Help with raw glaze bubbles

[tomhumf]

I've been working on a raw glaze recipe for application to leather hard pots. I'm firing to ^8 in a new gas kiln. The tests pictured had some reduction during firing. The clay is speckled stoneware. 

My recipe currently is :

Hyplas71 ball clay 30

quartz 15

calcium borate frit 15

wollastonite 15

Soda feldspar 30

total 105

It's looking ok, I just really don't like the bubbles where it's thick - see the top of handle attachment. 

Any ideas on getting rid of the bubbles? Maybe more frit or wollastonite? 

thanks! 

[tomhumf]

[tomhumf]

Bubbles are worst on the inside. I've just read it could be to do with firing schedule. Maybe I need to hold at lower temps before glaze is melting while gases are escaping from body. 

[Soren]

I just had a similar issue to this with a stock Laguna glaze at ^06 (see topic if interested).  I cannot say that it is an exact parallel (due to cone difference) but my bubbling issue went away almost completely when I raised firing temperature from ^06 to ^04 and allowed a ~20-minute soak period right after the kiln sitter switch dropped on my kiln.  Thicker application of glaze seems to increase the chance for bubbling to be noticeable after firing is complete.  Slower firing time may help in allowing time for the bubbles to be absorbed or released during the fluid stage of the glaze.
The bubbles got worse on the clay I stained with Manganese Dioxide, which apparently off-gasses even worse at ^04, causing bubbles in the glaze to become extreme rather than eradicated.  If off-gassing is your issue, I have no advice how to avoid the bubbles.

[neilestrick]

Are the bubbles sealed over, like they're down in the glaze, or are they open at the surface?

[Magnolia Mud Research]

My thoughts:
 Item 1:
there are two sources of gas for the bubbles: (1) from the open pores between the particles in the clay body, and (2) from gases being generated within the glaze as the glaze ingredients are forming a melt.  

 Item 2:
The glaze appears to be reasonably stiff, since the glaze line at the bottom of the mug is crisp without a 'jelly-roll.' 

 Item 3: 
The inside of the mug is most likely cooler by several degrees K than the outside of the mug unless the mug has been heated for a significant and has equilibrated.  
 
 Item 4: 
The bubbles are more present in the thicker regions implies that these are gases from item 1.2 above.  

 Item 5:
My strategy would be to increase the hold time at the peak temperature, or to raise the peak temperature a little bit.  The hold time would give the glaze time to 'de-gas', the higher temperature would lower the surface tension and viscosity of the glaze melt.  A bit of both would not be bad either.  

LT

 

[tomhumf]

Thanks all. I can see firing higher could help, but  already burning through more propane than I'd like so hopefully I can fix this with the glaze. 

The bubbles in question are all sealed over. There are a few tiny pinholes in other areas but I'm not too concerned about those, I'm not expecting a perfect finish with this body.

I would have thought increasing the fluxes and reducing quartz a little would create a more fluid glaze that would get rid of the bubbles. Problem is the CaO and B2O2 levels are already over the suggested limits according to insight live.

https://digitalfire.com/4sight/education/limit_formulas_and_target_formulas_206.html

Perhaps this is contributing to the problem. I'm thinking maybe another flux like talc or frit3110,  may be needed...mainly because I have those in stock.

[neilestrick]

Lots of good information HERE. The stiffness of the glaze is the likely culprit. It's just too stiff for the bubbles to migrate. Try lowering the clay content in 3% increments until it is fluid enough for the bubbles to pass.

[High Bridge Pottery]

There is a good thread here but after trying to reread it now it is not the easiest thing to follow what's going on with some of my posts. If I was to sum it up in one post it would be this one below from curt.

I think your glaze for me has too much silica and I would try switching to a potassium feldspar.

On 11/3/2015 at 4:52 AM, curt said:

Happy Melbourne Cup Day.  Like a dog with a bone, cant let the bubble thing go, not yet.  The usual dogged internet surfing has turned up some interesting explanations and causes of bubble formation, which I want to set out here, lest they all leave my brain as I sleep tonight...  I am hopelessly out of my depth scientifically speaking, but as my wife will attest that has never stopped me before.... 

 

Warning: If you don't care about how glaze melts and why bubbles form stop reading here!

 

The most interesting and useful information by far comes from our colleagues in the field of glass, who deal with many of the same materials and issues that we do in ceramics.  In particular I found Introduction to Glass Science and Technology by Shelby to be very helpful (turned up on Google books).

 

Bubbles come from lots of sources, including the ones we know well like the decomposition of different glaze ingredients, but the ones we should be worried about are carbon dioxide and sulphur products (sulphide and sulphate).  During glaze melting, bubbles form around grains of silica sand as they react with things like Lithium, Sodium and Potassium Oxides that we introduce through our glaze ingredients. 

 

As temperatures rise and the melt proceeds more and more silica melts and creates little localities in the glaze where the prevsiously happy bits of feldspar now start giving off carbon dioxide and other gases..  However, at the same time, more and more silica is making the melt more and more viscous!  For a while, many of these gases simply get absorbed (saturated) into the surrounding liquid and disappear.  However, when the concentration of a gas is too high to be any longer absorbed into the melt, it is said to be supersaturated, and, voila, bubbles of that gas start to form.  Carbon dioxide is the principle one that does this in silicate melts like our glazes.

 

Before you eyes slowly shut and you drift off before banging your head on the desk (oops did that already happen?), why does this matter?????  It matters because carbon dioxide bubbles will keep emerging out of the now supersaturated glaze liquid LONG AFTER all the initial glaze ingredients have dissolved.  There was me thinking oh, well, when LOI is over I am safe.  LOL, not by a long shot.  This strongly suggests we can stop worrying about the breakdown of glaze materials in the early stages of the firing, and start worrying much more about how much silica we have in the glaze and whether or not it is all melted yet....  And, it means that key time for bubble management is definitely at the end of the firing.

 

Oh, and don't forget "reboil."  That is where a previously-formed good mature bubble-free glass or glaze starts to generate bubbles where there were none, before due to higher and higher heat.  This is a problem particularly with sulphur contaminants in our glazes and clays, which undergo a 3-fold increase in their supersaturation potential between 1100 C and 1400 C.    Kind of makes you wonder how clean those glaze ingredients really are....  However, more to the point, lowering your temp for a hold wont just allow more time for big bubbles to pass out of the glaze.  It ALSO may cause some of the sulfter to be reabsorbed into the melt, ie, bubbles disappearing by themselves rather than having to be passed out of the glaze! 

 

As a related issue, particle size of your glaze ingredients also seems to matter.  And there is a "right size," because if particles are too large then decomposition persists into the latter stages of the firing creating glaze defects and bubbles,   But, if particles are too small, this increases the likelihood that glaze particles group together early and trap gasses.  However, my take is that most of us are erring on the size of glaze particles that are too large if anything.  Hence, ball milling seems to hold some promise....

 

Little bubbles (less than 0.4mm) are called "seed" in glass-land, and anything bigger is called a bubble.   This matters because seed is much more difficult to get rid of than bubbles, but mercifully much less visible as well.  The process of getting rid of seed and bubbles is called "fining".  It can take 50% longer to fine out small bubbles than large ones...  The bigger the bubbles are, the faster they pass.  Sometimes in glass-land they use specific chemical additives called fining agents which makes lots of big bubbles which collect the seed on their way to the surface of the melt. 

 

Whether or not bubbles pass out of a liquid depends on their size (bigger is better!), how viscous the melt is (this changes over time during a firing of course), how dense the melt liquid around the bubbles is (higher temps give less density, which is good for bubble passing but too high may also cause reboil, which is bad - what temp you hold at starting to come into focus now - maybe not the highest?) 

 

Want to melt all the bubbles out of your silica-based glaze by force??  Better get a better kiln because you will need to go to 2200 C to get commercial-grade bubble-free silica glass.  Silica melts very slowly. 

 

We tend to think of lithium having super-powered melting properties in a glaze, but research shows that whether you use potassium, sodium or lithium matters far less than the amount of whichever one you use.  Decreasing effect after 10% in glass-land and virtually no additional effect after 20% R2O.  This would seem to match up with what I have seen on currie tile experiments.

 

The glass forming temperature when you use calcium is about 200 C higher than if you used sodium, potassium or lithium due directly to the viscosity of the melt.   Of course eutectics probably mean a mix is even better.  All you whiting users whose glazes lean heavily on calcium be advised.

 

OK, will stop there and take a breather.

 

Conclusions: 

 

1.  The end of your firing is where you will have the biggest impact in getting rid of bubbles.

 

2.  Use the cleanest glaze ingredients you can get to reduce the chance of bubbles

 

3.  Use glaze ingredients with small particle sizes

 

4.  Holds:  definitely, because

 

a) The more viscous the melt, the longer the hold, the better.  And since the smaller bubbles are, the longer it takes them to pass (so if you have to have bubbles, hope for big ones, cause at least you can do something about them!)

 

and

 

if you lower the temp for your hold you have a good chance for bubbles (particularly sulphur ones) to be re-absorbed into the melt).

 

I found in interesting after hoovering this kind of stuff to go back and look at the bubble photos again.   Is it just me or is there lots of unmelted silica floating around in some of them....?

 

As ever, very interested in the reflections and practical experiences of forum readers on this stuff....

 

 

[Joe_L]

Another possible cause might be due to raw glazing as inadequate burnout of the body can cause out-gassing through the glaze as it melts, the firing should be slow in the 600-800C zone before the glaze melts. (If you're new to raw glazing it might be worth posting your firing profile). That said, it looks like only the thicker and inside areas have the problem and the rest of the glaze is ok so I agree with the others it's likely to be a problem in the glaze department.