Jump to content
Sign in to follow this  
carolross

Clear Glaze Forms Bubbles At Edges & Rims

Recommended Posts

Hello,

I'm new to these forums and would appreciate some help!  This isn't a new problem but I still don't know what causes it.

 

On this particular problem piece, I used:

Flint Hills "Porcelain" ^5-6 

slip made with mason 6308 (Delphinium) mixed with the Flint Hills clay body

Amaco HF-9 Clear Glaze.  

 

I do an 8-10 hour bisque to ^04; glaze firing is ^5 or ^6.

 

I'm attaching a photo that shows a sample of bubbles that formed all along the edge of this platter.  It happens most often on blue underglaze or slip, but I've had the problem with other colors.  Sometimes a mug's rim will come out all bubbly and rarely the bubbles are on the main section of the piece.  

 

I dip glaze my work and carefully smooth out any pinholes or bubbles - and as far as I can tell, THESE bubbles along the rim are not visible before firing.

 

Everything else in the kiln with this platter came out great!  

 

Here's what I've done so far:  I've tried other colorants - several blue underglazes from Amaco & Mayco.  I tried clay body slip with mason stain.  I've tried other clay bodies.  I slowed my bisque time to 8-10 hours.  Called Amaco, but they weren't able to help.

 

Does this look like out-gassing that didn't complete during bisque?  Do larger pieces require longer bisque firings?  Do you have a clear glaze recipe or suggestion that might "fix" it?

 

I'm really hoping you all can figure this out - offer some advise or a solution (yes!)  Usually it happens on platters that I've spent hours carving (I sgraffito the work) and I'm so frustrated  :( and tired of re-making work :angry: - Please, can you offer advise or a solution?

 

Thanks so much!

Carol R

 

post-73928-0-55372000-1472497212_thumb.jpgpost-73928-0-41827000-1472497238_thumb.jpg 

post-73928-0-55372000-1472497212_thumb.jpg

post-73928-0-41827000-1472497238_thumb.jpg

Share this post


Link to post
Share on other sites

LT - I'm not sure what you mean? "If the rim is white" - do you mean on this piece by grinding off the bubbly mess? Or on a new piece, by leaving the edge bare - or applying white underglaze?

 

I have ground off the bubbles on other platters, down to bare clay. I've then reapplied underglaze, dabbed on glaze and re-fired. The result? Usually more bubbles...

Share this post


Link to post
Share on other sites

I would try making a few simple new pieces.

 

1) bare clay no slip, bisqued as usual and then dipped in clear. Check to see if it bubbles. If it does it might mean your glaze is incompatible with your clay.

 

2) clay with WHITE slip applied, bisqued as usual and then dipped in clear. Check to see if it bubbles. If it does it might mean there is an issue going on with your slip and the glaze.

 

3) clay with a colored slip applied any color BUT this blue, bisqued as usual and then dipped in clear. Check to see if it bubbles. If it does NOT it might mean there is an issue going on with the blue you are putting in your slip and the glaze. If this is the case try using a different stain to color your blue slip and test it out.

 

The last thing I am going to mention, and this may or may not be your issue so I would test for this as well. You may have your clear glaze too thick. I use Amaco Zinc Free clear and it is very touchy about how thick it it applied. Too thick and it can change stain colors under it, make colors run, crawl, pit, get cloudy, etc. To test for this I would make up a 6x6 test tile, cover the entire thing with your blue slip then divide the tile down into strips and apply 1, 2 and 3 coats of your glaze. Fire and check to see if only portions bubble or the entire thing does.

 

Good luck and I hope you get it figured out. It's so frustrating when a lovely piece is ruined and you are not sure why.

 

T

Share this post


Link to post
Share on other sites

It is very hard to see in the photos but it looks less like bubbling and more like crawling to me. Which could mean your glaze is too thick, as Pugaboo said. Also the other suggestions were spot on, I would try a thinner application first personally. :) I've had lots of issues with crawling. Yay me. ;) 

Share this post


Link to post
Share on other sites

Hey everyone - thanks for your suggestions! I can add some info which might help...

 

1. I did a test with this clay, slip and glaze - No problems. Actually, it was from the first time I made the platter and the rim cracked :( So I bisqued a 2" X 5" section, dipped it in glaze and fired. It was good, so I made the 2nd & current platter.

 

2. Someone suggested a hydrometer so I got one and have been testing the glaze every time I glaze for a month or so. It's been at 1400 every time. I know anything between 1400 and 1550 is good - ago does this indicates that it's possibly on the thin side - or thick side?? I was also cautioned not to use glaze that's too thin! My head is spinning!

 

3. This same problem occurred on a mug's rim that had a different color on it. rim was really rough - attaching photos.

 

Does this info & these questions change anyone's suggestions? And please explain about the damned hydrometer that comes with no instructions!

 

Thanks Tons!

post-78621-0-86597700-1472533672_thumb.jpeg

post-78621-0-75103600-1472533732_thumb.jpeg

post-78621-0-60153700-1472533754_thumb.jpeg

post-78621-0-86597700-1472533672_thumb.jpeg

post-78621-0-75103600-1472533732_thumb.jpeg

post-78621-0-60153700-1472533754_thumb.jpeg

Share this post


Link to post
Share on other sites

I think you may eed to add some frit to your coloured slip as it is not "melting" into the glaze.

I have had such a prob with some blues,

Your glaze seems fine on uncoloured areas just a prob with some colours?

Share this post


Link to post
Share on other sites

carolross, could it be you are adding too much mason stain?  I had that problem when I first started using stains.  My instructor was not a measuring sort of person, so we dumped some in.  The yellow bubbled and bubbled when fired.  So I learned to measure.  Just a thought. 

 

Roberta

Share this post


Link to post
Share on other sites

Roberta - it's absolutely possible that I've used too much stain. But... that wouldn't explain why the same thing happens with commercial underglazes, would it?

I'm wondering if holding the mug or platter upside down over the glaze bucket to drain might be collecting too much on the rim?

 

Babs, I think it could be crawling, maybe... I've had areas where the glaze AND underglaze are not adhered at all. They just lift off...

 

I think I've been fighting this for abt 4 years! I fired a blue vase, same slip as the platter, and it was great? There just doesn't seem to be one thing that explains it

Share this post


Link to post
Share on other sites

If this has been going on for a time, I am fairly certain you have looked at how you wipe off your bisque ware before glazing, but throwing that idea out there.  And you said you have changed clay bodies.  But have you changed clay companies?  I do have one brand that I simply struggle with.  Almost all of my glazes pit, bubble or have some issue.  As much as I like the clay to throw with, it's just no bueno for me. 

 

Roberta

Share this post


Link to post
Share on other sites

Babs - Thanks for the suggested changes to the slip.  From what you said, I could add a bit of clear glaze to it?  And yes, I was so angry when the platter came out of the kiln, I just started grinding it down.  You know - I'll show you, damned glaze!  I do soak at the top of the firing for 10 minutes.  But I'm spoiled now with an electronic kiln, so I don't know the rate.  If it's important, I can probably find out.  I set it to do a medium speed ^6 firing.  And, no, the platter was at least 2" from elements & I'm guessing there's at least 1" between mug & top shelf.  I've been trying to keep up with orders (from Etsy) and so sometimes fire with 1/2 empty kiln  :wacko:

 

Neil - I have also changed from commercial underglazes (Amaco & Mayco blues both did the same thing) and most recently I've tried a clay body slip to which I added 30% mason stain.  I think that's probably high, although it was suggested as an upper limit in something I read...  I want the color to remain strong - If I reduce it, will the color likely get pale?

 

Hi Roberta - I admit i don't always wipe the bisqueware because I usually unload the bisque firing and glaze it on the same day.  Do you think it's still necessary?  I've been using this particular clay for a long time, although I did briefly try another white stoneware from a different company.  

 

Okay - another question...  I'm thinking about doing a longer, slower bisque.  Would the out-gassing possibly take longer at the edges, by any crazy chance?  Any thoughts?  

 

Thanks so much for all your help!!

Share this post


Link to post
Share on other sites

Looking at the pictures, particularly the second one, it almost looks like deposits of something cracked or possibly crystallising around the edges of those little craters. Reminds me of how around the high flux corner on a Currie tile the glaze just crawls up the walls of the cell, and which often looks crazed due to the excessive flux content.

 

I am wondering if you have a solubility problem.

 

There could be some soluble material either in your clay itself, or introduced by you in your slips or other parts of your making process (even possibly a frit or stain) Soluble materials, eg the sodium in nepheline syenite, start as a solid, but eventually dissolve into the moisture of the liquids that they are exposed to. Once incorporated into the liquid, these soluble materials then migrate through the clay with the water as it moves around while the piece drys. Since the rims or outer edges of plates and other vessels dry first, the soluble-carrying water in the clay prefers to migrate there on its way to evaporation. Of course the water evaporates into the air, but it dumps its passenger - the soluble material - at the last stop, which is at the edge of your plate. This would explain why this problem only appears around the edges. As this process continues while you are working on (carving) your slowly drying piece, more and more soluble material builds up. When you fire your work, since this soluble material is a flux, it mixes with your glaze and makes it much more fluxy in that area of your pot. Glazes which are too fluxy bubble a lot when because they are effectively being over fired in that area. And voila!.... Bubbles in just one place.

 

Look at Digitalfire and see the DFAC test, which is about drying stresses, but can also reveal the presence of soluble materials in a clay. Egyptian Paste and other self glazing ceramics also come to mind, as they exploit this mechanism as part of the making process. If this is indeed happening, you may have seen yourself the occasional buildup of a light dust (almost looks like white mold) around the rims of your pieces, which seems easily brushed away. Actually these are small salt crystal structures, which are effectively the soluble material resolidifying. Those of us who recycle clay on plaster bats will surely have seen this.

 

If this is indeed your problem, you need to go through your raw materials and see which of them contains things which might become soluble over time. If (when?) you find them, try eliminating them and see if that helps.

 

Hope this helps you resolve your issues. Good luck!

Share this post


Link to post
Share on other sites

Could be Babs, but even so, what really needs to be explained is why these bubbles are appearing on the rim and not elsewhere. That is what I was focused on above.

 

Then again, I could be totally wrong...

Share this post


Link to post
Share on other sites

carolross, the amount of colorant seems VERY excessive.  you can talk to the technician at mason stain co and get some ideas from him.

 

i use lots of slips and have never used more than 10 percent of anything and that is pinks.  blue is a very strong colorant and you should be able to test how little to use by simply trying various percentages a little increase at a time.  since this happens on edges, make several test plates, just make discs with a round cookie cutter and push them into a shallow bowl. separate the clay from the bowl with saran wrap or a similar thing.

 

start with 1/2 % cobalt or mazerine blue stain or whatever colorant you are using.  increase the percentage by 1/2 each time until you get to the point the problem happens.  you will be surprised by how little makes the correct color.

Share this post


Link to post
Share on other sites

I want to update this thread - everyone has been so generous sharing ideas and possible fixes! I still have some tests to run, but I'm hopeful... First, I have to say - Curt, when I read your post about soluble materials, I was ready to hide under the bed! The phrase "when/if you isolate", well it made me really think about crawling under that bed! 😱 LOL I just stayed on my feet and kept working, though.

 

The last glaze firing I did revealed some interesting things... In the kiln, fired at medium speed to ^6, were a couple of mugs and a couple of small plates - made with the clay body I've been using, with Amaco underglazes and Amaco clear glaze. These pieces fired without any bubbles or residue along the edge. Also included was a blue slipped & sgaffito'd platter with Mayco glaze brushed on. And a 15" birdbath once-fired with carved Mayco Stroke & Coat glaze (I've had good results with this method but it does leave areas unglazed and isn't suitable for too many pieces). Also one small flower pot & saucer.

 

Here are the results:

1. the platter bubbled all around the rim. It had clay body slip and Mayco glaze, 3 coats brushed on (not the Amaco I've been using).

2. The birdbath bubbled in several places, not exactly on the rim, but where the rim curves into the bowl and I'd left a 1/2 gap between colors. This was the Stroke & Coat glaze which had been perfect till this firing. :((

3. The flower pot also developed bubbles at the upper rim - Mayco underglazes and Amaco clear glaze.

 

So, I'm absolutely not a science person ( could you guess?) but what I'm pretty sure that the Flint Hills clay body was the only thing these pieces had in common. So, back to you, Curt - I think I may have isolated the soluble material!! Everybody touch wood now, please! I have a couple bags of Laguna 609 and (would it be asking too much if, after you touch wood, you cross your fingers?) I'm going to start testing. The problem of course is that the pieces with bubbles seemed to be random - usually the platters and larger pieces, but also on mugs, plates, flower pots - all intermittently.

 

So I think it's a firing at a time to see if it was the clay - but after feeling so bummed out earlier this week, I'm once again optimistic about finding that solution! So, if you see huge flaws in my scientific method, be gentle, please, when you explain how I've proved nothing!😳 Thanks everyone! I'll report back later...

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.