Pedestal bowl/Cake stand sinking down in the glaze firing

[Gazal]

Hi, 

I really want to make a pedestal bowl but every time I made one the edge of the bowl sunk down, same happened with cake stands, does anyone know what am I doing wrong? 

Thank you!

 

[Kelly in AK]

As clay reaches its maturation temperature (becomes vitreous, changes from bisque ware to stoneware/porcelain ) it becomes “pyroplastic.” Soft from the heat, in other words. I feel that has a significant bearing on your problem.

People solve this in various ways. One is to adjust the form to compensate for the sag, another is to add structure in the design to support wide flat areas. Still another is to calculate thin and thick areas for greater support. One very direct way would be to use a clay that matures a little higher than you’re firing it, or, better still, a clay known to keep its structure at maturity. I suppose even some kind of supports could be fashioned to make it work. 

Porcelain and highly vitrified clays are notoriously pyroplastic. The weight of a handle will pull a mug into an oval shape. 

Some peoples favorite ceramic word is eutectic. Mine is pyroplastic. 

[kswan]

You can also use your own body as an analogy about feeling the weight of gravity. If you hold heavy weights, you can hold them longer straight up as opposed to straight out. The same happens with clay, it needs support to not sag. An upward curve at an edge instead of horizontal will help. If your edge is thick, it is heavier and will sag down too. As Kelly said, thick in the support area like the base is important, and then thinner edges where possible.

[Pres]

@Gazalposting a picture might help with your description problem leading to a more specific solution.

 

best,

Pres

[Gazal]

Thank you Pres,

unfortunately I do not have any photos,  I tried to show what happened in a sketch.

top one shows how cake stands sinks down on the edge of the plate

and

bottom one shows how where pedestal (foot) is attached, bowl deforms.

I hope it helped?

[Gazal]

21 hours ago, Kelly in AK said:

As clay reaches its maturation temperature (becomes vitreous, changes from bisque ware to stoneware/porcelain ) it becomes “pyroplastic.” Soft from the heat, in other words. I feel that has a significant bearing on your problem.

People solve this in various ways. One is to adjust the form to compensate for the sag, another is to add structure in the design to support wide flat areas. Still another is to calculate thin and thick areas for greater support. One very direct way would be to use a clay that matures a little higher than you’re firing it, or, better still, a clay known to keep its structure at maturity. I suppose even some kind of supports could be fashioned to make it work. 

Porcelain and highly vitrified clays are notoriously pyroplastic. The weight of a handle will pull a mug into an oval shape. 

Some peoples favorite ceramic word is eutectic. Mine is pyroplastic. 

that was great help, thank you Kelly

 

[Gazal]

16 hours ago, kswan said:

You can also use your own body as an analogy about feeling the weight of gravity. If you hold heavy weights, you can hold them longer straight up as opposed to straight out. The same happens with clay, it needs support to not sag. An upward curve at an edge instead of horizontal will help. If your edge is thick, it is heavier and will sag down too. As Kelly said, thick in the support area like the base is important, and then thinner edges where possible.

Thank you! makes sense  

[kswan]

It could be that your clay may never be able to hold that cake stand shape. Many people make a cake stand without the pedestal by having a downturned rim. That would be simple enough to throw on the wheel upside down. You might try that and fire a pedestal separately and epoxy them together if you want, or just have the shorter cake stand. 

The bottom of your bowl should be thicker where the walls are curving upward, and then attach a wider pedestal there. If the attachment point is horizontal, it will be more likely to sag. You could also attach a coil of clay around the outer seam where the bowl meets the pedestal for extra support. 

If possible, fire in the coolest part of your kiln to reduce warping.

I'd also suggest making slightly smaller versions of these items as you learn the physics of them and the limits of your clay. Good luck with them! I hope you'll share your successful results.  

[Pres]

I would consider throwing the plate upside down with a rim perpendicular to the plate, could be decorative cut rim. Throw the pedestal with a bow shape pulled inward then flaring out to provide a larger support area for the plate. This will distribute the weight and allow for a gripping area to lift the pedestal.

 

best,

Pres

[Russ]

I think Id try wedging in some #60 silica sand to give your clay body some strength. Its quick, easy and worth the experiment .

[Hulk]

With the rim turned down per kswan and Pres's suggestions, you could throw a support ring for the platform - just for firing, bare clay; the turned down platform rim would hide the unglazed portion underneath.

The support ring would have to be very near the same thickness as the pedestal, so they both shrink the same, else the platform wouldn't come out level ...or, let the pedestal hang, support the platform only?

Firing upside down should make the platform level, however, it would have to be left bare clay.

I like Pres's flared pedestal.

 

 

[kswan]

@Pres That's a fabulous shape! Good design.

The flared pedestal could work for the bowl as well.

Edited January 4, 2023 by kswan

[Min]

Another option would be to use a lowfire clay and glaze. Less likely to have pyroplasticity issues. For the most part lowfire white claybodies tend to warp less than mid or high fire vitrified bodies.

Edited January 5, 2023 by Min

[oldlady]

do you have the skills to throw the whole thing at once?   upside down with a slight slope so the "flat" part is not flat on the wheelhead or bat.  join the stem by bringing the center upward into a pedestal.  try it slowly with a small piece and work up to something bigger.   as the piece dries, catch it at just the right point to use one finger pressing down on the outer edge to make it flat  or just a small upward rim.

are you planning to make many of these or is it just a one off?

[Pres]

One other option is to make the two pieces and not join them together, Just have foot ring that fits into the opening of the base. The foot ring would be thrown into the base of the top pieces as an inner rim.

 

best,

Pres

[Min]

13 hours ago, Pres said:

One other option is to make the two pieces and not join them together, Just have foot ring that fits into the opening of the base. The foot ring would be thrown into the base of the top pieces as an inner rim.

 

best,

Pres

That's a good idea. Could go one step further and add locking keys, flange inside the plate part with a keyhole and 2  tab keys on the top of the base.