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sweaty bottoms! ---( bottoms on pots incase you were about to delete haha


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 Ok everyone, hope someone can explain why my stoneware and indeed earthenware vases filled with water and flowers seems to 'sweat'  and leave a damp patch beneath? Even some of my mugs leave a mark too? I' d rather not glaze the bases then have to grind away the splinters of stilt  that's usually razor sharp after firing, I quite like the contrast of the clay and glaze, especially with a nice foot rim so is there any other way to prevent this please? what am I doing wrong or not doing right? 

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It's because you clay is still porous and therefore absorbent enough to soak up water and leak. A well fitting non crazed glaze that covers the entire surface of the pot is a good idea for earthenware, which means stilting. Using terra sig on the bottoms can help reduce weeping too. 

For stoneware, getting a clay that fires to under 1 1/2 - 2% absorption is ideal and you don't need to rely on the glaze to stop the pots from weeping. Even a craze free glaze can have micro fissures which, over time, will allow them to weep but if the absorption of the clay is below approx 1 1/2% the pots won't leak even with no glaze on them.

I've noticed that in England it's common to see wide firing range claybodies, if this is the case with your clay are you firing your stoneware to the top of it's firing range? If not by being immature the absorption rate will in all likelihood be higher than it would be if fired to maturity.

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Ahh thank you Min...so if I bisc fired higher? would that make a difference..? the clays I've been using are stoneware (1120 - 1280 and  the other 1150 - 1280 firing range....I  usually bis at 05  or 06.  

Also re:-  ' getting a clay that fires to under 1 1/2 - 2% absorption'  ?

how besides doing the soak test do I know or find this out? I probably sound a right idiot for asking but that reply actually makes sense and I wish I'd asked the supplier before I just took delivery of 250k of new clay..

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56 minutes ago, dondon said:

Ahh thank you Min...so if I bisc fired higher? would that make a difference..? the clays I've been using are stoneware (1120 - 1280 and  the other 1150 - 1280 firing range....I  usually bis at 05  or 06. 

Looks like both of your claybodies are wide firing range bodies. This can be a problem if they aren't fired to maturity, which is 1280 (approx cone 10) for both bodies. I have noticed there are midrange claybodies on some of the UK pottery supply sites but there seems to be a prevalence of these types of wide firing range clays. Thing is to be mature at the upper end means they will be immature at lower temperatures. This is fine for sculptures and non functional pots but not for functional ware. Bisque temperature / cone won't have any impact on the absorption, it's the final glaze firing that needs to go higher.

1 hour ago, dondon said:

how besides doing the soak test do I know or find this out?

It's a good idea to do the soak test using your firing conditions rather than going just with what the supplier says it is.

Weigh a small bar or slab of 1/4" or so thick of clay that has been fired to whatever you are going to fire to. (weigh it straight out of the kiln) Now soak it in water for 24 hours (if you want to get really precise simmer it for the first 8 hours). After the soak dry it off really well with a towel and weigh it again.

For the math: for example if the dry unsoaked weight of the test piece is 200 grams and the soaked weight is 218 grams. Subtract the dry weight (200) from the soaked weight (218) = weight of absorbed water. In this example it would be 18. Now take the absorbed water weight (18) and divide it by the dry weight of 200.     18 / 200 = 0.09  Move the decimal over 2 places to the right and you get the absorption, in this example that would be 9%

If you don't want to do the soak and just want a rough idea if the clay is fairly absorbent, you can put a drop of food colouring on an unglazed part of the pot. Leave it sit for an hour or two then rinse it off. Ideally there won't be a stain from the food colouring, darker the stain the more absorbent the clay is. Not very technical but it's a quick method to rule out a clay or firing temperature/cone.

1 hour ago, dondon said:

I wish I'd asked the supplier before I just took delivery of 250k of new clay..

This problem of wide firing claybodies and underfiring seems to come up here quite a few times. If firing to cone 10 (approx 1280C) isn't an option would you be able to swap the clay for a midrange body? Firing to cone 10 is much harder on electric kilns than firing to midrange (approx 1220C).

 

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This won't help for selling or giving pots to others, but what I did for home use when I first encountered weeping bottoms in my early vases was to simply use a sealant, like Flexseal, and heavily coat the inside about a 1/8  of the way up & the outside bottom--problem solved. 

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