Why does my Bone China have a Green tint?

[Jarman Porcelain]

Hello folks!

Curious if people are still checking this thread? I started throwing bone china about a year ago and am getting some decent results. I'm looking for more online resources, as well as colleagues to discuss with. I simply cannot find either of those. Bone China people seem very guarded, it is so nice to see someone helping others in here.

I learned the hard way about oil from your fingers being enough to resist glaze.

Is it just me, or is glaze fit unpredictable? I've had glazes that craze normally shiver, glazes that craze normally fit well, but also had glazes that fit normal ceramics also fit bone china. There seems to be no rhyme or reason, it strikes me as odd. 

Edit: Oh, I almost forgot, my bone china glows green! I've googled and searched everywhere, but cannot find any information at all. I've tried with florescent, incandescent, and sunlight, all green. What color does yours glow? 

Edited April 8 by Jarman Porcelain
forgot to ask about something

[Hulk]

Posting a reply to push this (newly split?) thread to the top of "Topics" and "All Activity" lists...

[Jarman Porcelain]

I'll add in my other image, which compares my bone china to laguna's frost. The quality of the white is totally worth all the hassle. 

Edited April 17 by Jarman Porcelain
"worth" not "worse", big difference there.

[Min]

This thread is a continuation of this one. Some comments regarding the green tinge on  page 2 of that thread. 

 

[Kelly in AK]

A quick glance tells me Iron in oxidation sums it up.. See other thread for more details. 

[PeterH]

49 minutes ago, Kelly in AK said:

A quick glance tells me Iron in oxidation sums it up.. See other thread for more details. 

Any idea of the likely source? Or the likely level of contamination?

 

[Min]

5 hours ago, Kelly in AK said:

A quick glance tells me Iron in oxidation sums it up.. See other thread for more details. 

If it was iron I would hazard a guess we would see this more frequently. From my searches I haven’t been able to find any other cases of it.
I find it very puzzling, wondering about colloids and phosphorus and the water used. 

 

[Jarman Porcelain]

5 hours ago, Kelly in AK said:

A quick glance tells me Iron in oxidation sums it up.. See other thread for more details. 

If this were the case you'd see this in commercial bone china as well. I'm using NZK, Grolleg, Standard Kaolin, Veegum, bone ash, silica, frit 3110, and Neph Sye. Those are all materials used in industry, and are all noted for they exceptionally low iron content. As I said in the other thread, I'm not ruling it out, but it seems extremely unlikely as this oddity is apparently unique to me, but none of my materials are. I'm also sieving the slip, as well as running high powered magnets through it to remove even more iron (something industry also does). 

My suspicion is some kind of crystalline silicate of calcium is forming, the body becomes very fluid in the kiln, and the body has an absurd abundance of calcium. It could either be green in colour, or blue in colour and then giving green when light passes through those molecules and the yellow we all are used to from oxidation firings. Again though, the problem I'm facing with this explanation is much the same, if I'm getting this, why does it not occur in other's bone china?

I plan to make a small batch with distilled water, hopefully this week if possible, that seems like a very worthwhile avenue of exploration. 

[Jarman Porcelain]

20 hours ago, PeterH said:

Starting a new thread with an eye-catching title such as  "Why is my bone-china green?" would probably draw a wider audience.

In desperation:
- I cannot see how it could be a kiln issue, but can you get somebody else to fire a test-tile for you?
- It also would be interesting to see how a low-fired test-tile came out.

I have had it fired in different kiln, all the same result. I have occasional access to a gas firing, so I'm going to ask if they can put a test into the next firing for me. It might not shed any light on this, but more data is always good.

What do you mean by a low-fired test tile? I've certainly seen it in bisque a lot, but I assume you mean something else.

[PeterH]

1 minute ago, Jarman Porcelain said:

What do you mean by a low-fired test tile? I've certainly seen it in bisque a lot, but I assume you mean something else.

I meant a something like a normal bisque temperature, rather than the high-bisque used for bone china.

In a hand-waving sense the more refractory ingredients wouldn't have entered the melt at that stage. OTOH the bisque nature of the test-tile might impede looking for the colour.

[Jarman Porcelain]

28 minutes ago, PeterH said:

I meant a something like a normal bisque temperature, rather than the high-bisque used for bone china.

In a hand-waving sense the more refractory ingredients wouldn't have entered the melt at that stage. OTOH the bisque nature of the test-tile might impede looking for the colour.

Oh yes, I've done both actually. The bisque looks normal, a slight pinkish hue, but less pink than my halloysite porcelain. No trace of green. I've looked at cut samples of bisque and vitrified pieces under magnification up to 80x, I can't see anything green.

[Min]

Do you think there is a link between the Veegum  and the bone ash or TCP? (Veegum being magnesium aluminum silicate) Interesting read here https://www.researchgate.net/figure/Concentrations-of-phosphorus-pentoxide-and-magnesia-in-Roman-emerald-green_fig1_277089354 possibly linking the two with a green colour.

[Kelly in AK]

The two photos are from Luxurybonechina.com and narumi.co.jp. 

A link to another photo is here: https://simple.wikipedia.org/wiki/Porcelain#/media/File:Transparent_porcelain.jpg

I get the sense a greenish tinge to bone china is not so unusual. The green of the piece @Jarman Porcelain posted is tilted away from a yellowish I would expect from iron, but not so much to make me think iron’s not primarily responsible. The clay body is a hair’s breadth from being a glaze at maturity (the ultimate clay/glaze interface: the pot is the glaze). Grolleg and Standard both have over 0.5% iron in them, perhaps that’s enough to show when it’s all in the melt? NZK has half the iron of those two, so I don’t know.

I found this interesting, since I feel like I hardly know what I’m talking about: https://www.jonsinger.org/jossresearch/tjiirrs/017.html

I certainly imagine getting some into a reduction firing, being able to compare, would provide useful information. 

[PeterH]

1 hour ago, Kelly in AK said:

I found this interesting, since I feel like I hardly know what I’m talking about:

Your not alone there.

White is just about the first adjective I think of when bone china is mentioned. It's an interesting idea that it might really be objectively bluish but subjectively  "whiter than white" (like many detergents).

The change from "whiter than white" to a more visible greenish tint then doesn't seem so implausible. Possibly due to some subtle change in the nature of the glass in which the (iron?) chromophore is embedded.

From @Min's reference.

PS a test-tile without Veegum seems an increasingly interesting idea.

BTW how are small test-tiles for different body compositions usually made? Using volumetric mixing ideas from glaze tests would seem sensible if you need to do lots. Perhaps drying the over-wet samples on plaster or in plaster moulds - or applying as a slip to a biscuit tile (although that might make inspection more difficult).

PPS Can you confirm that you have observed the green tint under several different lighting conditions. So we can exclude any light-spectrum related issues (c.f. neodymium glass).

Edited April 16 by PeterH

[Min]

4 hours ago, PeterH said:

PS a test-tile without Veegum seems an increasingly interesting idea.

I would go back to square one and change just one variable at a time.

Original recipe of 50 bone ash (one test with real and another with TCP) 25 Cornwall Stone (if @Jarman Porcelain doesn’t have any I think I have a small amount left that I can share) and 25 kaolin (grolleg). Mix that up dry then  split into two and mix one batch with tap water (after flushing the tap for a few minutes) and the other batch with distilled water.

At the same time I would run the same two tests with Veegum included.

If these test show no green (or far less) then the field is narrowed down to something in the Cornwall sub being used.

 

[Min]

I have a Shelly bone china set from my Grandmother, it's at least 100 years old. I shone a (halogen) flashlight into one of the teacups, very translucent! However Shelly made their bone china I'm not seeing any green in it. (except for the on-glaze work)

[Jarman Porcelain]

Thank you everyone for all the replies! This is amazing. I've not found people elsewhere this helpful, you folks know your stuff.

Kelly! I'm overjoyed to see another bone china piece that seems to have a touch of green in it! I was a little unsure at first, the plate you linked is possibly porcelain, and could be glazed green, but the Narumi piece has a green band around the footring where thick, but a yellow body. I did some digging and I found some 1980s bone china of theirs online with a distinct green tint that matches mine very closely. Super exciting, you must have done some serious googling. I will say green is not common at all, nothing I have that wasn't made by me  is green, nor has anyone else's that I've checked. It is clearly out there though. I'm put at ease seeing it in something made professionally, it can't be a sign of fault. It must just be some arcane material interaction. It couldn't be iron, as all other bone china would have iron in similar amounts. My bone china has only slightly less iron on average than vintage English bone china (they entirely use nzk now I'm told), so I'd imagine I'd see similar results if iron were the culprit.

That leads me to look at differences, I use Veegum T and don't use Cornish  stone or a sub. It must be something in either of those. I've already tested real bone ash vs TCP, both giving greens, so that isn't it. My mind zeros in on the Cornish stone, I can't be sure of much else, but I can be sure that Narumi wouldn't import Cornish stone, they'd find another flux. As they produced ware with that green as far back as the 80's, whatever it is needs to be available back then. That possibly rules out Veegum T, but maybe other refined Smectites were available? As min points out those contain magnesium, that could be it, and is easy to test for! In my work, I've been playing with combinations of frits, soda spars, and silica in the 25% of the recipe that would have been Cornish stone. Min, I will absolutely test your suggestions, but you don't need to ship me any materials. That is too kind, but I share space with a long running large production studio (also where I work), odds are I can dig some vintage stuff up in there and work something out. I'll get the sub on my spring order.

For Peter, yes I can confirm I've tested it in many lighting conditions. I use neodymium a bit and have lots of lights to show it off, made that test easy.

Thanks everyone, I've got some serious testing to do now. I'll check back frequently!

[neilestrick]

I'm voting iron as well. I think that also accounts for the pinkish tint in low fire bisque pieces. If we were able to have a slab that thick of our clear glazes we would also see it to varying degrees. Window glass is also green for that reason. It look pretty darn clear when we look through it, but if you put some paint on it you'll see just how much color it really has. They make super clear glass, of course, but it's expensive. If I remember correctly they call it Water White glass. Standard  plate glass and such is quite green. I worked for a glass shop for a few years after grad school, and we would occasionally have to replace opaque colored glass panels on buildings that were no longer available from the original supplier, so we just had to paint the backsides. Getting an accurate color match was nearly impossible due to the tinting effect of the green. You couldn't just have the paint color matched to the old panel.

[Magnolia Mud Research]

When we produce a ceramic material that allows "light" to pass through that material we are working with a "glass" even though us potters call the material a clay body or a glaze.  

Todays "Ceramic-Tech-Today"  article: Colored glass: From alchemy to empirical chemical design  
https://ceramics.org/ceramic-tech-today/education/colored-glass-from-alchemy-to-empirical-chemical-design/   
has some useful information and background that might help determining why the bone-china pots show a color when seen in a light source.  Remember that the source of the "light" is also an important variable.  

LT
 

[Callie Beller Diesel]

If we’re taking bets, I call iron in the water being the culprit. It’s the one constant variable across all OP’s tests so far. Very interested to see how the distilled water variation turns out.

[Jarman Porcelain]

I wouldn't get your hopes up on that. We don't get iron staining from our water, and the iron impurities in my kaolins are guaranteed to be significantly higher than whatever might be in my water. Public testing results are under 0.03mg/l, so only trace amounts. We also know for a certainty that iron gives a yellow discolouration in oxidation whiteware bodies. There's a solid chance it is caused by something in my water, but I don't think iron makes sense as we'd see this in all bone china worldwide.

Creeped your insta, my god your slip looks delicious, only way to describe it haha! That pink slip just looks amazing, luminous.