Bubble Bubble Toil And Trouble

[High Bridge Pottery]

I have removed the second row and second to last row from the currie ml chart The top row numbers still match and now the sides are duplicates of that.. The tiles are not even testing what he sets out to test as his results wouldn't lock the si/al along the axis. Here are the four recipes for that tile. The other two tiles are 5 line blends instead of 4 mixed glazes. I think the tile is more squished than actually losing 30% of the data.

 

[David Woodin]

I have removed the second row and second to last row from the currie ml chart The top row numbers still match and now the sides are duplicates of that.. The tiles are not even testing what he sets out to test as his results wouldn't lock the si/al along the axis. Here are the four recipes for that tile. The other two tiles are 5 line blends instead of 4 mixed glazes. I think the tile is more squished than actually losing 30% of the data.

 

I can see why you would want to change the grid, because the Currie grid produces a lot of tiles way out of any limits.  Another way to get more meanfull results is to pick a limit that is at the cone you are interested in.  For example Matrix at cone 6, than A= AL .56, Si 1.9  B= AL .56 Si 4.2, C= AL .21 SI 1.9, D= AL.21 Si 4.2 or Matrix at cone 10 A=AL .7 Si 2.63, B=AL .7 Si 5, C=AL.27 Si 2.63 D= AL.27 Si 5.

David

[curt]

I have removed the second row and second to last row from the currie ml chart The top row numbers still match and now the sides are duplicates of that.. The tiles are not even testing what he sets out to test as his results wouldn't lock the si/al along the axis. Here are the four recipes for that tile. The other two tiles are 5 line blends instead of 4 mixed glazes. I think the tile is more squished than actually losing 30% of the data.

 

 

OK, starting to get the picture.  Not what I guessed you were doing   so glad I asked.  

 

So your Row 1 is a Currie Row 1, and your Row 5 is a Currie Row 7.  And your rows 2, 3 and 4 are the same as Currie's 3, 4 and 5.  Is that right? 

 

Not sure I understand what you mean when you say "the sides are duplicates."  Do you mean that the cells along each edge of the tile correspond directly to the cells on a standard currie tile?

 

Looking this morning at Currie's books Stoneware Glazes and Revealing Glazes  it appears that the basis for the shape of his grid (and why it is taller than it is wide) is related to the idea that the magic 8:1 ratio between silica and alumina is the middle of the road in producing shiny, "matured" glazes.   And, further he wanted to see the effect of 100% flux in corner C moving all way up through the shiny zone to corner B (what he calls the gradient), where, at only 35% flux you are really not going to get a good melt at all much beyond this.   And cell 18 or thereabouts is the sweet spot for many glazes as far as a desirable flux-alumina-silica blend is concerned. 

 

If I understand you correctly, your modification to the standard currie grid still lets you see those extreme corners, but probably saves some time and material by focusing more on that sweet spot around cell 18.  Does that sound right?

 

Assuming that I have understood what you are doing, I am wondering why you have Kaolin in all 4 corners.  In a standard currie tile corner C and D would have no Kaolin.   Is that deliberate?

 

 

Dave, I don't think Currie tiles are about producing functional glazes per se.  The Currie technique is more about exploring the impact of systematically changing alumina and silica for a given set of fluxes.  Some glazes on any given currie tile are likely to be functional, but many will not be, but could be just as desirable as decorative-only glazes.  That is part of the attractiveness of his technique.  Whether or not one sticks to "food-safe" limits is a different issue (if that is the kind of limits you are referring to?).  The various sets of those limits I have seen seem to vary in eyebrow raising ways as to how much of which materials is acceptable, and the differences are big enough that I see them as guidelines rather than absolutes. 

 

Anyway, of course there is nothing sacrosanct about the standard currie grid.  Even currie himself comfortably discussed "non-standard" currie grids.  His format can be changed in many ways to suit the purpose at hand, but at least one thing sacrificed would seem to be comparability of results across different sets, and subsequent interpretation of results becoming more difficult.  

[High Bridge Pottery]

Yes that is right. I meant they are duplicates in that it blends-
24 | 0 16 | 8 12 | 12 8 | 16 0 | 24 across the top and bottom line and

 

24

0

 

16
8

12
12

8
16

0
24
Down the sides, so after taking out 2nd + 6th they now copy each other. I also halved  all the values to use 24ml instead of 48.

I think the tile still lets me see exactly the same results but obviously misses out a few data points in the middle. If you want to focus more on the sweet spot then that is about changing the four glazes not how you blend them.

The kaolin is deliberate. It took some clever manipulation to only increase alumina when you go up a square and only increase silica going right one square. Still not perfect.

I have come to see the method as you can pick whatever four glazes you want with chemistry you want to blend together and graph the glazes between those chosen values. I have just shrunk my graph paper.

[curt]

I found these coffee cups out of a recent wood firing to be quite instructive as I pondered where bubbles come from and what causes them so I thought I would share them. 

 

These cups were fired in an Olsen Fastfire to something like cone 10, cant be sure exactly the temp due to temperature differentials around the kiln, but these were in a place in the kiln that we know to be one of the hotter spots.  Medium reduction, but plenty enough for the iron in the clay body of these cups to be active, as evidenced by the warped rims.  This figures into the story below I think.

 

The clay body is a red porcelain I am working on.  These cups are thrown fairly thin on the walls, but with a bit more weight in the foot for balance.  The outside of these cups is unglazed, save for a modest amount of salt from small pots set around the kiln when loading, and of course the ash from the wood fire (eucalyptus).  The inside of the cups is glazed with a celadon glaze poured in and swished around just long enough to cover, but in the end applied somewhat thicker than usual for various unintended reasons.

 

What interests me about these cups is where the bubbles appear to be and where they appear not to be.

 

 

Now you can begin to see the celadon liner glaze.  It looks to have run down the walls nice and fluid-like and settled in the bottom of the cup.  On the walls of these cups there are no bubbles visible to the naked eye. 

 

However, there are bubbles in the bottom of the cup, clearly visible even in these photos from this distance. 

 

 

OK, now for some closeups. 

 

First the walls of the cups.   Bubbles here for sure, even though you cannot see them without magnification.

 

First picture is 200x magnification with a digital microscope. 

 

 

and now 800x.  If you look closely the same cluster of bubbles is evident in both photos near the center just to the left of a crazing line.

 

 

Now for the bottom of the cup, first 200x magnified.

 

 

 

And now 800x magnified.

 

 

 

 

 Interestingly, the bubbles in the bottom seem to quickly disappear near the outer edges, even though you can tell from the darker color that the glaze is still several times thicker there than it is on the walls of the cup.  Where the bubbles thin out seems to correspond to where the thickness associated with the feet of these cups underneath ends, but that is just a guess. 

 

Tentative conclusions, in no particular order, and not necessarily consistent with one another  : 

 

1.  If a bubble forms in a glaze, but no one can see it, is it really there????

 

2.  I have strong suspicions that the iron in this clay body is adding some additional fluxing power which is helping this glaze flow and pass bubbles (and craze, ooops, however I think we can fix that).  However, the same iron seems to be increasing the frequency of bubbles where the glaze is thick (compared to cups where the clay body has a lower iron content - I made a few bodies and varied the iron). 

 

3.  Bubbles seem to be much more prevalent - and larger - where the clay body is thicker.  Maybe this is due to more offgassing where clay is thicker.  Or, possibly this has something to do with heatwork in those areas as compared to other (thinner) parts of the pot.  Or both?

 

4.  These cups add more evidence that thicker the glaze, the more visible the bubbles will be, simply because there will be more of them, and they may quite possibly be larger bubbles than where the glaze is thinner.

 

5.  Revisit conclusion 1.

 

 

I have tried to base these conclusions on what I can see from these cups, but I am open to other interpretations.

[curt]

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....

[High Bridge Pottery]

Great research there Curt and it all pretty much adds up with what I have been seeing. Bubbles appearing from nowhere! Are you saying that in the melting of silica and a flux there is gas produced or in that process gas is absorbed into the melt from the atmosphere? or that there is impurities everywhere in the glaze and these release gas?

Going to have to reread your post a few times for it to sink in, and read that book.

 

It is interesting after what you have said that I am thinking maybe a glaze that does melt early and trap lots of gas could end up with less bubbles in the end because it is more fluid and in the melted stage for longer with more bubbles to make big easy bubbles to rid the glaze from. hmmm...

 

Looking back at all my glaze tiles, anything that has melted has bubbles in the surface no matter how much silica or alumina. 

 

Hard to say  but I don't think it is unmelted silica, probably some kiln of feldspars forming in the melt.

[curt]

Yes, yes, and yes.  

 

At lower temperatures plenty of carbon dioxide is present in the silica melt, but due to the presence of alkalies, it is in the form of various carbonates which are soluble and don't make bubbles.  However, as the temperature rises the localities around melting silica grains becomes silica rich, which (don't ask me how) converts the soluble carbonates to much less soluble carbon dioxide.  Once the melt become supersaturated with carbon dioxide, bubbles form.   That (the supersaturation) is why the bubbles continue to appear so long after all the carbonates in the glaze materials have broken down.  

 

And yes, gas can also be absorbed from the kiln atmosphere, and some of this gas could be coming from kiln refractories (shelves, props, bricks, etc,) breaking down and their pores opening up to release gases.

 

I agree that all else equal a more fluid glaze is probably better for getting rid of bubbles.  The small problem with this is getting that fluid glaze to stay on the pot.  Look at the celadon glaze on my red porcelain cups above.  Yes, very fluid and no bubbles visible where it was allowed to run.  If I had glazed the outside of those cups it would have been all over the shelf.  Oh, and there is the other small problem of crazing when flux levels are too high, as corner C of any currie tile always demonstrates, and so did my cups.

 

Yes, not sure what those snowflake colored bits are in back of the bubbles in the pictures... could be feldspar but I guess I supposed feldspars would already be well melted at that point,   The most refractory thing by far in a glaze is alumina....could it be?  Back to the books....

 

.

[Tim T]

Just to add to the discussion, I've been working on a low fire (1120C) lead glaze on earthenware with bubble problems.

Initially I did a Currie tile, and C had sparse large bubbles (perhaps 2-3mm diameter), but as you moved away from there the bubbles quickly became much smaller and numerous, so much so that it made the glaze milky white. The glaze was applied quite thickly.

From then, other things I have tried to date, with no difference to the bubbles, are:

- higher firing temp and/or longer soak to let the bubbles disperse

- lower temp to see if the bubbles are being formed late in the firing

- the flux has small quantities of Gillespie Borate, whiting and feldspar in so I've switched these to non-gassing versions using frits and/or Wollastonite, and also tried removing each one. This should have made a difference, but none visible

- adjusted the glaze thickness - a thinner glaze definitely gives fewer bubbles, but is less glossy and doesn't cover as smoothly

- in case the bubbles came from the clay body, I've tried different earthenware and stoneware bodies, and also done a normal (1000) and a high (1100) bisque, with both a normal firing (4 hrs to 600 then max to top temp then soak 15 mins) and a slow firing (150/hr to max temp), with no difference

 

Next things to try are increasing the amount of flux beyond the initial formulation to get a more fluid glaze, and also trying a finer silica in the glaze that will melt earlier.

If those don't work, I've no idea what next!

[curt]

Initially I did a Currie tile, and C had sparse large bubbles (perhaps 2-3mm diameter), but as you moved away from there the bubbles quickly became much smaller and numerous, so much so that it made the glaze milky white. The glaze was applied quite thickly.

Makes sense since Corner C is 100% flux in a standard currie tile.  Moving away from C in any direction is a more refractory mix with either more silica or more alumina or both, so more seed bubbles seem almost inevitable.  My gut reaction is fire higher, but there are of course practical limits on that journey....

 

But I am going to do a bubble hunt on some of my currie tiles now that you have said this, and will report back with any substantial findings...

 

Thick is good in my book, exactly because is pulls back the covers on the kind of issues you are discussing here. 

[High Bridge Pottery]

Thank you for sharing your research Tim Seems like you are in the same area as me, started with the obvious removing LOI and trying a few different firing curves with no avail. Interesting that the same is happening be it low fire or high fire. I will be interested to know what happens using a finer mesh silica.
 
Here are a few more of my tests. I made up these glazes to try and have a few different alumina values with similar(ish) flux and not have any added silica or kaolin.
 

 
Glaze A is actually bubble free to the eye, you can't see any, glaze B has many bubbles and glaze C has the odd one or two. C is the most runny with A pretty close and B far away from the two but it does have much more alumina and silica.
 
I fired 150c/hour to 1140 then 60c/hour to 1260 with 15 hold. From there I did a fall, 60c/hour to 1200 with a 30 min hold.
 

[High Bridge Pottery]

I then added, 2, 4, 8, 16 and 32% extra of silica or frit to each glaze. Increasing silica increases the bubble count which seems to match up with what has been said. Adding the frit increased fluidity and also helped the bubbles escape. I think maybe a slower fall to 1200 or a smaller fall with longer hold could get the last few bubbles out. These were all about 3g balls of glaze. Need to run the tests again but don't want to make all these tiles again   
 
First shot is a little out of focus, my bad.
 

[curt]

Outstanding work, Joel.  This thread is punching way above its weight in terms of information per line!

 

Just to be sure, the first line is frit and the second line is silica for all three series? 

 

Also, these were all fired in oxidation, right? 

 

Also, to get a sense of scale, how big is the physical size of the test tiles?

 

On the edge of my seat to hear any other conclusions from this run of tests...

[curt]

The forums once again deliver the fruit, as you have come at this from a completely different direction than I would have.  I get the general thrust of what you were tyring to do, but I am curious how you decided to try these three specific recipes if you have a moment to comment.   Did you have any epiphanies about the alumina when reviewing the results?

 

Given that your pictures have some clear lessons on bubbles, my immediate reaction is to extend this learning by exploring how to make these into viable glaze recipes from a limits point of view (in this case Cone 8 - 10 leadless traditional).  I have punched your recipes into Insight (had to hunt for formula for BPS Calcium Borate Frit and create a new material but no biggie - my goodness it has some boron!) to do some further analysis.

 

From your pictures, Recipe 1 seems to hold the most promise in this regard.  For cone 8 - 10, you have just enough alumina according to the limits, but are a bit low on silica.  If I put in 16 silica this gets you just up to the lower limit for silica.   Interestingly, we can see from your pictures that that is just about the point where Glaze A starts to general some bubbles...  Also from your picture, the glaze melt for 16 silica has a considerably smaller footprint on the tile than 8 silica, suggesting that this is also about the point where you start to get a glaze that does not run right off the pot when applied to vertical surfaces.  If I could magically zoom in I would be very curious how this glaze behaves in 1% increments above and below 16%.  On a standard currie grid, my modified version of your Glaze A that would put it somewhere around cells 23 or 28, and normally one might be worried about crazing in that (relatively high flux) area, but you don't seem to be getting that with Glaze A which is good news.

 

Glaze B really brings the problems with excessive silica into the spotlight.  As you pointed out, even your starting Glaze B has lots of bubbles, and proably more than your glaze A with 16 silica added, do you agree?.  While the recipe has no added silica, your pictures suggest Glaze B still needs to dump some silica, probably by reducing the Cornwall Stone in favour of adding some of your frit, or maybe another flux source.  Calcium and Boron and a bit of lithium are main flux drivers here, and adding maybe some potash feldspar would be the obvious option.  Thermal expansion is not too bad, so maybe some potash or sodium wouldn't hurt...  Anyway, this glaze and even the whole series you did are underscoring for me the idea that one needs to get the silica down as low as possible, probably stopping just short of getting crazed glazes (revisit currie tiles to see where that point is I think....)

 

Glaze C looks like a giant calcium party!    At flux unity you are .82 (and almost 22% molar!) on the calcium, a whopping great number I think, being blasted in by the wollastonite, whiting, and frit in that order.  Even the Cornish stone is adding a bit of calcium lol.   I can not add enough silica to this glaze to get it into the limits without ending up well on the right hand side (ie, very white) of your picture. Looking at your picture, with this particular mix of fluxes the silica seems to be waving the white flag at around 8% and has well and truly left the building by 16%     This series/picture is very instructive for me, as it reminds me of the bottom row of some of my currie tiles as silica increases.  Making me think that silica and calcium have a definite line in the sand past which they do not work well together.  

 

Conclusions I draw from your latest test series.

 

1.  Get silica and alumina down as low as possible, and add as much flux as possible, to reduce (and maybe even eliminate?!?!?!) bubbles.

 

2.  The big difference between glaze A and the others is potash (it has much more).  Wondering if potash has extra good effect when working with higher silica amounts....

 

3.  There is an effective upper limit to how much calcium you can have in a high silica glaze...

 

4.  Glazes B and C, which have the most bubble issues, seem to have very high Cornish Stone in common

[High Bridge Pottery]

Yes first line is the frit and second is the silica in all three series. Oxidation fired and the tiles are 2-3 inch or about the size of half a credit card.

 

I had a few rules I wanted to hit when making up these glazes. Have a few different values of alumina so I can see it's effect with viscosity, not contain magnesium oxide or zinc oxide and have no added silica or alumina. I wanted no zinc so it would happily go in reduction and I have been reading about chrome-tin pink needing no magnesium to work well. That's a colour I would like to go for with this glaze.

 

Glaze A I wanted a good mix of sodium an potassium and they both seem to have the same alumina in each so I added both along with wollastonite and frit till I had a chemisty that looked ok. 

Glaze B was me trying to duplicate the chemistry but get more alumina and silica in from the feldspar so used cornish stone instead.

Glaze C was a slight wild card with really high calcium and frit to keep the silica and alumina real low.

 

Glaze C on it's own is the most fluid but has visible bubbles where glaze A, less fluid, lost all visible bubbles.

 

As I was adding silica anyway I thought it was ok to have mismatching silica values. Don't know if it is an epiphanie but interesting that A has more silica and alumina but less bubbles, making me wonder if there is a sweet spot for the silica/alumina melt to saturate with or desaturate quickest.

 

I notice that too, around the 8-16 silica for A there are many more bubbles but that is where limit formulas tell me the minimum silica should be. Looking at the frit if I also added 16 frit it could counteract the silica. It would also give me a bit too much boron. I think I will have to run some kind of durability test and see how it holds up to acids/bases and such.

I think the crazing is controlled with the lithium and high calcium, be good to do boil,freeze tests too. So many tests....

 

I agree that glaze B has far more bubbles than A16 silica. I think this is part down to silica but also the alumina. Down at the C tiles adding silica matts out the glaze but doesn't add to many bubbles.

When I was working with the cornwall stone it was obvious it is made up with some clay to achieve the old chemistry. Glaze A was by far the hardest to work with as I had to ram press it into pills, the others I could ball up after 5 seconds on a plaster batt.

Might try this run again but with a few more alumina values and all my feldspars.

 

Interesting your thoughts about the potash feldspar, I hadn't even noticed glaze A was highest in potassium. Could that be helping the bubbles or is it the Al/Si values. Probably both

 

Be good to try the run again with single feldspars and mixes to see if the cornish stone is causing bubbles or not. I think it probably isn't the material itself but it is in the chemistry that it brings to the glass.

 

I hope this makes sense as I am not great with explaining myself and writing it down succinctly first go. Really happy with these results, I think with a bit more work on the firing cycle a lot more of these could have been bubble free. Some were so close, I think a longer drop to 1200 would do it or maybe drop to 1220 and an hour hold. I say bubble free, glaze A still has bubbles under the microscope but there is no way you can see them with mk1 eyeball.

[curt]

Great clarification and observations.

I see why you chose these three, and it makes it more interesting that at least two of them are intended to be real glazes as opposed to testing vehicles.

10% more alumina in Glaze B than Glaze A does sound like a meaningful difference and agree that it is a candidate to account for the differences between these two. I also just noticed something that when you add 16 silica to Glaze A, its chemistry suddenly looks very similar to Glaze B (!). With that in mind interesting to look again at those two tiles in your tests (Glaze A Silica16 and Glaze B   Seems that more potash and less sodium, along with a bit less alumina and a bit more boron make for a less bubbly glaze. I know many potters who hate soda spar and love potash spar for reasons that have never been quite clear. And, is it the extra boron that is counteracting the extra alumina? That would seems to prove up your viscosity suspicions?...

In Glaze C as you point out, there is a lot less silica and alumina. The corollary is that there is ALOT more flux. In fact I would venture that the amount of flux is surplus to requirements to melt the silica and alumina, hence the calcium borate opalescence and probably the lack of bubbles due to overfluxing. I guess I am not sure how comparable the silica additions in Glaze C are to those of A and B. Even with 16 silica added Glaze C is still short of being inside the limits, at 18 silica I just get over the line on silica but would still need to add a bit more cornwall stone to get the alumina to minimum limits. So not sure whether the lack of bubbles in C is due to alumina or silica, it seems a bit hidden behind all that flux.

Interesting observation about the clay in Cornwall Stone. I continue to think clay is a prime suspect in offgassing, and if you are right B and C would tend to corroborate that. It also means that testing the cornwall stone directly would have some value, particularly if you are planning to use a lot of it...

Again, awesome work and really enjoying the analysis and discussion. Got virtually nothing done in my workshop today but feel like it has been a day well spent nonetheless! I spent a lot of time looking at currie and other test tiles with the microscope, but the pillow calls so will report on this shortly.

[curt]

OK, just when I thought the bone was bare I found few more pockets of meat to chew on....

 

For anyone who has followed the thread thus far I strongly recommend a re-read of Hamer and Hamer under the headings Viscosity and Surface Tension, and if you are really keen Valency (because that provides the underpinning for the first two).  Joel's tests and results make even more sense after this.

 

Here are some salient points I got out of those passages:

 

Turns out Potassium is indeed a magic flux, which kind of explains why Joel's first series of glaze tiles above is probably the best at handling increasing silica additions.  Due to its mono-valency,  Potassium has elite status as the most fluid flux of them all, even more fluid than boron at stoneware temperatures.  It also holds the crown for lowest surface tension of any flux, which means it is going to melt down better (and crawl less) than any other flux you can add.  So pound for pound it is going to produce a more fluid, better spreading glaze, and almost certainly pass bubbles better due to these features.

 

At the other end of the spectrum is alumina oxide, the stiffest, most viscous oxide of them all.  Its complex molecular structure and high valency make it very viscous and also give is the highest surface tension of any oxide available to our glazes, so it is going to severely inhibit the passing of bubbles.  Joel, this tends to support your intuition on keeping alumina levels low in your test glazes, thereby keeping it out of the way of what the silica additions were going to do. 

 

Silica is the benchmark by which all other oxides are judged on their viscosity and surface tension, but let it be said that on the spectrum of stiffness, silica is ALOT closer to alumina than it is to potassium.  So as has been said elsewhere in this thread, minimizing silica and alumina will, all else equal, make a glaze more fluid and better able to pass bubbles.

 

Regarding the calcium overload in the third series, it turns out that as a flux, calcium is relatively viscous and relatively high surface tension.  It is also the kind of flux which, when overloaded, will make glazes stiffer (yes, increases viscosity!) rather than more fluid.   This probably helps explain why the right hand side of the calcium silica series is actually looking stiffer, even though it really has too little silica and alumina to be a proper stable glaze.  

[High Bridge Pottery]

I could never find hamer and hamer, is it a certain book? That's how people seemed to reference it. I actually feel like this thread has nearly helped me rid visible bubbles from a glaze   hopefully everybody else can learn too.

 

Going to be a good idea to test out just my potash feldspar and see what happens there. I will snap a few better comparison shots of A + B glazes that have similar chemistry. Doing this run of tests has really started me thinking about why the silica limits are set. I have fired to cone9 but the best bubble free glaze is at the bottom of cone6 limits for silica and oversupplied with flux.

 

Is there something I am missing, if the silica is pushed up to the bottom limits then the glaze has so many bubbles and probably unmelted/melting silica. Is it time that I am missing, is my small electric kiln too fast for the silica to melt and bubbles to disappear. Maybe I do need to ball mill my silica and produce the finest powder known to man.

 

I really need to start doing useful tests on the glass to see how durable it is with lower silica.

[bciskepottery]

I could never find hamer and hamer, is it a certain book? That's how people seemed to reference it.

 

 The Potter's Dictionary of Materials and Techniques by Frank and Janet Hamer.  Fifth Edition is the latest, I believe.  A & C Black Publishers, London. 

[High Bridge Pottery]

Ok, not got any money to spend on books right now 

 

Here is A16 and B. I would say bubble density is about the same but a slight difference in A has more bubbles breaking the surface and B they are mostly contained within the surface. It sure seems like silica is making the bubbles.

 

 

[Min]

Ok, not got any money to spend on books right now 

 

I can scan the pages Curt is referring to and email them to you if you like. (just PM me your email)

[curt]

Cool, Joel, thanks for reposting A 16 and B.  I also later looked again at A32 and B16, which in a similar way are also comparable (that is, A 32 has a not dissimilar amount of silica to B16).  If we step back and look at the complete series, it is easy to see B Silica as just an continuation/extension of A silica.  Looking at it this way triple underscores the lesson on silica and viscosity.  Ignoring for a second the potassium difference, the only real (and pretty major) difference in the chemistry of A and B is silica!

 

On a mole% basis, comparing the two glazes, B has about 10% more silica than A.  However, as a percentage of each whole glaze recipe, about 6% more of Glaze B is made up of silica than in the case of Glaze A.  May not sound like a lot, but it pays to remember that for every additional 1% of silica we wanted to get in the recipe we had to sacrifice 1% of something else.  In this case, since we weren't changing alumina that meant we were giving up fluxes 1 for 1 to get in more silica.  Now, since all the fluxes combined were only about 30% of the entire recipe for Glaze A, giving away 6 of those percentage points of flux for Glaze B amounts to an effective 20% reduction in the size of the flux package!  So Glaze B not only makes the road steeper with more silica to melt, but it simultaneously takes away some of the flux engine's power to climb that melting slope.  

 

OK, maybe not the clearest way to put it, but picture a line blend for any glaze where all you do from left to right is reduce flux and substitute in silica, and you get the picture. 

 

Anyway, where I was heading with this was to agree with you Joel that we need to be HIGHLY sensitive to amount of silica in our glazes.  As your test series demonstrate, each additional mole% of percent of silica requires massive additional flux firepower to digest effectively, otherwise it just doesn't melt, looks hazy, leaves bubbles, dimples, etc, etc.  Time, firing speed, top temp reached, soaks, etc will all help, but they are probably all much less effective at getting a good glaze outcome than just simply getting the recipe fine tuned correctly in the first place.

 

Contemplating this is effectively a revolution on my thinking about glaze composition, and particularly AMOUNTS of silica and WHICH fluxes and how much of them I use, because it is now pretty clear that all fluxes were not born equal in their fluxing power. 

 

I agree Joel, that the limjts have a lot to answer for.   I think the reality for most of us is that if you don't really know (from a ceramic engineering perspective) what constitutes a strong, safe functional glaze, the limits are the only guidance available, and all we do is end up discussing which set of limits we will agree to follow.   However, cant help thinking there has got to be a better, smarter way...

[Tim T]

Although a slight tangent to the discussion here, one thing I have been experimenting with a bit to make glaze tests a bit more rational is to take the Currie tile method as the basis (so I can use the same test pieces), but instead of varying Si and Al to vary the flux composition and hold Al and Si fixed. So in making up the test glazes you'd keep Si and Al constant so the ration of flux to these 2 is constant, but then in the flux you would (say) take K2O and Na2O from low to high along the 2 axes, and this also varying the rest of the flux so it totals 1. If they went from 0 to 0.1 each then you'd have as the 4 corners: K2O 0, Na2O 0, rest 1; K2O 0.1, Na2O 0.1, rest 0.8; and K2O or Na2O 0.1, the other 0, and rest 0.9  This seems quite a good approach in having a single test tile showing the effect of varying aspects of the flux, complementing the traditional Currie approach.

[curt]

Hi Tim, sounds interesting, can you post a photo?   So if I understand, the C corner is a much-trimmed down flux package which will in effect overemphasize minor flux ingredients, but up in the B corner you are getting back to a balanced flux package.  Does that sound right? 

 

I guess this presupposes that the silica and alumina amounts are fundamentally correct, something that I am now much more suspicious about with my own glazes.  Also, as a practical issue, how are you keeping the Al and Si constant across the grid, since many fluxes also have one or both of these? 

[High Bridge Pottery]

I did a tile similar to what you are talking about. It would be hard to vary KNaO and keep silica/alumina the same I think. Take some messing about with the values. It seems to be a constant dance between all the oxides that we are trying to choreograph.

 

I can't remember the exact details right now so might update later. Zinc along and Talc up the way.