supporting unstable forms in the kiln?

[youpital]

2 hours ago, PeterH said:

How do you achieve the taper on the conical section, solid casting?

... and how thick is it at the rim?

This would achieved with a fully closed mold, however I still have to work on that part, the general hight of the rim by just slipcasting in one half of the mold is 2,5-3mm

 

27 minutes ago, Kelly in AK said:

Pardon me if this has been addressed. Clay on the outside of the form will shrink on and crack, left to its own devices.  Clay on the inside will shrink away from the mold and survive. Clay fired on the outside of a sitter has a similar risk if tolerances haven’t been calculated. 

I am not entirely sure if I understand it yet, is it because of creep? And beside that how can I calculate it so that the object would get out of the setter in one piece?

[Callie Beller Diesel]

On 7/17/2023 at 3:58 AM, youpital said:

is it because of creep

By creep, do you mean clay shrinking? If so, that can be measured. If you’re using a commercially prepared mix, your supplier can give you wet, fired and total shrinkage rates. It’s a good idea to verify those under your own making and firing conditions if you need precise measurements.  If you’re mixing your own recipe, you’ll have to make a bar test and fire it anyways. This article from Digitalfire gives details on probably more test than you need, and you can probably skip the parts about wheel throwing and porosity. If you want more info though, there are other links at the bottom of the page to relevant topics.

What Kelly is referring to is that on a curved mould or kiln support, if you form clay (or fire it) over a dome shape, the clay will shrink while the form won’t, causing the clay to crack. If the clay is draped or supported inside a bowl shape, your piece is free to move away from the walls of the support. 

[youpital]

11 minutes ago, Callie Beller Diesel said:

On a curved mould or kiln support, if you form clay (or fire it) over a dome shape, the clay will shrink while the form won’t, causing the clay to crack. If the clay is draped or supported inside a bowl shape, your piece is free to move away from the walls of the support. 

The inner side of the cone would be indeed curved very slightly but the outside of the lampshade (the part that actually touches the mold) is linearly sloped so I think that wouldn't cause any problems right?

[Kelly in AK]

I was thinking of firing shrinkage rather than creep.

I just looked more closely at your setter design and realized I saw it wrong. I imagined there was a cylindrical projection on top that isn’t actually there.

The principle behind my thought still holds though, clay will shrink in firing and if it’s shrinking “onto” your setter that’s a possible crack. If it’s shrinking “away from” the setter that stress isn’t set up. 

Edited July 17, 2023 by Kelly in AK
Clarity

[youpital]

10 hours ago, Kelly in AK said:

I was thinking of firing shrinkage rather than creep.

I just looked more closely at your setter design and realized I saw it wrong. I imagined there was a cylindrical projection on top that isn’t actually there.

The principle behind my thought still holds though, clay will shrink in firing and if it’s shrinking “onto” your setter that’s a possible crack. If it’s shrinking “away from” the setter that stress isn’t set up. 

Oh now I get what you mean, thank you for the advice!

[Kelly in AK]

@Callie Beller Diesel creep is a materials engineering term that refers to materials exposed to stress deforming over time. We’re doing some accelerated action in a kiln, it might be likened to pyroplastic deformation (I’m no engineer, but I play one in this forum).

An example of creep, though there are probably less obscure ones, is Frank Lloyd Wright’s historic house “Fallingwater.” His plans for the cantilevered concrete slabs showed they were to be cast with a reverse camber, to compensate for natural creep of the concrete. In my crude sense, it means to cast them curving up a bit because in time they’ll tend to curve down, even though we think of concrete as an absolutely rigid material. They were instead cast flat, and over many years began to droop, then crack, requiring great expense to remediate the historic building. 

Of course in a kiln this all happens in minutes and hours, not years. Our rigid material shows exactly where it would “creep” under the force of gravity by warping and slumping. 

[PeterH]

> And beside that how can I calculate it so that the object would get out of the setter in one piece?
Note that the bone-china reference I sent used a one-time internal setter. As this was made of the same body as the pot being fired their shrinkage should be similar at all stages in the process. Obviously this isn't true for kiln-furniture style setters. As I've said previously, my concern with an outside setter is managing to keep the slot-ridges undamaged.

> This would achieved with a fully closed mold
I take that to mean you will be using a 2-piece mould, with the each piece defining one face of the cone.
This mould would presumably also fully define the slot and its associated ridges. (Concerns about de-moulding the thin ridges and "dropping-out" a form with a slot.)

I've done almost no solid casting, and worry that the thinness of your walls might be an issue when filling the mould.

 

[youpital]

1 hour ago, PeterH said:

> This would achieved with a fully closed mold
I take that to mean you will be using a 2-piece mould, with the each piece defining one face of the cone.
This mould would presumably also fully define the slot and its associated ridges. (Concerns about de-moulding the thin ridges and "dropping-out" a form with a slot.)

 

I have managed to do so by instead of casting the two lips apart, connecting them (by using a plaster mold with the same profile as the setter) and then cutting out the slit afterwards.