast forms.

ps. after the inner support form has been high fired, cone 6, I keep it on the shelf and re-use it when necessary. The kiln wash prevents it from sticking to the piece of the kiln shelf.

Hi Jeff, thanks for the response! The support idea is a great one! The support would need to be quite cleverly engineered in my case, the bottom is not exactly open.  And to make my life even more difficult it's quite important to me that the whole piece is black when seen from any orientation . Throughout this project I've been wondering why black kiln wash doesn't exist.

 

Edited July 22, 2022 by George S

[Mark C.]

Black is from an over saturation of colorants or stains. So to make black kiln wash one would need to oversaturate  or stain. Often this comes with fluxing ( think sticky) and thats exactly what you do not want with kiln wash

[Min]

5 hours ago, George S said:

Actually I fired these works at 1200c, originally at 1300c but I decreased the temperature in a attempt to decrease the slumping. As for firing them unglazed and upside down, do you suspect this is how many commercially made ceramic "side tables" would be made?

I don't know how ceramic stools are made commercially. I'm guessing (and it is just a guess) many are made from lowfire clay that are cast. High fire porcelain functional ware in industry can be fired unglazed to maturity in a same shaped form, ie bowl fired in a bowl of a claybody that won't slump, then inspected for flaws and sprayed with lowfire glaze and fired off to glaze maturity.

Really long history behind ceramic stools, I doubt the original ones were sprayed. Perhaps they accepted more wabi sabi back 1,000 years ago than what some consumers accept these days. Form of them was traditionally more barrel shaped which would also play into warpage during firing.

[Callie Beller Diesel]

I suspect too that manufacturers tailor their casting bodies so they’re not pyroplastic. I’ve seen it done for things like brick murals. 

[Jeff Longtin]

Hey George,

You've set yourself up for a challenging process. The only other thought I have is to cast the top portion/slab separately from the rest of the piece and then assemble them in the leather hard state. 

Perhaps by casting separately the particles would align in a way that minimizes warping in the horizontal direction? You could also add slivers of clay that could stabilize the slab?

Good luck!

[PeterH]

A question rather than a suggestion. What would be the effect of casting the seat thicker than the sides (by manipulating the slip level)?

[George S]

2 hours ago, PeterH said:

A question rather than a suggestion. What would be the effect of casting the seat thicker than the sides (by manipulating the slip level)?

I'm not sure what you mean by "seat"

[PeterH]

46 minutes ago, George S said:

I'm not sure what you mean by "seat"

The flat surface with holes that you sit on. Didn't want to say top because of the possibility of bisque-firing inverted.

[George S]

51 minutes ago, PeterH said:

The flat surface with holes that you sit on. Didn't want to say top because of the possibility of bisque-firing inverted.

Ah ok. The "wall" thickness varies from top to bottom of the piece, on the vertical walls the thickness gradually changes, while the top or bottom have a mostly uniform thickness respectively. While the piece is being casted the bottom wall builds the fastest therefore becoming the thickest wall. The vertical walls are tapered in thickness, this gradual change in thickness is mostly due to the rate I can physically fill the mold ( a bit slow-I have fairly rudimentary equipment.) During casting, this mold contains 80 litres of wet slip, lots of slip weight is compacting and encouraging the development of the walls towards the bottom. Towards the top of the mold, the walls are developing more so by how much moisture the plaster can draw out of the slip, without as much assistance from gravity.

So if I were to flip the piece after casting, the bottom ( which would now be the top) would be thicker than the vertical walls.

phew, gives my head a spin!  does this answer your question?

[PeterH]

On 7/19/2022 at 5:46 PM, George S said:

does this answer your question?

 

I was asking if you changed your casting process to make the seat significantly thicker would the seat then be less inclined to slump on firing. 

Two ways to achieve this increased thickness might  be:

1) After emptying the mould immediately add enough slip to - say - double the final thickness of the seat.

2) Only partially empty the mould  and let the remaining slip increase the final thickness of the seat.

 

[George S]

5 hours ago, PeterH said:

 

I was asking if you changed your casting process to make the seat significantly thicker would the seat then be less inclined to slump on firing. 

Two ways to achieve this increased thickness might  be:

1) After emptying the mould immediately add enough slip to - say - double the final thickness of the seat.

2) Only partially empty the mould  and let the remaining slip increase the final thickness of the seat.

 

Hi Peter, yes one could do as you proposed to increase the thickness on the bottom of the mould, but for my purposes I don't believe this would prevent the slumping. 

[George S]

On 7/7/2022 at 6:55 PM, Min said:

I don't know how ceramic stools are made commercially. I'm guessing (and it is just a guess) many are made from lowfire clay that are cast. High fire porcelain functional ware in industry can be fired unglazed to maturity in a same shaped form, ie bowl fired in a bowl of a claybody that won't slump, then inspected for flaws and sprayed with lowfire glaze and fired off to glaze maturity.

Really long history behind ceramic stools, I doubt the original ones were sprayed. Perhaps they accepted more wabi sabi back 1,000 years ago than what some consumers accept these days. Form of them was traditionally more barrel shaped which would also play into warpage during firing.

Hi, yes I see your point, good guess! I visited a shop where I could inspect a large plinth/stool/table and it quite likely is a lowfire clay.

"High fire porcelain functional ware in industry can be fired unglazed to maturity in a same shaped form, ie bowl fired in a bowl of a claybody that won't slump, then inspected for flaws and sprayed with lowfire glaze and fired off to glaze maturity." - This in particular is very interesting for me thanks for this info!

Do you perhaps know where you read about the history of ceramic stools?

Thanks!

[Min]

21 hours ago, George S said:

"High fire porcelain functional ware in industry can be fired unglazed to maturity in a same shaped form, ie bowl fired in a bowl of a claybody that won't slump, then inspected for flaws and sprayed with lowfire glaze and fired off to glaze maturity." - This in particular is very interesting for me thanks for this info!

Do you perhaps know where you read about the history of ceramic stools?

I made the comment about bisque firing to maturity then low fire glazing before I saw a picture of your pot. I think it would be difficult to spray all the surfaces on the inside of the stool to get an even coverage so not sure this would be the best approach. If it was just the outside that needed glazing I think it would be more successful. Glazing a mature clay is difficult, warming the pot up and adding gum to the spray glaze would help. Once the first coat is on subsequent coats go on easier.

Can't recall where I read about the history of ceramic stools, a search of Chinese ceramics should turn up something.

[Pres]

On 7/6/2022 at 9:55 AM, George S said:

 I've already  made it so the top surface is somewhat covex prior to firing, this helps slightly but not much.

 

 

 

I wonder if making it more convex would meet your needs. Seems like the piece is shrinking more at the top, thus seeming to slump, but if you give more surface to the top by making it more convex then you may meet your needs without the extra weight of the thicker top.

Just a thought.

best,

Pres