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Clear Glaze Chemistry

clear glaze chemistry bubbles

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#1 amoqt

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Posted 01 April 2017 - 05:51 PM

Hi guys. I've started my aquaitance with ceramics and glazes half a year ago. I still have a very basic understanding of chemistry and I have several problems I'd like to ask about.

 

First is clear transparent glaze. I made around 10 of them taking different recipes, but all of them contain bubbles. I've tried bisque firing in various temps, from 600 to 950C and second firing slow cooling, dropping and slow cooling, but bubbles are there and their quantity is similar in all conditions. I know commercial glazes don't have such problems. Please help me to figure out what is going on. My thought is that my raw materials are the problem. I have very few materials for glazes, those are: feldspar, whiting, borax, dolomite, talc, silica and kaolin. Substituting whiting with wollastonite didn't help. The runnier the glaze the less bubbles it has, but running is provided with whiting and borax and I have white clouds coming along with bubbles. Less flux - no clouds, but still bubbles.

I imagine it is kaolin that gives a lot of small bubbles. Any chances to fix that?

I have stains and wish to use them in underglaze painting, so I need to make a decent covering glaze.

For underglazes I bought a few frits, I don't know their formula, seller keeps it in secret, only mentioning temp range. I have 1 lead flux frit melting on 710C and other leadless on 1100C. I need to fire on 1220C, so I guess some kaolin may help to lift temp of frits melting so the stains won't burn out, but more bubbles again?

 

If you have info how to win the fight with bubbles please share.

Sorry if made any mistakes.

 

 



#2 JBaymore

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Posted 01 April 2017 - 06:09 PM

Welcome to the forums.

 

What is your glaze firing profile?

 

best,

 

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#3 High Bridge Pottery

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Posted 01 April 2017 - 06:44 PM

Amoqt, where are you? The seller should give you that information. I haven't found kaolin in glazes to be the problem.

 

Take a look at this thread, no concrete answers but lots of talking about bubbles http://community.cer...l=+bubble +toil


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#4 glazenerd

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Posted 01 April 2017 - 11:58 PM

Welcome to the forums Amoqt.

 

You are firing to cone 6 +/-, so let me ask you about your clay? Stoneware or porcelain? The clay is rated to what cone/s? White, grey, or brown clay color?

 

Are the bubbles consistent in size? Do any of them break open?   Pics would be nice.

 

Nerd



#5 bciskepottery

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Posted 02 April 2017 - 05:38 AM

Pete Pinnell has a column in the new Spring/Summer Clay Times on "Dealing with Bubble Troubles." They are offering free access to the digital magazine through 4/15.  http://www.claytimes.com/

 

He offers some good suggestions on dealing with the problem.  Page 23. 


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#6 Chilly

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Posted 02 April 2017 - 10:59 AM

Bisque firing to 950C may be too low.  Try firing hotter, say 1050C.


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#7 amoqt

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Posted 07 April 2017 - 08:36 AM

Thank you all for answering!

I use semi-porcelain, the color is greyish white, which I guess is similar to stoneware. Announced that it has around 6% porosity. Firing range from 1180C to 1220C. I fire to 1220 it's about 6 cone, but I don't really use cones.

The reason why I think its kaolin because I 've read digitalfire article displaying small bubbles from kaolin. Of course they may be from anything else, but I'm pretty sure it's not the clay. I've had samples of commercial glaze and used it in the same conditions on the same clay and it came out absolutely perfect glossy bubble-free transparent and thick in application.

 

I attached 2 pics, hope the quality is enough.

 

Bubbles tend to move up to surface and I have needle size holes. I understand there's a moment where I need to hold temp so they move out, but I couldn't guess when exactly. -__-  

 

 

 

 

 

Attached Files



#8 High Bridge Pottery

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Posted 07 April 2017 - 01:02 PM

What's the glaze recipe? I have found reducing silica can help.

 

There is a point where the surface tension of the glaze beats the size/buoyancy of the bubble and you can probably hold for as long as you want and not move the bubble.


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#9 amoqt

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Posted 07 April 2017 - 02:41 PM

Here are some recipes:

 

FFF feldspar - 34

Silica - 18

wollast - 17

borax - 17

Kaolin - 10

dolomite - 5

 

 

FFF feldspar - 51

Silica - 28

whiting - 18

ZnO - 3

Kaolin - 11

 

Silica - 30

whiting- 10

borax - 15

Kaolin - 15

dolomite - 8

Talc - 3

 

These recipes may seem weird. I mix whatever I have and see what's going on.

 

Reducing silica is a brand new idea for me. I thought the more the better! Without silica it's not glossy and tends to matte. I'll think what I can do, thank you!

I've tried to reduce the surface tension with borax but it gives white clouds. It's a pretty effect actually, lovely white streaks when runny :) But not what I want in this case.



#10 Diesel Clay

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Posted 09 April 2017 - 11:37 PM

I've and a lot of the same problems, and I'm not gonna lie, it's an ongoing battle. I find application and firing cycle are excellent places to start. My clear can't be too thick, or it clouds up horribly with bubbles. I ran tests varying the specific gravity of my glaze, and had some success. I should note that the bubbles are still there, but because the glaze layer is thinner, they're less apparent. Firing hotter and with soaks at critical points also helped a bit.

#11 glazenerd

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Posted 10 April 2017 - 07:01 AM

 

I use semi-porcelain, the color is greyish white

According to your description of a "greyish" white: then this is not a true porcelain body. "greyish" indicates to me that some ball clay/s have been added, otherwise it would be a pure white color. "greyish" also indicates the presence of carbons; and can also indicate the presence of higher levels of magnesium. Ball clay/s that have a "greyish" color usually means the presence of lignite (coal): which means a high sulfur content, or in this case: sulfides. Sulfates burn off easily, sulfides do not.

 

Run your bisq program up to 1850F ( 1020C) or so and do an extended hold ( 20 minutes)

Run your glaze program up to 2230F, with an extended hold. ( 20 minutes).

 

or door #3..find a new clay body.

 

 

Announced that it has around 6% porosity.

Way out of line for a porcelain body: most run 0 to 1% porosity if fired correctly.  Rather poor performance even for a stoneware body. (2-4%) Given the color of "greyish", along with this porosity number: I would suspect this is a 25% kaolin, 25% ball clay, 25% silica, and 25% feldspar body. ( the old 1/4 standard porcelain). Try the recommendations given, if the problems continue then I would recommend finding a true porcelain body if you are going to continue doing functional pieces. The 6% porosity alone is enough to disqualify this clay for functional use.

 

Nerd



#12 amoqt

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Posted Yesterday, 07:46 AM

Diesel Clay thanks, that's something I can work on too! I'll try different applications.

 

glazenerd I never told it's a porcelain! I wrote semi-porcelain, a term probably not used anywhere else and I don't know to explain it in english. It's pretty cheap and easy to handle so I chose it for my ceramic start. I find greyish color beautiful and I don't want to use white porcelain so far. If you are curious about it:

SiO2   67,0

Al2O21,8

Fe2O 0,47

TiO0,5

CaO  0,45

MgO  0,3

K2O  1,7

Na2O 0,9

 

 

I'd like to have stoneware body in future but all I do now is dozens of glaze samples and fit recipes to temp around cone 6 so it doesn't matter to me. And what do you mean that it's not functional if the piece is covered in glaze?

I gave a reply up there that bubbles do not appear with commercial glaze and I think the body is not the problem.



#13 glazenerd

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Posted Yesterday, 06:18 PM

Semi porcelain would be better known as 50/50 porcelain. It is one of the oldest clay recipes around. 25% kaolin, 25% ball clay, 25% silica, and 25% flux. Grey color in natural clay typically means carbon; the clay is usually dug up / located around coal seams. Grey can also indicate magnesium, but usually a higher carbon content. Carbon will usually burn out with a higher bisq temp, as suggested above. However, looking at your sample photos; the clay could also have larger particles of natural feldspar minerals that are still off-gassing. It may be the picture, the lighting, or the angle: but I am seeing a greenish cast in the glaze which should not be there if it is a clear glaze.

 

Nerd

 

** There are many deviations on the old 50/50 recipe..



#14 amoqt

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Posted Today, 02:07 AM

glazenerd your point is - it's definately the body and definately not the glaze? I would love that outcome, because I like my raw materials and changing contents would be troublesome.

I'll try firing bisque at 1020C right away and check glazes :)

No, there's no greens in glazes, it's a little yellow in some of them, if the layer is thick some clear are more yellow than another, but in daylight I can't tell without specific close up watching. I didn't put any colorants, maybe iron in some materials?



#15 glazenerd

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Posted Today, 07:00 AM

Do the higher bisq fire and check the results. If you still have off gassing issues; then use this schedule in your next glaze fire.

 

Use your current firing schedule up to 1120°C - no need to change any of that.

 

50°C an hour from 1120°C to 1220°C with long peak hold: or,

 

If feldspar minerals in the clay body are causing this: this schedule will resolve that issue. A crystalline friend in Paris uses silica that has higher iron content: he often gets secondary iron crystals.I know Joel has had some issues with silica as well. Just a matter of going through a corrective check list until you resolve the issue.

 

Nerd







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