Agateware Cups Warped

[Benzine]

I am experimenting with a couple new mid-fire clay bodies I got at the beginning of Summer and thought I'd try to mix them together for a marbled look.  I made several cups with the mixed bodies, and noticed that some warped during the bisque.  I am almost positive none were warped prior to the bisuque.  If it were just one or two, I'd just say that they warped while drying, and I just didn't notice before loading them.  But it is was more than just one or two.

I did some tests with the clay bodies before hand, including making some test tiles with the marbled clays.  Both bodies on their own fire to cone 6, have a shrinkage of 10% and have an absorption if 1% or less.  The test tiles came out well, as did a smaller test cup I made and glazed.  No defects that I could see.

The cups are thrown evenly and were all dried together in the same condtions, and fired together (upside down on their rims).

Any idea what caused the warping?  Could it be something that happening in the forming that didn't show up until they were fired?  I would blame it in an issue with the two clays not being compatible, but over half had no issues (thus far).

I've never had anything  warp in the bisque before.  Are they guaranteed to get worse in the glaze firing?

[Bill Kielb]

My thought, pictures might shed some more light and I never fire a cup on its rim, ever. If it catches slightly while firing, it’s definitely going out of round right at the rim. I don’t even dry cups on the rim, just put them on a paper towel and lightly cover to dry slow. The paper towel let’s everything slide and wicks away the water from the foot so it drys pretty much as fast as the rim. This works best when I throw the cup on the wheel and move it to a paper towel on the ware board right from throwing. I need very round cups for custom orders so this has worked for me. Same for reasonable size bowls as well. Cutting them off a batt is a thing of the past for me.

[Hulk]

I'll dry upside down - after trimming and handling, when the rim portion is cheesy hard - mugs and cups, not open ended stuff, like bowls. The openings stay round; I do run a tapered rounder on it a few times, just checking, and if there's somewhat out o' round, the rounder trues it back most o' th' time. I also fire rimside up, having put some effort into making it smooth and just so. The foot I'm expecting to have to dress off - polish, or sand and polish. 

[Min]

5 hours ago, Benzine said:

Are they guaranteed to get worse in the glaze firing?

Yup, unfortunately that's always been my experience. There isn't a lot of shrinkage that happens between bonedry and bisque so the drag of the rims on the shelf might not have been the culprit. From bisque to a cone 6 glaze fire firing on rims without a waster slab can absolutely cause rims to go out of round. Do you flip them over to dry the bottoms prior to putting handles on? If so were the rims still fairly soft when you flipped them? Was there a draft while drying?

[Benzine]

@Min

I am firing them right side up for the glaze firing.  As long as they don't get worse, I can live with it.

These were not handled.  I let them dry uncovered overnight, in my basement studio.  This time of year things dry slow.  I took them outside to dry in the sun.  Once they released from the bats I flipped them to dry slightly before trimming.  Once trimmed They dried upside down overnight in the studio, then dried outside in the sun again, still upside down.  I also attached a stamped medallion after trimming (I doubt this had anything to do with it, just listing all the specifics).

Then as I mentioned, they were fired upside down for the bisque.

Firing is finishing up now, so fingers crossed...

[Callie Beller Diesel]

@Benzine did you happen to take shrinkage measurements at the dry and bisque stages? It’s possible for 2 clay bodies to have the same end shrinkage, but one will shrink more while drying and one will shrink more in the firing. Which in theory could create stress.

[Min]

4 hours ago, Benzine said:

I am firing them right side up for the glaze firing.

Yeah, I figured they would be but just trying to make the point that bisque firing upside down with the rims on the kiln shelf is less problematic than glaze firing like that. (like butter dish lids sometimes are)

 

[Benzine]

@Callie Beller Diesel I did do several tests, when I first got the bodies, because I was always planning on trying to combine them, and also because the supplier doesn't list specifics like shrinkage and absorbption.

Both bodies have a shrinkage of 10%.  I will say, he brown body has a more noticeable grog, and rougher texture associated with it.

[Min]

1 hour ago, Benzine said:

I will say, he brown body has a more noticeable grog, and rougher texture associated with it.

Have you seen the work of Les Manning? He did landscape type thrown sculptural pots that were mimicking layers of geological formations using heavily grogged clay combined with porcelain and everything in between. I went to a workshop of his years ago, slow drying was something he really stressed. (picture of one of his landscape pots below)

If the only difference in process between these cups and your test pieces was the application of the medallions I would be looking at that. 

[Benzine]

So I just pulled them out of the glaze firing and no additional warping (And worse breaking) occured.  I forgot to mention that only the inside was going to be glazed, so I did worry that might contribute to future warping as well.

Luckily they turned out well, and the warps aren't too noticeable.  

In the future I will dry them slower and just to be safe, fire them upright.  

[Babs]

Do they have handles?

Sometimes the attaching of handles esp on thin walled pots can cause warping  

[Benzine]

6 hours ago, Babs said:

Do they have handles?

Sometimes the attaching of handles esp on thin walled pots can cause warping  

Nope, no handles. 

As I mentioned earlier, I did add some stamped medallions, so perhaps that did something similar?  The medallions are about an 1/8th of an inch thick, and made out of *only* the brown clay.

I will say that some of the medallions were getting a bit stiff, when I attached them (despite me keeping them in a damp box) so maybe it is possible that those medallions and their memory pulled on their side and caused the warp.

[Babs]

9 hours ago, Benzine said:

Nope, no handles. 

As I mentioned earlier, I 

Maybe get s.thing which can support the wall from the inside when applying the medallion, keeping the pot in shape....

Putting outside before taking off bats is fraut. Bases can't move much. Rims drying faster...

Slow drying away from draughts.

I guess change one thing at a time....

Maybe your laidback throwing from the couch has to stop. :-))