What is wrong with this pot?

[SouthPark]

Originally Posted by Ray

Did you not understand my explanation of the iron oxidation state and how the processing can affect that?

Not all of it. I didn't understand all of it. One question I have is how do they make that particular region all black-coloured, while that other cross section is almost entirely grey-coloured.

I won't put a question mark. But the above is the question.

In other words - why is the thicker region entirely black-coloured, while the thinner region is pretty much entirely grey-coloured? What's the mechanism behind that?

If I had understood this process, I would have explained to the O.P. the reason behind such behaviour, or result. But I don't. That's why I said it was a good question. The OP's question is a good one.

[Ray]

The color changes due to the oxidation state of the iron.

That is affected by the chemistry of the atmosphere around it and having enough energy to change - it is in a crystalline form, after all. Plus, there is a factor of time.

When fired, control of the air/fuel ratio determines if the atmosphere is reducing or oxidizing. Excess fuel = reducing.

My speculation here is that the pots were fired under reducing conditions that led to the darker color. When the fuel flow was stopped, the atmosphere immediately becomes oxidizing, leading the the color change.

The color change starts at the external surfaces of the ceramic body and moves inward as the oxygen diffuses in from the surfaces. That takes time, but while that is happening, the pots are cooling, which slows the process more and more. In this case, there simply wasn't enough time for the oxygen to diffuse in far enough to fully oxidize the thicker regions before it got too cool for the chemical reaction to happen.

[SouthPark]

Thanks for your time for explaining that Ray. The chemistry and processes involved is really interesting! Thanks Ray.



Note the significant effect of OrchidBoard's image compression - or image re-compression software, which automatically processes our uploaded image files.

The word 'black' in the image text is supposed to be super clear and good resolution in the original image on my computer. But after uploading, OrchidBoard's software has re-compressed and made it look blurred like that.