trouble with big pots

[CactusPots]

This pot was thrown in 2 pieces, first a bottomless ring then placed on a matching cylinder.  Total weight was a full 25 lb bag.  The first bisque on another similar pot cracked the bottom.  There were other pots stacked inside, so I fired this on on the rim with other pots inside, but not touching.  Fired on the very bottom of my electric kiln.  I don't know what to think about this cracking.  Either a fault in throwing or somehow a kiln issue.  Doesn't seem like a typical rim crack to me.

[Bill Kielb]

2 minutes ago, CactusPots said:

This pot was thrown in 2 pieces, first a bottomless ring then placed on a matching cylinder.  Total weight was a full 25 lb bag.  The first bisque on another similar pot cracked the bottom.  There were other pots stacked inside, so I fired this on on the rim with other pots inside, but not touching.  Fired on the very bottom of my electric kiln.  I don't know what to think about this cracking.  Either a fault in throwing or somehow a kiln issue.  Doesn't seem like a typical rim crack to me.

Seems shrinkage exceeded the shear strength of the material at the time of cracking. Because of its flared shape upside down definitely adds a horizontal component force to this so not sure how thick this section would need to be to resist that effectively. IMO I would fire this right side up for that reason, nothing stacked inside, it’s heavy enough.  Just curious if a decent dose of grog or alumina was on the shelf so this could slide easily. Was the firing schedule reasonably slow as well? Just asking.

[CactusPots]

This is Soldate 60 from Laguna.  Pretty much my go to clay.  Firing schedule is 5 hours to 400 (lid cracked) and then 5 hours to cone 06.   Works good for thick hand built as long as it's dry.  Shelf had a good layer of grog, so should have slid ok.  I'm zero for 2 on bisque firing these.  I have one more to try.  I hate to waste space on either bisque or glaze firing by not having anything inside. 

Maybe try loading the pot in the middle of the kiln rather than the bottom?

[Bill Kielb]

1 hour ago, CactusPots said:

This is Soldate 60 from Laguna.  Pretty much my go to clay.  Firing schedule is 5 hours to 400 (lid cracked) and then 5 hours to cone 06.   Works good for thick hand built as long as it's dry.  Shelf had a good layer of grog, so should have slid ok.  I'm zero for 2 on bisque firing these.  I have one more to try.  I hate to waste space on either bisque or glaze firing by not having anything inside. 

Maybe try loading the pot in the middle of the kiln rather than the bottom?

Five hours from 400 - 1945 is 300 degrees per hour. Even most fast bisque schedules don’t exceed 200 an hour through quartz inversion etc...  Maybe next one you limit top end speed to 200  per hour or less just for this form. Might be worth a try and an extra two hours. I would still fire this right side up though, nothing inside of course, rather than have all the weight bearing on the rim. Short of that, my last idea that strikes me, maybe I would try rolling the rim over while throwing  and thickening it if I Still could not keep it together. 

[Pres]

When throwing/assembling this pot did the top stiffen up much? Did you rib the inside to expand the volume when finishing, Looking at the cracks, they really look like formation/throwing problem, INMO.  What are the measurements of this 25# form?

 

 

best,

Pres

[CactusPots]

The pot is about 9" tall and 15" wide.  It's a pretty heavy pot for the size.  I was hoping the thicker walls would help with warping and cracking.  Planters should be heavier than bowls, etc.

The 2 pieces when they were being thrown together were a good inch thick and pulled the last with  stiff ribs inside and out.  Then expanded out to shape.  They got a couple days under plastic, then a couple more under towels before direct air drying.  

The other thing is that I softened up the clay using a trick from this forum of pushing holes all the way into the block of clay and adding water that way.  Then wedging the bubbles out.  Even if there was a streak of softer clay, I can't see it doing this

[CactusPots]

1 hour ago, Bill Kielb said:

Five hours from 400 - 1945 is 300 degrees per hour. Even most fast bisque schedules don’t exceed 200 an hour through quartz inversion etc...  Maybe next one you limit top end speed to 200  per hour or less just for this form. Might be worth a try and an extra two hours. I would still fire this right side up though, nothing inside of course, rather than have all the weight bearing on the rim. Short of that, my last idea that strikes me, maybe I would try rolling the rim over while throwing  and thickening it if I Still could not keep it together. 

Let me be more specific.  (1)   40 degrees per hour to 120  hold 1 hour  (2) 80 per hour to 200 hold 1 hour   (3)  250 per hour to 1000  (4) 300 per hour to  1823 degrees.

I think I will make some changes on this schedule.  I'll make (3) 200 per hour to 1100 (quartz inversion is 1063)

Do you really think this is a quartz inversion crack?  It's not a regular rim crack, it spirals down.  Something about the throwing, I think.

[Bill Kielb]

2 hours ago, CactusPots said:

Let me be more specific.  (1)   40 degrees per hour to 120  hold 1 hour  (2) 80 per hour to 200 hold 1 hour   (3)  250 per hour to 1000  (4) 300 per hour to  1823 degrees.

I think I will make some changes on this schedule.  I'll make (3) 200 per hour to 1100 (quartz inversion is 1063)

Do you really think this is a quartz inversion crack?  It's not a regular rim crack, it spirals down.  Something about the throwing, I think.

I’m only guessing but it looks like the clay simply did not have sufficient strength  and failed  while shrinking. Any damage to the rim would have been a great start but you likely would have noticed.  I am not sure what all the ribbing is about but really believe in compressing final shapes to be more dense which for rims and sidewalls means compressing downward as practical after they have been thinned and stretched out to their final shape. For bottoms of bowls it definitely includes compression from perimeter to the center.

When we throw we stretch everything  out and up,  ribbing  can compress in one direction but the end result is to make thinner often by stretching a bit more. The pot gets taller and wider. So to me this can be compression in one direction but also tends to stretch axially a bit as well.

This is especially important to me on unconstrained sections or terminal ones if you will. So for me it’s imperative to compress the rims downward actually to thicken them and the wall below else from  experience they begin to tear just because they do not have sufficient strength. Not saying that is your issue but something I would focus on for the next go around. I still feel firing this on its rim put way more stress on the rim than firing it on its foot.

It will be interesting what you find out. I am not a big quartz inversion guy  and usually note clay is tough and can take it. The  material is undergoing considerable physical change in the 1000 degree range so I would expect the internal stress to be a maximum at some point so not exceeding the speed of some established schedules likely would not hurt.

Last thought, thicker walls add strength but also create a bigger gradient when heating with faster schedules. What is fast? Million dollar question actually, depends on thickness  - thermal mass, radiant energy, thermal conductivity ......... lots of stuff. Going slow does help reduce the gradient though.

All speculation though.

[Mark C.]

I would fire right side up in middle of kiln empty (I know seems wasteful) Compress the rim extra when throwing as well.

Did this pot have any reclaim in it?? or was it new clay?

[CactusPots]

47 minutes ago, Mark C. said:

I would fire right side up in middle of kiln empty (I know seems wasteful) Compress the rim extra when throwing as well.

Did this pot have any reclaim in it?? or was it new clay?

New clay.  Soldate 60.  Softened using a puncture method.  What do you think about the spiral nature of the cracks?  They tail away from the counter clockwise motion.

[Sorcery]

Sticking on shrink all day IMO.

This "waster slab" idea is fantastic and I won't try another larger pot without one.

Considering throwing faults is just muddying the waters I reckon.

Sorce

[Bill Kielb]

10 minutes ago, CactusPots said:

New clay.  Soldate 60.  Softened using a puncture method.  What do you think about the spiral nature of the cracks?  They tail away from the counter clockwise motion.

Just thinking, every pot on the planet likely has some spiral alignment. That’s how it’s thrown. What makes this one different?

[CactusPots]

10 hours ago, Bill Kielb said:

Just thinking, every pot on the planet likely has some spiral alignment. That’s how it’s thrown. What makes this one different?

The pot wasn't really thrown in one piece.

What makes this one different is that before attaching to the bottom half of the finished pot, the top half was centered and then pushed out entirely to the width of the bat.  Then it was opened all the way to the edge with no bottom.  It seems to me that I could have created a fault or fold in the clay moving the clay from the center to the edge.  Rim cracks are one thing, but the spiral nature of this crack is unusual to me.

Does anyone else use this method for throwing larger pots?  I find 25# of clay too much to center and open.

[Sorcery]

It looks as if it stick here.

The left damage is because of the sticking.

The right damage is stress relief.

Sorce

[CactusPots]

I don't know how it could be sticking on a layer of grog, but ok.

Didn't I read here that there isn't much shrinkage in bisque?  So the waster slab would be bisqued along with the pot and then high fired with it also?

If I get a successful bisque, then I'd expect you could treat it normally for the high fire.  I'm mainly thinking about stacking other pots inside.  If I have to high fire this pot by itself, I won't make any more this size.  Too much kiln space = too much cost.

[Mark C.]

Ok I read your post again and firing on the rim is not a good idea-no matter what its sliding on. Since the two piece part did not crack on seem-I think its non issue. Firing on the rim for any bowl (planter)  this size is not a good idea

[Min]

When reading Frank and Janet Hamer's Potters Dictionary they bring up a few things that I think apply to these cracks that point to a forming or drying issue rather than a firing one. Looking at the edges of the cracks they are not clean breaks, according to Hamer if the edges are frayed or rough then that indicates the crack(s) occurred slowly and probably happened during the making and drying stages even if they didn't show up until firing. I'm seeing some displacement of the rim which makes me wonder how thick the rim is compared to the rest of the pot. If there was a dormant crack from forming or drying and then during firing the cracked rim couldn't take the stress I think you would see this type of displacement. 

There isn't much shrinkage between bone dry and bisque but if there was a lot of weight resting on a relatively thin rim (in proportion to the rest of the pot) then I wouldn't be firing these upside down. Break it open and look at the wall thickness.

[Sorcery]

1 hour ago, CactusPots said:

shrinkage

Was it dried on the rim?

 

Sorce

[Babs]

Middle of kiln for sure. On its foot. How close to element?

You ask how to make bigger pots. Some folk  centre and form pot then add thick rings of clay and pull up , and repeat till pot size wanted.

Or throw your pot, throw another pot with same dimension . Key then to gether and "open up top pot to finish off. Are threads here and I am sure videos of both these processes. 

Looks like you expanded an already hardened pot

That is a lot of clay in that pot.

 

[CactusPots]

19 hours ago, Sorcery said:

Was it dried on the rim?

 

Sorce

At least for a while.   The bottom was my big concern.

[Sorcery]

On 6/29/2020 at 8:56 AM, CactusPots said:

At least for a while.   The bottom was my big concern.

I don't understand what the second part means exactly....but....

I believe drying shrink can begin these cracks that firing exposes. So I'm still certain these are shrink snag cracks.

The only thing that changes with larger forms, is the distance they must travel to shrink.

There is nothing I'm your throwing technique that effects a larger form. 

I see your throwing technique is fine. So it is fine across the board. Sure you did a little sausage adding or whatever..... but that doesn't change your good judgement and throwing intuition.

Shrink shrink bo Bink....

Just don't call me no fink!

Sorce