Throwing teacups with deep foot off the Hump: how to compress to avoid S cracks

[Babs]

Haven't had this for a while but last batch of teacups thrown off the hump, cut with generous base to enable trimming a deep foot  developed cracks on drying. Would like someone to run through how clay is compressed enough on the hump to lessen this fault. I know in some I trimmed not enough so base was a bit thick.

[Callie Beller Diesel]

I’m not one to throw a lot of teacups off the hump, but I do all my jar lids like that, and I never get s cracks. I make a shallow form with a large “foot” that gets trimmed into a knob, so there’s some similar principles at work I think. I’ve used this method on lots of fine toothed clay bodies, including B-mix  which can be notorious for this kind of thing. 

I try not to throw with too much water, or let it linger at all in the bottom. I don’t spend time going back and fort across the bottom of the pot, but for lids I do use a flexible metal rib to get the curve I want, and to remove all slip. I use a heat gun to stiffen up the rim enough that it won’t adhere to the jar gallery and let the pieces dry together for most small jars, but larger ones I’ll let stiffen up for a couple of hours before I put them together. Doing this makes them dry very evenly because of the trapped humidity on the inside of the jar. I almost never have to cover them overnight. Unless the weather is unusually dry, if I throw them sometime in the afternoon they’re ready to trim the next morning. 

For teacups, to replicate something similar I’d just flip them as soon as they’ll hold up to it, and let them dry upside down after trimming if the problem persists. 

When I trim them, they also get a good burnish on the knob side of the lid with a silicone rib at the end. This step is for aesthetics and might not contribute a lot to crack prevention, as I didn’t get them before I started doing it. But maybe worth a mention

[Rae Reich]

Definitely trim the feet asap after throwing, as soon as the rims are firm enough. The longer you wait, the more chance of the foot’s clay  drying unevenly. Cover the cups with plastic if you have a bunch to do, keep the bases moist until trimming. 

[Babs]

Thanks,  @Callie Beller Diesel, and @Rae ReichI did put these outside as wanted to " get on with it, flipped asap but I feel that out in the sun would have stressed everything, then sitting on hardiplank woulf have sucked the foot extrriors also, oh well will see what the bisque fire did tomorrow a.m.

[Pres]

Years ago, I had problems throwing tea cups and bowls off of the hump. After considerable frustration trying to fill an order for 2000 vessels I found a video of a Asian gentleman throwing off the hump. He separated a ball on the hump, and pulled a platter/pancake outward, then using both hands opposite sides-fingers underneath the platter he curled it up to make a cylinder. then continued to pull as normally we do. So looking in Hamer, I found a section about the clay particles and it showed a correct alignment that went from the flat bottom to curving in the corner to become the wall. Made sense to me and now all of my chalice bowls are thrown that way as were all of the honey jars I used to make and the lids for the honey jars.

best,

Pres

 

[PeterH]

5 hours ago, Pres said:

Years ago, I had problems throwing tea cups and bowls off of the hump. After considerable frustration trying to fill an order for 2000 vessels I found a video of a Asian gentleman throwing off the hump. He separated a ball on the hump, and pulled a platter/pancake outward, then using both hands opposite sides-fingers underneath the platter he curled it up to make a cylinder. then continued to pull as normally we do. So looking in Hamer, I found a section about the clay particles and it showed a correct alignment that went from the flat bottom to curving in the corner to become the wall. Made sense to me and now all of my chalice bowls are thrown that way as were all of the honey jars I used to make and the lids for the honey jars.

best,

Pres

 Perhaps this video, which you mentioned in an earlier posting.
http://mcgyakimono.blogspot.com/2010/08/making-teaware.html

PS Thanks for so many memorable postings.