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After wiping my bisque porcelain ware fresh out of the kiln with a damp sponge I noticed some yellow stains appearing.  I know the stains are sulfur. My question is whether or not they will fire out in the glaze firing?  Or do I need to do another bisque.  This is the first time this has happened.  We do use a water softening system for our water.  Could this have any effect on the pottery?  Thanks for the advice in advance.

Charlotte

 

 

 

 

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58 minutes ago, char wheat said:

After wiping my bisque porcelain ware fresh out of the kiln with a damp sponge I noticed some yellow stains appearing.  I know the stains are sulfur. My question is whether or not they will fire out in the glaze firing?  Or do I need to do another bisque.  This is the first time this has happened.  We do use a water softening system for our water.  Could this have any effect on the pottery?  Thanks for the advice in advance.

Charlotte

 

 

 

 

I have had this happen on occasion and never have had it affect the final product. I mostly use very white porcelain.

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I have the yellowing on bisque every time I use a liner glaze and leave the pot dry overnight before glazing the outside. Soluble minerals carried through the clay from the liner glaze, perhaps a bit from the water in the glaze. Never seen any negative effects from it.

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I have had sulfur stain change the color on a matt glazes ever so slightly firing to C6.   I started cleaning my pots with a dry microfiber cloth  and then going over the piece lightly  with a sponge and distilled water.  The sulfur problem was pretty bad,  it was worth buying distilled water at a dollar per gallon.   Denice

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I have had the same issue as Min, and as I sometimes leave the exterior of pieces unglazed, I find that it IS a problem for me. I fire to cone 5.

After it's fully dry, I wipe away anything visible (a couple of times, letting it dry in between) which sometimes works, but not always. When it doesn't it will fire with a dingy grey or yellow effect - kind of fine if there are no exterior decorations, but if I've used any kind of tape mask to add an iron stain or wash, I've got problems - a sharp line where the soluble materials were not able to emerge due to the tape. 

I've tried a 50/50 vinegar solution, hasn't been 100% effective, so I'm considering getting a masonry efflorescence cleaner to do a more effective job - has anyone tried that?  

This one and this one both use Urea Monohydrochloride instead of Muriatic acid. 

 

 

Edited by kristinanoel
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What has brought you all to the conclusion that it is sulfur causing the yellow stain? I had filled a bisqued Laguna Raku ^10 clay pot with water and let it stand overnight. The water seeped through the pot to the point where tiny droplets formed on the outside of the pot and the pot had a slight but noticeable yellow tinge to it with a little more intense yellow on the rim. I refired it to ^04 wondering if it would burn off, but it didn't. I would think that if it was sulfur it would have gone away. That's why I ask...why do you think it's sulfur?

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10 hours ago, JohnnyK said:

What has brought you all to the conclusion that it is sulfur causing the yellow stain?

That isn't what I think it is in my case and for those whose water doesn't contain an appreciable amount of sulphur and have had a clean bisque firing. As I said above I think in my scenario it's soluble minerals from the glaze, and perhaps a tiny bit from the water, being carried through the clay wall from the liner glaze. If I wet my bisque before glazing with a sponge I don't get the yellow so I rule out it coming from the water itself. It only appears if I either wash a glaze off a pot or use a liner glaze and let it sit to dry then glazing the outside of the pot.

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I think as a test I'll fill one of the bisque pots with distilled water to see if it goes yellow...I know that my water here is running through galvanized pipes that are more than 100 years old and there's some rust in it which may or not be reacting with the clay...

Edited by JohnnyK
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staining can be seen on bisqueware for several reasons coming from a bisque-in a gas kiln it can ahppen from flame lick or hotter temps on one side that the other. Usually not seen in electric kilns. The other is you said you wiped with water So I suspect thats the issue. Its not a problem and should cause to issue with the glazing .

It also can happen when the inside glaze can leach thru the body and leave a yellow look to the outside-again no big deal.

After about 100 years the inside diameter of a galvanized pipe is diminished but about 85%. by rust build up. Time to move up to more modern copper or pex pipes.

I threw out all my galvo pipe about 40 years ago and put in all copper.The sawzall is your friend on this job .

Edited by Mark C.
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  • 4 months later...

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