Teapots cracking just above foot when filled with boiling water.

[MaineMorrisPottery 2]

I’ve made at least 5 teapots.  All but one have cracked when boiling water is poured into them.  The crack is always in the same spot, right above the foot and cracks pretty much all the way around the bottom.  None of the teapots had any cracks after they were bisqued.  I’ve used 2 different kinds of clay.  The teapots were not all fired in the same firing.  I made sure the kiln was completely cooled to room temperature before opening the kiln.  If anyone has any ideas I would really appreciate some help.  Thank you.

Nancy

[Bill Kielb 1,333]

1 hour ago, MaineMorrisPottery said:

I’ve made at least 5 teapots.  All but one have cracked when boiling water is poured into them.  The crack is always in the same spot, right above the foot and cracks pretty much all the way around the bottom.  None of the teapots had any cracks after they were bisqued.  I’ve used 2 different kinds of clay.  The teapots were not all fired in the same firing.  I made sure the kiln was completely cooled to room temperature before opening the kiln.  If anyone has any ideas I would really appreciate some help.  Thank you.

Nancy

Pictures for sure would help with opinions can you post some 

[oldlady 2,971]

do you use a lot of water when you make the teapots?   do you dry it out of the bottom before it dries?    just wondering if that might cause a problem later.

[MaineMorrisPottery 2]

I don’t think I use a lot of water.  I will make sure  that I dry out the bottom thoroughly when throwing.  Thanks for your thoughts.

[Hulk 937]

Hi Morris!

ditto on pics, and approximate the thickness on both sides o'th' footring - if you trim rings, else both sides of the cracks - my guess would be the bottom is much thicker than the base of the wall? 

[MaineMorrisPottery 2]

I’m trying to track down the one teapot I saved so I can post a photo.  I think you have hit the nail on the head.  From everything I’ve read the different thicknesses make the most sense.  Thanks for confirming my suspicions.

[JohnnyK 733]

To follow up with what Hulk said, section the next teapot you throw to check the wall and base thickness to make sure you are not causing undue stress when the pot dries. It may not show up during the bisque and glaze firing but when you pour hot water into the teapot, the wall will expand more quickly than the foot area causing the cracking if you have uneven thickness.

[Pres 2,537]

Hi, So you are pouring boiling water into your teapots? Hmmmm! Wonder if it wouldn't be better to fill pot with hot tap water before, let warm up pot, then try your boiling water. . . after it has stopped boiling. I always do this with my own, and tell customers the same, as heat shock can really stress the pot, especially around the base as that is the thickest part of most teapots and  the thermal expansion between walls and base is not the same. Pictures will help, but I think that is your problem.

 

best,

Pres

[Denice 842]

I am with Pres,  I am not a tea drinker but I was told that you warm up any ceramic pot with hot water before you put any thing in it that is extremely hot.    Denice

[liambesaw 2,990]

Stays hot waayyyyy longer too

[Russ 63]

One possibility could be the clay body.  I had this same thing happening and found out it was the Hawthorn fireclay that had an extraordinary amount of super fine silica because of where they were getting it in their mine.  i completely changed my clay body formula so that no clay was  more than 20% of the formula. Fast forward to testing... finished glaze fired pieces were placed in the freezer for 24hrs  then removed and boiling water poured in.....  zero cracks in all pieces.

[Babs 1,262]

Varying thickness after triming the foot.

And what Pres said.

Always warm the pot...made for hotter tea and my old mum used to drink tea soooo hot but sthing to do with the tea making process rather than the pot as water poured in to warm pot was boiling...though just a little water

[MaineMorrisPottery 2]

Pres and Denice,  I do suggest pre warming the pot.  I even tell the client what my grandmother always did.  Put a knife in the pot then pour the boiled water into the teapot over the knife and it dissipates the heat.  The person who bought the last teapot l, after I tested it at home first, did use the knife and it still happened!

[MaineMorrisPottery 2]

Russ,  I wondered about the clay.  I don’t make my own.  I used laguna Bmix and also standard 66.  Talked to both companies and  they haven’t had any difficulties with temperature shock.  The freezer to boiling is impressive.  

[Min 3,805]

34 minutes ago, Russ said:

Fast forward to testing... finished glaze fired pieces were placed in the freezer for 24hrs  then removed and boiling water poured in.....  zero cracks in all pieces.

+1 for this.

Teapot shouldn't have to be mollycoddled to prevent cracks. For the brewing flavour or temperature warm the teapot if you like but no way should it crack without doing this.

[MaineMorrisPottery 2]

That’s what I was hoping.

[liambesaw 2,990]

40 minutes ago, Russ said:

One possibility could be the clay body.  I had this same thing happening and found out it was the Hawthorn fireclay that had an extraordinary amount of super fine silica because of where they were getting it in their mine.  i completely changed my clay body formula so that no clay was  more than 20% of the formula. Fast forward to testing... finished glaze fired pieces were placed in the freezer for 24hrs  then removed and boiling water poured in.....  zero cracks in all pieces.

Cristabolite inversion?

[Babs 1,262]

Maybe make one with no footring or start  by cutting a few trimmed ones to see how thin you are getting both sides of footring.  This could be the problem. Had a few high footed teamugs do this and this was the reason.  Just saying.

All teapots cracking or a few per batch.

[Babs 1,262]

2 hours ago, liambesaw said:

Cristabolite inversion?

 

Not at the temp of boiling water imo

Cracks occurring with same glaze combo?

Edited October 10, 2019 by Babs

[MaineMorrisPottery 2]

Most of them have cracked.  They were fired at different times. Now that I think about it. The ones that were fired according to preprogrammed cone 6 med speed cracked a couple that were a ramp and hold schedule with slow cooling did not.  Hmmmm

[Babs 1,262]

A photo would be great.

[douglas 31]

One other possible cause is too much glaze or using a runny glaze on the inside of the pot. Maybe when you are pouring the glaze out it is taking a while and the inside has a thicker than normal application? If the glaze is too thick at the bottom of the pot, and if the glaze doesn't shrink at the same rate as your clay body during firing, the thick glaze pooled at the foot is creating a stress crack and the sudden temperature change is causing it to fracture along the spot where the thick glaze ends and the clay wall begins. 

[curt 269]

Had exactly this happen with a 15” round casserole dish I made out of a nice porcelain.  First use in the oven the whole bottom cracked off just around the foot ring so neatly that I was able to save the (slip decorated) bottom and turn it into a nice cheese plate.  Fine, tight clay bodies with small particle size and lots of glass in them when fired, do not like thermal shock is what I learned from that.  So I would bet that the clay body is the issue in that use.

[Babs 1,262]

Curt did you trim a distinct footring or  did your pot taper/trimmed to a base?

[curt 269]

No foot at all really, Babs.  The outside line of the pot was just straight down the wall (right past the level of where the bottom was inside) with a slight turn in at the very bottom outside to create a small shadow line.  Amazing how it almost sheared off all the way around, just slightly below the level of the inside bottom.