Spots On Bottom Of Unglazed Tea Pot After Use With Sencha Tea

[Reto Zustand]

Dears

 

I recently made tea pots based on the old japanese houhin design. Hence it does not need a metal seeve. However, after using them some dark spots appear on the bottom side. Perhaps this is just the color of the tea slowly leaking through. I wonder if it could be an unhealthy fungus.

 

It was fired approx 1250 degrees.

 

For comparision, the inverted teapot on the top right of the photo does not have dark spots. Because it is glazed on the inside as well.

 

Any experience or ideas on that?

 

Thanks.

kind regards

 

Zustand

[neilestrick]

What temp/cone is your clay formulated for?

[Reto Zustand]

Dear Neil

I have seen samples of the very clay high-fired and it looked alright. It's local clay from a historic site.

Strangely only the pots that are unglazed on the inside have those spots after using them for tea. The other cups and pots that are glazed on the inside don't bear the spots.

 

R

[Reto Zustand]

ps: it was fired at 1250 celisius that is 2282 farenheit !

[Tim T]

If fired to 1250C it should be pretty well vitrified and non porous. If this is the case, nothing should seep through.

To test, make a small bar say 10 x 2 x 0.5cm and fire it as you normally would. Weight it (W1), then soak in water for 24 hours, pat dry to remove surface moisture, and weigh again (W2). The water absorption is then 100*(W2-W1)/W1. If this is below 1% then this is generally seen as non-porous, and no tea should soak through the clay.

You could also try taking a teapot you don't like, and making a "tea" using water with a strong food dye in it, and seeing if that seeps through.

Could it be that the bottom of the pot is damp, so water is collecting and mould forming in the warm, damp bottom of the pot?

[neilestrick]

ps: it was fired at 1250 celisius that is 2282 farenheit !

 

That's only about cone 8. If that clay vitrifies at cone 10 or above, then it might not be tight enough to be water tight. If it's just the local clay, with no added feldspar, then it may very well vitrify at temps even higher. You'll have to do an absorption test. 

[MatthewV]

I suspect it is tea seeping through.

 

1- the firing temperature might not have been high enough to fully vitrify the work.

2- when working with the wet clay, you might have had it a little dry and had very fine bending cracks.

 

It is likely to be mold only if you left them in a damp place for a few days.

Also  of note, unglazed pieces should be washed with just hot water and then left to dry.

[Reto Zustand]

Dera Tim T, MatthewV, NeilEstrick

 

I've put some hot water in a pot and let it rest for 2 hours. A paper towel placed underneath got a little wet. So it is definetly the content of the pot. Strangely is that the Sencha tea is light green yellowish in color and it only stays in the pot for 1-2 minutes before I pour it in a small cup. The spots are much darker than any tea spots I've ever seen. Perhaps it is something else that comes with the water out of the clay? Or just the long time it takes for the teawater to run through the clay.

 

The clay was probably not fired high enough and not the best industrial quality.

 

Would it be possible to glaze the same pot on the inside and fire it at low temperature 1000 celsius / 1830 farenheit. A low temp glaze may seal the inside?

 

I usually just rinse it with hot water and dry it off. Normally I use the pots just for one type of tea to retain it's flavour.

 

kind regards,

R

 

 

If fired to 1250C it should be pretty well vitrified and non porous. If this is the case, nothing should seep through.

To test, make a small bar say 10 x 2 x 0.5cm and fire it as you normally would. Weight it (W1), then soak in water for 24 hours, pat dry to remove surface moisture, and weigh again (W2). The water absorption is then 100*(W2-W1)/W1. If this is below 1% then this is generally seen as non-porous, and no tea should soak through the clay.

You could also try taking a teapot you don't like, and making a "tea" using water with a strong food dye in it, and seeing if that seeps through.

Could it be that the bottom of the pot is damp, so water is collecting and mould forming in the warm, damp bottom of the pot?

 

 

 

ps: it was fired at 1250 celisius that is 2282 farenheit !

 

That's only about cone 8. If that clay vitrifies at cone 10 or above, then it might not be tight enough to be water tight. If it's just the local clay, with no added feldspar, then it may very well vitrify at temps even higher. You'll have to do an absorption test. 

 

 

 

I suspect it is tea seeping through.

 

1- the firing temperature might not have been high enough to fully vitrify the work.

2- when working with the wet clay, you might have had it a little dry and had very fine bending cracks.

 

It is likely to be mold only if you left them in a damp place for a few days.

Also  of note, unglazed pieces should be washed with just hot water and then left to dry.

[Min]

If the paper towel got wet after just 2 hours I don't think low fire glazing it will seal it. Low firing it will not noticeably lessen the porosity of the clay. Also, a low fire glaze is likely to craze from water absorption into the clay so you will land up with the same situation. Need a tighter firing body that doesn't leak for teapots. Can you refire the teapots to a higher temp and see if the clay is more vitrified?

[MatthewV]

In general, refiring creates more problems than in solves. The correct way is to make another piece and compact the areas that will remain unglazed a little more. And maybe get to a higher temperature if that is possible. Potters fix problems by making new pieces :-)

 

There is no guarantee that this will be enough to keep some liquids from seeping through. At the very least you should be happy to know there is no health issue (that I know of) from what you are doing.

[Reto Zustand]

I would't try to refire it at a higher tempetature. I think the clay isn't made for non porous tea pots (ike the chinese and japanese originals). besides the handles (paperclay ) is for max 1260 celsius / 2282 farenheit

 

But I'd give it a try on a (1000 celsius) low temperature without too many expectations. Yes the nice thing is to make new pots and based learing from past ones. In the end ceramics are fragile so I hope to be able to let go of them at any point in the process. ;-)

 

After I have left plain water in the pot for 16 hours the spots appeared also on the top part of the pot and many more on teh bottom. Perhaps a substance get washed out of the pot? Or just some tea residue in the clay.

 

greetings

 

 

 

 

 

 

 

 

In general, refiring creates more problems than in solves. The correct way is to make another piece and compact the areas that will remain unglazed a little more. And maybe get to a higher temperature if that is possible. Potters fix problems by making new pieces :-)

 

There is no guarantee that this will be enough to keep some liquids from seeping through. At the very least you should be happy to know there is no health issue (that I know of) from what you are doing.

 

 

If the paper towel got wet after just 2 hours I don't think low fire glazing it will seal it. Low firing it will not noticeably lessen the porosity of the clay. Also, a low fire glaze is likely to craze from water absorption into the clay so you will land up with the same situation. Need a tighter firing body that doesn't leak for teapots. Can you refire the teapots to a higher temp and see if the clay is more vitrified?

 

 

 

 

 

If the paper towel got wet after just 2 hours I don't think low fire glazing it will seal it. Low firing it will not noticeably lessen the porosity of the clay. Also, a low fire glaze is likely to craze from water absorption into the clay so you will land up with the same situation. Need a tighter firing body that doesn't leak for teapots. Can you refire the teapots to a higher temp and see if the clay is more vitrified?

[neilestrick]

There is a method of staining wood to make it black called ebonizing. It's done by applying iron acetate to wood that is high in tannins, like oak. Woods that are not high in tannins can be ebonized by first applying strong tea to the wood, which provides the necessary tannins. I'm thinking maybe the tea in your pot is reacting with the iron in your clay to produce the black spots.

[Reto Zustand]

Thanks Neil , that is good remark. The clay was harvested locally and probably impure for any reasons. Pottery has been done there (Portugal) for centuries. I guess that is the charm and risk of a non-industrialized production