slipcasting plates, bump in the bottom

[Nata Pestune]

Hi everyone,

i have a problem slipcasting one part mold (plates/bowls with wider bottom), bottom of the objects are warping and you can see a hill formed inside of the plate or bowl (only after glaze firing. Basically, bottom of the plate arches. Maybe some of you know why this happens?

[Jeff Longtin]

Hello Nata,

Do you have a picture of the mold? 

Unfortunately, clay doesn't always act as we think it will. More than likely you will need to make a new mold to account for this warping.

While I don't fully know the physics, of what's going on, I think the clay is warping in the only direction available to it, up or out, away from the mold face. Sometimes it shows up in the unfired piece but sometimes it doesn't show up until the piece is fired. 

The situation you describe is why plates are usually made on a jigger press. The clay is compressed against the mold face during the process. This compressing process reduces warping in the fired piece. If you can figure out a way to compress the cast clay while it's still semi moist you might reduce warping as well.

Good luck!

[Mark C.]

Jiggering plates is fast and easy-better than slip cast plates

[PeterH]

On 10/4/2024 at 5:49 AM, Nata Pestune said:

Basically, bottom of the plate arches. Maybe some of you know why this happens?

@Nata Pestune I'm not certain if you are still here, so just an outline ...

It (probably) happens because the flat bottom of the plate "wants" to finish up larger than the outer rim of your plate. To do so it needs to arch, and the firing gives it an opportunity to do this.

It wants to do this because a "memory" of the past history of the clay is is held in the alignments of the clay platelets.  Which can then modify the clay's behavior during firing.

In your case stresses caused by uneven drying may well be contributing to the problem.

Presumably the shape of flat bottomed plates make them prone to uneven drying.
https://lakesidepottery.com/HTML Text/Tips/Clay drying and firing process.htm
Why is it important to let clay dry slow?
Be aware that the larger to piece is, the more movement of shrinkage will take place. For a example, if clay shrinks 5% during drying, a piece of clay that is 5" long, will shrink 1/4" and a piece that is 20" long, will shrink 1" during the drying. This is why it is beneficial to know that the larger the piece is, the more important it becomes to let it dry evenly (slow drying will yield even drying). See next chapter about Air Drying. If one part drys faster than the other part, the dryer part will shrink more on wetter part which will create stresses on the clay, thus, cracking could occur.

Can you post a photo of your fired plate and details of your casting slip? Do you have access to a pottery wheel?

PS Just measured my commercial household plates (bought in the china section of a retail store). When on a flat surface only 35% of the plates bottom surface doesn't have free access to the air. So its much easier to dry evenly that a flat plate with a raised rim, where most of the plates bottom surface doesn't have free access to the air, but all the rim does.

Edited October 6 by PeterH

[PeterH]

For the record.

Three Approaches to Slip Casting Plates
https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/three-approaches-to-slip-casting-plates#
... which mentions mold-throw ware (using a process which including both casting and some jigger-ish operations on a wheel).

 

[Hulk]

Thanks for that article link, and "...what you make and how you make it..." might be quote of the day. :-)