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amoqt    1

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.

 

 

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JBaymore    1,432

Welcome to the forums.

 

What is your glaze firing profile?

 

best,

 

..............john

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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.ceramicartsdaily.org/topic/12951-bubble-bubble-toil-and-trouble/?hl=%2Bbubble+%2Btoil

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glazenerd    816

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

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bciskepottery    925

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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Chilly    330

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

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amoqt    1

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. -__-  

 

 

 

 

 

post-84776-0-74603400-1491571871_thumb.jpg

post-84776-0-31046200-1491571881_thumb.jpg

post-84776-0-74603400-1491571871_thumb.jpg

post-84776-0-31046200-1491571881_thumb.jpg

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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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amoqt    1

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.

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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.

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glazenerd    816

 

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

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amoqt    1

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.

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glazenerd    816

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..

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amoqt    1
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?

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glazenerd    816

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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amoqt    1
glazenerd I've done tests and unfortunately it didn't help. I bisque fired to 1020 and then second fired slowly. My kiln does not heat up fast, it's a handmade thing, so it took around 3 hours to reach 1220 from 1100. I hold 20 min at top temp and let it cool as it goes itself. Probably I need to hold an hour maybe? Bubbles are still there, the same size, the same places. Thanks to bisque firing to higher temperature than usual glaze layed thinner, but I can still see bubbles in there and it's pretty obvious in spots I put colorant on. 

 

 

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oldlady    1,323

i have never had trouble with bubbles, but that white stuff happens if one recipe is applied too thickly.

 

have several recipes for clear which may need more ingredients.  one of my favorites contains barium carbonate which many potters refuse to have in their studio.

 

if i researched it correctly, the fear comes from the fact that it is used as rat poison.  history shows some really strange mistakes.  someone used it in place of flour in cooking for group of soldiers(?) and poisoned them.  they all recovered but the fear is strong.  i cannot imagine a potter trying to cook with ANY ingredient from the studio.  so after finding no fatalities online, except for a suicide, i decided i could use the small percentage of barium carb the recipe calls for.  it also goes over green underglaze without changing the color to grey, a big deal for me.

 

i would be happy to give you the recipe if you will send me a private message asking for it.

 

min has several good ones that i have tried and found just a little less attractive than mine but no bubbles in them, either.

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A few other things to try;

 

use frit for the flux

 

calcine the kaolin

 

decreasing the kaolin would make the glaze more fluid

 

use wollastonite rather than calcium carbonate (if you use it)

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amoqt    1

oldlady thank you, I have barium and I'm totally okay using it in glossy well-melted glazes. for whatever reasons found in the internet recipes of barium matte didn't work out (overburn and I'm too lazy now to correct the recipes) and I'd like to use barium since I have it.

 

Bryan Johnson  

I don't have a frit, they are quite expensive. I wrote list of materials I have in the first post and wollastonite didn't help. I believe my kaoline is already calcined, but I'd like to try. can you give me more info about it please? I haven't calcined anything before. Also, I'll try to decrease kaoline, thank you!

 

BTW here's the recipe I use now and it looks promising to work on.

 

Borax  31.91     Wollastonite 31.91     Calcined Kaolin 21.28     Silica 14.89

 

I reduced Sio2 as High Bridge Pottery adviced and it layes thin and has less bubbles than the others.

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This is a good picture I have to explain my thinking. Bottom row increasing silica, top row increasing frit.

 

Your glaze looks a little heavy in boron, I would try swapping a bit out for a little feldspar. Maybe 10 FFF and 20 Borax. If you can source a lithium feldspar or other lithium source and add a little of that.

 

Glaze T011A recipe is below and it's the only bubble free glaze I have managed to melt. Cone10.

Soda Feldspar 21

Potash Feldspar 30

Wollastonite 26

Calcium Borate Frit 10

Spodumene 13

 

gallery_23281_912_153204.jpg

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Min    783

You have a large amount of borax in these recipes. Borax is soluble, even if if you get a glaze to work using un-fritted borax you are going to run into problems if the glaze isn’t mixed and used straight away. Even then you will likely have problems using borax as it will precipitate to the surface of the glaze on the pots as the glaze dries. Is there any possible way to get either a frit with boron or else ulexite or gerstley borate? Boron is really quite necessary for a cone 6 (approx 1220C) glaze so you need some way of getting it in there in an insoluble form.

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RonSa    189

This is the recipe I just tested. It didn't bubble and there is no frit

 

Clear Base - Clay Art ^6
Gerstley borate       20
Wollastonite            10
Nepheline syenite    30
EPK                         10
Silica                        30

 

I'm guessing exchanging  Neph Sy to Cluster Feldspar might get you to ^10

 

Disclaimer: I'm a beginner and haven't risen to the level of an amateur glaze maker yet, so please, take my post with a grain of salt and proceed with caution.
 

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amoqt    1

Guys, thank you for trying to help, but I don't have any materials like gerstlay borate or spodumene or lithium or any other fancy stuff. I have very little of choice and I need to work with what I have.

 

Min The reason I use borax is this is my source of Na besides Feldspar. I didn't yet find any other source of borax, but i'll search for it. Thanks!

 

High Bridge Pottery  thank you! The feldspar that I have now is very roughly milled. :( I was unlucky to buy it in large amount. The layer I get with it is quite thick and grainy and the glaze itself settles very fast and uneven. Here we're talking about thin layers, right? I've just found another seller with finer feldspar and neph. sienyte and I'll check those in some time. For now I need to stop working on clear glaze, maybe?

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