Interesting pot damage from silicon carbide removal?

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Baiyun
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Fri Jan 20, 2023 11:52 pm

These images are from a fully handmade modern pot sold as xiaomeiyao zhuni which I recently received and returned prior to use due to the depicted damage.

Apparently this was caused by the silicon carbide paste they commonly apply to the rim of the pot to prevent the lid from sticking during firing, which may have gotten moist in this case and thus stuck to the clay in this area, and then a bit of clay was taken off the pot when they removed this anti stick compound after firing.
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I am surprised that the above mishap, if this extremely strong moisture bonding is a plausible cause, would take off such a thin surface layer and reveal such a porous layer of a different colour and texture underneath. I was able to scratch it a bit dusty with a tooth pick. The provided explanation for the difference in appearance is the temperature variation between the surface layer and the clay underneath.

The first thing I noticed when opening the pot was that it had tiny glitter particles inside, which I have never before seen in a pot, and which came from the exposed section that would also sparkle in the light due to these embedded particles. I am not sure if this could be a common mineral in the clay, however, the sparkles were far more frequent and smaller than the usual mica spots visible on the clay surface here and there. The clay on the pot exterior was a fine mesh with very few mica spots of the usual appearance. The ring of the pot was in line with expectations for the clay type and wall thickness.

Here is a shot of the inside out of the sunlight (glitter washed out already) as a reference.
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Since I don't have any chipped or broken xiaomeiyao pots, I have not personally seen freshly exposed post firing inside layers of this type of pot. I assumed the clay to be vitrified evenly, perhaps with a darker colour due to a lack of oxygen, but not to be as light and porous as this.

Has anyone come across such a defect, or can provide more insight?
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Bok
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Sat Jan 21, 2023 2:02 am

As I’ve mentioned before to you, this is quite unusual to me… the existence of tiny shiny mica spots in the core of the clay would suggest it’s not proper Zhuni, even a coating of the pot. Also not convinced by their explanations… very odd.
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Baiyun
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Sat Jan 21, 2023 3:33 am

Bok wrote:
Sat Jan 21, 2023 2:02 am
As I’ve mentioned before to you, this is quite unusual to me… the existence of tiny shiny mica spots in the core of the clay would suggest it’s not proper Zhuni, even a coating of the pot. Also not convinced by their explanations… very odd.
Yes your opinion was duly noted, and thank you for repeating it here. One question would be what today's xiaomeiyao mined zhuni clay sourced from yixing stockists looks like in that state, and whether it simply has those mica levels characteristically absent (?) in pre-modern zhuni pots, since they are not from the same source.

Another question would be if the anti-stick compound used around the rim can actually fuse to the clay, in the presence of moisture, in a way that removing it from the pot breaks out a surface layer of the clay with it, rather than still separating cleanly from the surface of the clay. Someone familiar with that exact process may be able to shed some light on this likelihood. The vendor stated that this was 'not uncommon'.

The pot has been returned and refunded, so this is just about education and a fair assessment before any alarm bells are rung.
.m.
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Sat Jan 21, 2023 9:51 am

Not an expert, but it looks to me like the pot has a coating.
maple
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Mon Jan 23, 2023 8:13 pm

The major difference of modern YiXing and vintage YiXing is kiln.
Since they have "electric kiln" to control roast temperature, most of modern teapot are fired in different temperature (from low to high).
Tea pot makers fix the lid to fit the neck of body perfectly in different temperature.

They will put some "aluminum powder" between lid and neck before put the pot in the kiln.
If there're impurities in the powder, impurities melt and stick on the pot.

I guess....it's how this damage come from.
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Baiyun
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Tue Jan 24, 2023 5:36 pm

maple wrote:
Mon Jan 23, 2023 8:13 pm
They will put some "aluminum powder" between lid and neck before put the pot in the kiln.
If there're impurities in the powder, impurities melt and stick on the pot.
Yes a version of this was what the vendor said happened here. Do you have experience with the process and think that the exposed under-layer could look like this, much lighter and more porous in texture?
Andrew S
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Tue Jan 24, 2023 7:26 pm

Just adding to Maple's comment, it seems like the use of aluminium trihydrate / alumina hydrate is common in the ceramics world to prevent things sticking together, including pot lids:

https://digitalfire.com/material/alumina+hydrate

https://ceramicartsnetwork.org/daily/ar ... ood-Firing

Interestingly, I also found a comment on some ceramics forum saying that " it seems to stain my red clay with a dusty white that doesn't come off, I have to use sandpaper": https://community.ceramicartsdaily.org/ ... ax/page/2/

I couldn't see equivalent articles for silicon carbide, but perhaps they're out there somewhere.

Now, I don't know anything about ceramics; all of that is just based on a quick search in case it helps, so it's all subject to what people who actually know about the topic can add.

Andrew
DailyTX
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Tue Jan 24, 2023 7:48 pm

Just wanted to add to @Andrew S info.
silicon carbide is also called 金刚砂. Its purpose in Yixing has to do with after firing the pot for the first time, the craftsman would use silicon carbide paste to sand the pot opening for better lid fit, and then fire the pot for the second time.

Here is a video that shows the process:
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Baiyun
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Wed Jan 25, 2023 12:22 am

DailyTX wrote:
Tue Jan 24, 2023 7:48 pm
金刚砂
Yes this was the compound cited.

Whether this stuff can cake on or not, I can't find any pictures of broken yixing where the exposed sub-surface clay layers have such a different appearance to the surface clay. Always the same, or very similar, or perhaps even darker, exposed clay I expected to see based on the various chips and broken pots I've seen over the years - seems to be the case across the clay colour spectrum.

It just never looks like there is a very different thin outer layer with a much lighter core.
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