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PotsbyChar

Yellowing Of Clay As It Dries

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recently I made up a batch of low fire clay using a recipe from John Britt.

WHITE VITREOUS Body Cone 05

 

12.00 Tile #6 Clay

 

20.00 EPK Kaolin

 

26.00 C and C Ball Clay

 

10.00 Nepheline Syenite.

 

28.00 Ferro 3124

 

2.00 Gerstley Borate

 

2.00 Bentonite

 

I did not have enough tile 6 clay to make 12% so I substituted EPK Kaolin to make up the difference. I absolutely love this clay body for handbuilding and throwing. So far it has done everything I have asked of it. There is only one small problem. The clay starts out a kind of pearl grey in the wet form but as it dries it turns a kind of cream color. However, in places where my hands have touched it or I have used water on it to smooth it out it turns a kind of golden yellow color when it dries out. I have not fired any of the pieces yet but I am hoping that when I do fire them the color will be white. Anyone have any ideas what might be causing this? Was it the substitution of Kaolin for tile 6? Or is it some other reason causing the discoloration in the clay. Any information will be greatly appreciated.

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Guest JBaymore

Soluble sulphur compounds in the water (picked up from the dry materials) traveling to the surface and being deposited there as the water evaporates.

 

It can cause glaze defects with certain sulphur-sensitive colors.

 

Fire your bisque SLOW and making sure thaqt ther is good air flow through the kiln. Because you know that sulphur is there you know the kiln will be producing noxious sulphur di and tri oxide. (expect the smell.... and make sure to have good ventilation.)

 

best,

 

...............john

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It could also be salts from the nepheline syenite or frit (both are somewhat soluble) collecting on the surface during evaporation (probably in conjunction with sulfur as JBaymore has noted). This is common in many clays, especially, i have noticed, porcelains that use nepheline syenite as a flux instead of feldspar. Do let us know how the clay comes out after firing - I'd be interested in trying it out myself.

 

 

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Thanks so much for the replies. I will be firing some pieces this coming weekend so will let you all know how they come out. I am so pleased with everything else about this clay. I really hope this discoloration is just temporary for the bone dry stage and will disappear in the firing. This clay has worked excellently for carving and piercing also. In fact it has performed well in all the forming techniques which I have tried. I am anxious to see how it performs in the kiln.

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Thanks so much for the replies. I will be firing some pieces this coming weekend so will let you all know how they come out. I am so pleased with everything else about this clay. I really hope this discoloration is just temporary for the bone dry stage and will disappear in the firing. This clay has worked excellently for carving and piercing also. In fact it has performed well in all the forming techniques which I have tried. I am anxious to see how it performs in the kiln.

 

Just unloaded the kiln and the pieces came out beautiful. They are a very soft cream color. All of the discoloration present in the greenware as is dried has disappeared. I have started glazing some of the pieces that they are taking the glaze very nicely. Now to see how they turn out after the glaze firing....still keeping my fingers crossed. However, I am very pleased with this clay. It throws extremely well and handbuilds just as well.

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Thanks so much for the replies. I will be firing some pieces this coming weekend so will let you all know how they come out. I am so pleased with everything else about this clay. I really hope this discoloration is just temporary for the bone dry stage and will disappear in the firing. This clay has worked excellently for carving and piercing also. In fact it has performed well in all the forming techniques which I have tried. I am anxious to see how it performs in the kiln.

 

 

Just unloaded the kiln and the pieces came out beautiful. They are a very soft cream color. All of the discoloration present in the greenware as is dried has disappeared. I have started glazing some of the pieces that they are taking the glaze very nicely. Now to see how they turn out after the glaze firing....still keeping my fingers crossed. However, I am very pleased with this clay. It throws extremely well and handbuilds just as well.

 

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