Curious about porcelain

[liaemars9]

 

 

[curt]

The cream is almost certainly due to a small amount (fractions of a percent) of iron in the clay.   I assume you are firing in an electric kiln when you see that cream color.  In a gas kiln the cream would instead be a blue with a bit of reduction.

As for the pink, not sure...  Others may know.

pictures?

[glazenerd]

Grolleg has 1% combined total of iron, magnesium, and titanium. These three metallic oxides can cause color shifts in porcelain; iron being the most notable. However, grolleg is very clean in this regard; so the color shift has to be coming from the plasticizer. In this case that plasticizer is ball clay. With ball clay comes higher levels of iron, magnesium, and titanium; plus organic and inorganic carbons ( sulfides in this case). The cream color can come from iron, but it can also come from titanium; or a combination of both. The pink is carbon residue discoloration. Commonly found in earthenware at bisq, but burns out during the glaze fire. Which makes me question your firing cycle.

Thermal expansion: I will use analogy that is not remotely accurate, but it gets the point across. At room temperature you put a 2" piece of ice and a 4" piece of ice on a table; obviously by the time the 2" piece of ice melts, half of the 4" piece of ice remains. They are unequal in size, so they melt at different rates. Now reverse that process: and put the liquid from each into a freezer, and again the 2" container will solidify much faster. Now add COE numbers: the 2" piece of ice is 5.25, and the 4" piece is 8.75.  No matter how much you control the melting or the cooling, you still have unequal reactions which result in crazing, dunting, shivering, etc. etc. 

As to your grazing issue on one body and not the other; even though they are both grolleg? The answer is feldspar, Nep Sy to be specific. The one that grazes has a higher % in the recipe as compared to the other. Sodium spar has nearly double the COE of clay and silica: as the feldspar levels increase: so does the COE of the body.

for this and other useless information you can contact me at: neveraskanerd.com

T

[neilestrick]

7 hours ago, glazenerd said:

In this case that plasticizer is ball clay.

Respectfully, maybe, maybe not. Most  commercial grolleg porcelains I have used do not use ball clay, but rather white bentonite or VeeGumT (or similar). Most folks who use grolleg porcelain do not want ball clay in there.

[tinbucket]

I'm not sure about this particular porcelain, but Vee Gum is a plasticizer give clay a pink color when fired. It is expensive but also very low iron and very plastic so it is used in some porcelains.

[glazenerd]

Just now, neilestrick said:

Respectfully, maybe, maybe not. Most  commercial grolleg porcelains I have used do not use ball clay, but rather white bentonite or VeeGumT (or similar). Most folks who use grolleg porcelain do not want ball clay in there.

Neil: absolutely correct, but in this case I am following the evidence. Neither white bentonite or V- Gum would account for the color hue. If either had been used as the plasticizer: the OP would have a pure white porcelain @ C6. V- gum at 2-3% and bentonite at 3-4%. Neither would add enough molar % to discolor. Grolleg with V-gum is in the 0.90 cents a lb? range! bentonite in .60-70 range. But hey- it's clay! 

Triolaz: I use V-gum and Macaloid on a regular basis, and a speciality smecite clay.

[neilestrick]

4 hours ago, glazenerd said:

Neil: absolutely correct, but in this case I am following the evidence. Neither white bentonite or V- Gum would account for the color hue. If either had been used as the plasticizer: the OP would have a pure white porcelain @ C6. V- gum at 2-3% and bentonite at 3-4%. Neither would add enough molar % to discolor. Grolleg with V-gum is in the 0.90 cents a lb? range! bentonite in .60-70 range. But hey- it's clay! 

Triolaz: I use V-gum and Macaloid on a regular basis, and a speciality smecite clay.

Thank you for explaining your reasoning on that. Another possibility is that they're cutting the grolleg with some domestic koalin. ART Clay used to do that, using Sapphire kaolin, which was higher in titanium.

[glazenerd]

Neil: that could also be a possibility for the one body with a "cream" hue. The one that was "pinkish" would be carbonaceous, which would bring a ball clay into the mix. The only other  possibility would be they mined through a contaminated vein of kaolin. Highly unlikely, but not impossible.

T