Slip cast stoneware ripple like a naan bread after glaze firing…

[Linchimb]

Hi, has anyone experienced rippled / distorted stoneware plate after glaze firing? The plate was thinly cast using stoneware casting slip. Bisque looked fine. I placed it quite close to the kiln wall which might have caused some stress…the plate didn’t break and I’m wondering whether it will eventually break at some point or is this type of rippling kind of permanently settled. The distortion on the upper and bottom side is somehow symmetric.  
 

thanks very much. 

[neilestrick]

Looks like it might be slumping due to over-firing. What's the cone rating of your clay, and what cone did you fire to?

[Linchimb]

6 hours ago, neilestrick said:

Looks like it might be slumping due to over-firing. What's the cone rating of your clay, and what cone did you fire to?

Hi, the clay can be fired up to 1260 and I fired to 1250. But it was quite close to the  kiln elements. 

[neilestrick]

Do you know what cone it's rated to? If your kiln was climbing slowly, it may have over-fired if you're firing by temp instead of heat work. What was the rate of the final ramp to peak temp?

[Russ]

This looks like delamination bloating. Have you broken the piece to see if theres a bubble or void?

[Linchimb]

3 minutes ago, Russ said:

This looks like delamination bloating. Have you broken the piece to see if theres a bubble or void?

Ah it is probably bloating. I haven’t broken it because of the painting on the plate and I was wondering whether I can keep it as decorative piece…? The bloating is quite symmetrical so I guess there must be void in between. 

[Linchimb]

34 minutes ago, neilestrick said:

Do you know what cone it's rated to? If your kiln was climbing slowly, it may have over-fired if you're firing by temp instead of heat work. What was the rate of the final ramp to peak temp?

I didn’t put cone in…it was ramping at 100 degrees per hour until 1250 with a 30min soak. 

[neilestrick]

9 minutes ago, Linchimb said:

I didn’t put cone in…it was ramping at 100 degrees per hour until 1250 with a 30min soak. 

Is that the firing schedule you typically use, and this sagging pot was an odd one? If so, then it's probably a thickness issue. Otherwise, it's probably over-firing. That 30 minute hold is adding heatwork equivalent to going another 1.5 cones hotter.

[Mark C.]

looks like bloating to me.-yes you can keep it (do not sell it) but  if you break it open you will know for sure thats it bloating as the inside will have a bubble in it.

[Linchimb]

14 minutes ago, neilestrick said:

Is that the firing schedule you typically use, and this sagging pot was an odd one? If so, then it's probably a thickness issue. Otherwise, it's probably over-firing. That 30 minute hold is adding heatwork equivalent to going another 1.5 cones hotter.

yes that's my normal firing schedule. There were other pieces made from the same casting slip (from scarva) and they were fine. That's why I was wondering whether the position in the kiln also contributed to the overheating of this particular plate. Maybe I can drop the soak next time.

[Jeff Longtin]

At first blush this looks like the rippling that occurs between the top and bottom halves of a two part plate mold when the slip is too thick.  The air bubbles have no where to go so they stay trapped between the two surfaces. At some point a vacuum is created and it sucks the clay wall inward.

I never knew it could happen until I started casting two part plate molds myself. The solution is to either thin the slip or redesign the mold?  I chose to redesign my plate molds instead. 

[Linchimb]

On 6/22/2022 at 6:30 PM, Jeff Longtin said:

At first blush this looks like the rippling that occurs between the top and bottom halves of a two part plate mold when the slip is too thick.  The air bubbles have no where to go so they stay trapped between the two surfaces. At some point a vacuum is created and it sucks the clay wall inward.

I never knew it could happen until I started casting two part plate molds myself. The solution is to either thin the slip or redesign the mold?  I chose to redesign my plate molds instead. 

Hi Jeff, thanks for your suggestion. I think this might well be the case. I use a two part mould for the plate where the main part is solid cast with an open back to pour the slip out. It often creates void and it’s not easy to have a complete fill. Maybe I need to redesign the mould…

[Jeff Longtin]

Another option, though hugely tedious, is to keep the mold moving, after you have filled it with slip. That way the bubbles are kept moving and can't settle in the plate wall.

You don't need to do it for the full 30 minute set period, but a few minutes, to move all the air bubbles, will usually suffice.  Filling the mold only half way, during this period, is the most effective. Once you've swirled the mold a few times then fill it to the top.

Years ago I delivered a few molds to a customer who used a "roto-caster" to cast large cement table bases. By keeping the mold in rotation they were able to cast large cement pieces with thin walls. I realized it might also work with casting slip and air bubbles. It does.