slow leaking pots

[LewisCzech]

Hello, I’m a newbie here (and I’ve only been doing ceramics for 2 years).

Recently I’ve had an issue with my pots leaking. For the first eighteen months of me doing pottery there was no problem, but since June every single one of my pots/vases/cups has had a slow leak, like it’s sweating from the base, leaving a damp patch on the surface.

My great problem is that I have no control over the clay, firing or glazes used. I go to a pottery class, and our teacher is in charge of buying the materials and firing. I’ll talk to her more about the technical aspects of firing next week, but I want to get some opinions here before I do that. She says that there is no problem with the clay body, but the fact that this problem arose some six months ago in every pot, and not before that, makes me think otherwise.
I would be grateful for any input.

[ApprenticePotter89999]

Hello yes, Also a newbie here but quite curious. Do you know the name or type/types of clay, and can you talk about the techniques you're using in the class, or any details that may have contributed to this? Are these pots that you're using, or do they hang out in the classroom? Apologies for offering questions and no answers. 

[LewisCzech]

Next Monday I'll ask our teacher more questions about the types of clay, firing temperatures etc. It's the last lesson of the year, so it's likely to be  more about wine and chat than pottery. I know she's had problems with the kiln before, and it's been repaired a few times, but she swears it's working fine just  now.  She's got decades of experience teaching ceramics, so I'm pretty sure she knows what she's doing. 
I do take home and use my pots, or give them away as gifts.
I'm guessing that the clays will be unknown to the people on this forum because they are likely going to be local Czech clays or possibly German ones. I'll see if I can get the spec. sheet for them though.
There aren't really any good Czech forums covering ceramics, and that's why I cast my net a little wider and asked here.

 

 

[Roberta12]

I started in a community studio.  Many of my pots would leak as well.  And sometimes grow mold.  After researching the matter, I found that the clay was not vitrified.  The clay bodies being used were for a higher cone that what was being fired.  I was not able to address this with the person in charge, so once I had my own kiln, clay, glazes, I bought clay that was appropriate for my firing temps.  And made certain my kiln was firing to the proper temp.  Perhaps you could ask the teacher if she uses cone packs in each load?  And read up on vitrification.  See if you think it addresses your problem.  

Roberta

[LewisCzech]

2 minutes ago, Roberta12 said:

I started in a community studio.  Many of my pots would leak as well.  And sometimes grow mold.  After researching the matter, I found that the clay was not vitrified.  The clay bodies being used were for a higher cone that what was being fired.  I was not able to address this with the person in charge, so once I had my own kiln, clay, glazes, I bought clay that was appropriate for my firing temps.  And made certain my kiln was firing to the proper temp.  Perhaps you could ask the teacher if she uses cone packs in each load?  And read up on vitrification.  See if you think it addresses your problem.  

Roberta

I rather think that this may be the case.

[neilestrick]

Chances are your clay is not vitrified. The absorption rate of the fired clay needs to be under 2% to prevent weeping. Find out what cone your clay is rated to, and what cone it's being fired to.

[Mark C.]

Most likely now one knows whats really going  on testing wise

absorption/vitracation  testing needs to occur and may be beyond the scope of the program?Or the person in charge

Neil summed it up well above .

[Babs]

Take a mug and piece of paper with you. Leave it full of water for duration of class. Sometimes eyeballing the evidence can be very effective in supporting your questions.

May be teacher doesn't use this clay for her own work OR fires it to a different cone.

Get the name and firing info from box or company....May be a big differece in cone reached over the shelves of the kiln..

Good luck.

 

[Stephen]

Just tossing it out there but is it possible that beyond the possibility that they bought some high fire clay they are firing too low six months ago, that the kiln is not hitting temp? I realize elements wearing out would be gradual but if one or two stopped working altogether and the kiln being on a timer is shutting down when expected. Obviously I am making a leap that first its electric and second that no one is checking the cone in the sitter. If it was gas though it seems unlikely they would fire to anything but 10. Some times teachers inherit these task with a sheet of instructions. My suggestion is further stretched because the OP didn't mention the glaze being affected and it surely would be in color and appearance if the pots were under fired enough to weep. Also with wearing out elements and a gradual under fire to timer shutoff instead of sitter might take a long time to reach the level of everything weeping so maybe it was gradual but when it hit that point seemed instant. 

[Bill Kielb]

I’ll throw in, if this is low fire clay (cone 04’ish) then the leakage would be very contingent on the effectiveness of the glaze. If this was midfire (cone 6’ish) then this could be underfired claybody and the glaze  is not perfect as well. For high fire, cone ten, we have the same issues as cone 6 or midfire stuff.

so to start

• what type of clay? Low fire, midfire, or high fire? ( to start we really do not need to know the specifics about the Clay)
• Do they fire with cones?
• to what cone?

until we know these items everything else is speculation

oh, and a picture or two of something that leaks, inside and out as clear as you can take it would go a long way as well.

[Stephen]

Could someone explain to me how water weeps through glaze? I've never really understood this and have read some on it. Weeping seems to always be attributed to the clay not being fully vitrified but vases, cups and such almost always have a glaze liner. Is it because the glaze is not fully sealed and relies on the vitrified clay underneath to stop moisture from seeping through?  Is it that any crazing will allow the seepage meaning that there are really two problems, non-vitrified clay and poor clay fit? Or Is it that non-vitrified clay will result in poor glaze fit causing crazing and then the resulting weep?

[liambesaw]

Glaze defects.  Bubbles, blisters, crawling, crazing, the glassy glaze may look immaculate but under a microscope could be full of defects.  A well fitting glaze is really important for unvitrified clay like earthenware.

I don't like unvitrified clay in general because of the mildew problem.  I don't use a dishwasher and air-dry my dishes in a rack so things can get gross pretty quick