Bone china glazing problems

[Hyn Patty]

First, glad to help if I am able.  Also, thank you for the kind words about my website.

Another issue I thought of that might be worth keeping in mind - but you may already be aware.  Do NOT handle bisqueware with your bare hands unless you have thoroughly washed them with soap first, or wear gloves.  I have to pre-fire all of my bisqueware clients send to me in a low fire before glazing.  They are always handling their bisques and getting skin oils and dirt over the surface.  This is not as huge an issue with earthenware but with porcelain bodies (and especially bone china) this can cause serious issues with the glazes, or even underglazes, to not stick to the bisque.  Especially in firing it'll encourage the glazes to crawl or flake off!

So, cleanliness is a must when handling bisqueware.  Any contaminants, even dust, can cause this problem.  Keep that in mind when you are ready to glaze.

Also the coefficiency of the glaze you are using vs the claybody can cause problems.  Just like in blending types of glass, if your glaze expansion and contraction rates are  not a good 'fit' for your clay it can cause issues later, though not usually the kind you are having.  Even so, it may be worth testing another brand of glaze to see if you have less problems.  Simply changing to a leaded glaze, or off of a leaded glaze for example can make a big difference.

Edited March 1, 2023 by Hyn Patty
Added comment at bottom.

[Ben10]

Hey  actually I was not aware of this, someone just told me a week ago. I usually clean them softly with a slightly wet sponge. I sometimes had little dots on my porcelain, where the glaze was missing, this might be the reason for this.

 

 

[Kelly in AK]

Damn! Who needs glaze when your clay does that? It’s already glass. Beautiful. 

[PeterH]

On 4/8/2024 at 4:07 PM, Jarman Porcelain said:

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 body are you using?

[Jarman Porcelain]

I'm using various recipes of my own creation, all closely based on the classic 25:50:25. Notably I'm in North America, so I've been testing subbing soda spars and various frits (mostly 3110) + silica to replace the cornish stone. For Kaolins I've been using NZK, Grolleg, and Standard Kaolin. I've used both TCP and bone ash, the real stuff burns whiter and fluxs much more in my experience.

I worry that green is a red flag that I'm doing something wrong, but the ware is very well vitrified and very strong. If it were a common problem I'd imagine I'd be able to find information on it. 



 

[Min]

What are you thinking is causing the green? Chromium or copper? Have you tried one of the recipes that does produce the green tinge with distilled water to see if it still goes green? 

Edited April 13 by Min

[Jarman Porcelain]

I don't have a good sense of what is causing it. I've mixed hundreds of tests now across many variations on multiple recipes, all produce the green. It isn't Chromium or Copper, neither are in any of the materials I'm using. I can rule out contamination as I first started working on this 7 years ago, but only got it really producing results in the last year. I've been using materials purchased at various points throughout, so the contamination would have to be consistent for years, and also not show up in my production that shares use of these materials.

Distilled water is a great test idea, I'll run that test soon. I would think I'd see the green in my porcelain too though if it was my water, unless something in the water reacts with phosphorus. It is one thing that is constant, and the water here is extremely hard. Maybe trace elements of magnesium? Thanks for suggesting this, it isn't something I was considering. 

I have considered that maybe some weird crystalline silicate of calcium is forming as I cool the kiln. There is so much calcium in the body, and it becomes quite fluid in the kiln, perhaps something green is forming (or blue and then combining with yellow from trace iron/titanium?). I always run a slow cool, but I have some work that doesn't need it. I'll save up enough of that work to run a cone 6 without a slow cool and see if that changes things. Another thought is how trace amounts of iron in reduction give us celadon greens. I'm firing in electric though, but maybe I'm getting some localized reduction? Seems unlikely.

I've come to really like the green, so if it isn't a sign I'm doing something wrong I'm going to keep it. I just really want to know why it shows up. 

Attached is an image comparing my bone china to Laguna's Frost. For those outside North America, Frost is a halloysite porcelain know for burning a nice white (for a NA porcelain). You can really see how the bone china is a cool white, with a distinct green tint even without light being directed through it. As I said before, I find it pleasing. 

[Magnolia Mud Research]

you will often get a greenish shine from a light on window glass at an angle. 
small amounts of iron in the glass is mostly the reason, but the source of the light and thickness of the glass are also a variables. 

take a look at: 
https://physics.stackexchange.com/questions/247939/why-is-glass-green 
question:
When I look at a glass block at an angle the edges appear green, but when I look at the edge sideways, so that is directly in front of my eyes it appears transparent. Why?
answer: 
Most glass contains iron oxide as an impurity which gives the glass a slightly green hue. When you're looking straight through a pane of glass you don't notice it because the glass is so thin. But when you look down the side it becomes apparent because of the thickness.

LT

[Jarman Porcelain]

7 minutes ago, Magnolia Mud Research said:

you will often get a greenish shine from a light on window glass at an angle. 
small amounts of iron in the glass is mostly the reason, but the source of the light and thickness of the glass are also a variables. 

take a look at: 
https://physics.stackexchange.com/questions/247939/why-is-glass-green 
question:
When I look at a glass block at an angle the edges appear green, but when I look at the edge sideways, so that is directly in front of my eyes it appears transparent. Why?
answer: 
Most glass contains iron oxide as an impurity which gives the glass a slightly green hue. When you're looking straight through a pane of glass you don't notice it because the glass is so thin. But when you look down the side it becomes apparent because of the thickness.

LT

I also have been considering this explanation, but this isn't glass, and if this were the case you'd see it in all oxidation fired porcelains. I won't rule it out, but it seems unlikely to me. The green is definitely in the body, not the glaze, as my unglazed bisque (fired to vitrification) gives the effect just as strongly as glazed pieces. Furthermore, if this were the case, commercial bone china would also have this effect.

If you can think of a way to test this I'd try it, but I'm also unsure how to test that. I don't think it is possible to produce a control that contains no iron.

 

[PeterH]

23 minutes ago, Jarman Porcelain said:

The green is definitely in the body, not the glaze, as my unglazed bisque (fired to vitrification) gives the effect just as strongly as glazed pieces

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.

Edited April 14 by PeterH

[Min]

I'll try and split this thread into a new one re Green Tint Bone China.

Arggg! I lost the initial post, I'm sorry but @Jarman Porcelainwould you be so kind as to  you repost your image/post from your first post here?

another edit: the split did work, it just took a while for it to go through. 

Link to new thread below, thanks for everyones patience!

https://community.ceramicartsdaily.org/topic/41608-why-does-my-bone-china-have-a-green-tint/#comment-256771

Edited April 14 by Min

[Jarman Porcelain]

Happy to, splitting this sounds like a great idea!

[Jarman Porcelain]

To give back to the thread, I just had great success adding 4% Veegum T ($$$) and 0.5% CMC gum to my LF clear and then gelled it further with a touch of epsom salts. The result is a bucket with about the worst rheology possible, unworkable by all rights. However, I dipped a couple pots through the glaze, shook them a bit, and got really good even application. It dried well without any cracking. I tested layering and that worked great too! I won't get a chance to fire for a bit, but thickness is good so I can't imagine how it could go wrong from here.

Up to you if ease of application is worth the VeeGum T tax.

Edit: It fired great! I even built up layers on one to have almost half a centimeter of glaze and it still fired well.

Edited May 14 by Jarman Porcelain
update