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carolross

Clear Glaze Forms Bubbles At Edges & Rims

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Actually, Babs, as I test the replacement clay, I'm going back to a commercial underglaze to start. The only slip I have is made from the clay body of the old clay :))

 

But I am confused about how much Mason stain to use. I've read articles by a several different potters who use anywhere from 1.5% to 30%, depending on the intensity of the color they want & get from the stain. I WILL do testing on much smaller amounts though, to see what's necessary to achieve a strong color - with sgraffito I think contrast between the clay body and colorant are key... I'm also going to check with Mason's customer service - obviously I need more info!

 

Thanks for checking up on me!

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Hi carolross,

 

If you are at the point where you are ready to test for soluble materials, you may want to do a test of your clay which exaggerates this issue to see if it is really the case. 

 

Visit this page (link below) on Digitalfire and scroll down to near the bottom.  You will see picture of a clay disk with a crack in it (it is the first one in the picture section) which is being put through the DFAC test.  Read the description next to this disk and you will see it refers to visual evidence of soluble materials.   You may also want to read about the SOLU test while you are there (link on the left of that page), but the DFAC test is really the main test anyway.

 

You may want to consider replicating this test for your clay.  It is not hard, and takes no special tools. Basically just put a heavy tin can in the center of a disk of clay and force the center of the disk to dry much slower than the portion not covered by the tin can.   This differential drying is what produces, among other things, the migration of solubles.

 

Then, once the disk dry (or bisqued? up to you), apply all around the outer circumference of the disk small patches of your slips, glazes, etc.. painted, poured, overlapped, layered, etc. or whatever other manner of application you want to test, basically any possible combination of surface treatments you think might be causing the problem.   Keep them all separate, number the patches, and and make careful notes about what you have done to each numbered area.  Essentially, applying these surface treatments all around the outside circumference of the disk where the soluble materials should now be concentrated should help you narrow down where the issues are at.

 

All I ask is that you post the picture when you do it so we can all learn from your test!  :D

 

Even if the problem is not soluble materials, this test should help you discover something about what is going on.  If you are really ambitious, do this for ever clay you are using or considering using.

 

Good luck and look forward to seeing the results.  Hope you are able to crack this little mystery!

 

https://digitalfire.com/4sight/tests/ceramic_test_drying_factor.html

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It is interesting that the problem only occurs on rims and handles when other parts with the slip and glaze look fine. I think the problem is a combination of too much colouring oxide and not enough glaze. Some places there is enough glaze to melt well with the colour and others like rims and handles, anywhere thin, is not able to suck enough glaze onto the surface when you are applying it.

 

I think it would be worth waiting a few hours after glazing and reapplying glaze to the areas that bubble before firing.

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Curt - okay I'll come out from under the slab roller where I've been hiding and I'll do it!  And will post the results!  Thanks for the suggestion. :)

 

Joel - that's an interesting thought...  I'll definitely give your suggestion a try.  'Cause if I fire this new clay and get the same bubbles and stuff...  thank you!

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Hi Curt! I did the tests - 2 disks, one of each clay - and I dried them in a convection oven (moving air) at 200 degrees.

 

I hope you're able to see everything clearly - if not, I can retake the photos, just let me know.

I'm really looking forward to your response - thanks you!

 

There are 6 jpgs - don't think I can get them all in one post, so please check the next one, too.post-73928-0-80368400-1474162284_thumb.jpgpost-73928-0-05736200-1474162292_thumb.jpgpost-73928-0-12643900-1474162287_thumb.jpgpost-73928-0-03010900-1474162289_thumb.jpg

post-73928-0-80368400-1474162284_thumb.jpg

post-73928-0-12643900-1474162287_thumb.jpg

post-73928-0-03010900-1474162289_thumb.jpg

post-73928-0-05736200-1474162292_thumb.jpg

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I'm having a really hard time following exactly where the issues are because you seem to be testing a lot of different variables at the same time. The original issue was the blue slip you make, correct? But now it seems like we're testing a bunch of commercial underglazes and two different commercial clear glazes, and nothing has changed with the original blue slip? I don't think we're narrowing anything down at all, but rather making it more complicated.

 

Any underglaze can cause bubbling/roughness issues if it's put on too thick, no matter what color it is. Some of the blue and red underglazes from many brands tend to be more prone to bubbling, especially if they're too thick. I don't think there's any issue with soluble materials here, but rather issues with application/thickness, too much cobalt in the slip, and incompatible commercial products. Not all glaze products play nicely together.

Juli Long, Babs and Roberta12 like this

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Hi Neil -

 

thanks for posting. My original problem (starting 3-4 years ago!) was with Amaco Med Blue Underglaze glazed with Amaco low zinc HF-9 glaze. That produced these bubbles... Wouldn't products from the same manufacturer be compatible? I switched to Mayco UG-97 Bight Blue and kept the same HF-9 glaze. Bubbles. Tried Mayco clear glaze - bubbles. I only recently tried the blue slip - obviously I have little experience making slips, but did some readin and said oh, what the hell? And mixed it w/ 30% blue mason. I'd tested other colors using under 10% and for the most part, they were washed out.

 

An engineer at Amaco looked at a greenware piece I'd put UG on a couple of years ago. He was surprised, but the inderglaze wasn't too thick.

 

From the tests I just posted and pieces I've fired recently, I think maybe I've used too little glaze...

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Hi Neil -

 

thanks for posting. My original problem (starting 3-4 years ago!) was with Amaco Med Blue Underglaze glazed with Amaco low zinc HF-9 glaze. That produced these bubbles... Wouldn't products from the same manufacturer be compatible? I switched to Mayco UG-97 Bight Blue and kept the same HF-9 glaze. Bubbles. Tried Mayco clear glaze - bubbles. I only recently tried the blue slip - obviously I have little experience making slips, but did some readin and said oh, what the hell? And mixed it w/ 30% blue mason. I'd tested other colors using under 10% and for the most part, they were washed out.

 

An engineer at Amaco looked at a greenware piece I'd put UG on a couple of years ago. He was surprised, but the inderglaze wasn't too thick.

 

From the tests I just posted and pieces I've fired recently, I think maybe I've used too little glaze...

 

Thanks for the clarity. Since it's happening with other underglazes, too, then I's say it's a firing or application problem. Have you verified with cones on the shelf that your kiln is firing to the correct cone?

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Hi Carolross,

 

First of all, congratulations on some very comprehensive testing! 😄 I have been staring at your test disks and looking for clues, but will probably need your help to interpret the results. Perhaps others will pitch in.

 

In case it wasn't clear, the reasoning behind the suggestion to test a number of your application methods and combinations at the same time across all the clays you use was this: since the problem was appearing in a number of different places across time with no particular pattern identifiable, a simultaneous test of all your methods, kind of a snapshot, might reveal some patterns. Maybe some find this confusing, but I think it provides a wealth of valuable information. It also helps avoid jumping around from test to test sporadically on a wild goose chase based on quick and easy diagnoses.

 

A few questions:

 

Did you dry the disks with something underneath or only the can on top?

 

I think I can see evidence on one of the disks as to where the can in the center was, but did you apply your slips etc into this center area? Would be helpful to see where the border between these two zones is.

 

Can you yourself see any difference in the quality of the surface between the covered area on each disk and the uncovered area, particularly:

 

1. On the raw clay with no glaze or slip on it. Does it have a sheen or shine on it in the uncovered area? That would be evidence of solubles.

 

2. On the clay with no underglaze or slip, but where you have dipped or applied clear glaze.

 

3. On areas with slip or underglaze, but no clear glaze.

 

4. On areas with a complete application of underglaze and clear glaze?

 

It is difficult for me to see in your pictures but perhaps you can make these differences out. I cannot see much systematic difference moving from the center of the disks to the outside edge, which suggest that in these tests soluble materials are not the issue, but then again I am not really sure where the boundary is between covered and uncovered.

 

Setting aside the solubles issue, I am struck by how many kinds of different problems/flaws seem to be appearing on these tests. However, they seem to be concentrated in the areas where you clear glaze overlays the underglaze. My overall takeaway would be that there seems to be some fundamental compatibility issues across the board between your underglazes and transparent glaze.

 

Perhaps others posters are right that it is the colorants. Or they are right that it is the firing schedule. Or they are right about both. Perhaps they will suggest some ways to test these diagnoses.

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The pink UG with glaze look pretty good to me, also the blue is not too bad but the slip and stain is going lumpy. I still think you want to drop to 20-25% stain and see if that helps.

The brushed glaze looks better but I think that's because the application is different.

 

The only time I have seen something similar is when I painted a kiln full of pottery with pure stain on the surface then glazed over and they all bubbled like crazy.

 

What temperature are you firing to? Any lumps in the slip/glaze?

 

Not sure about the brush on blue glaze, looks weird on one and ok on the other.

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