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Diagnosing Pin Holes Problems.


glazenerd

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600x of a pin hole less than 1/8th" in normal view. You can clearly see the clay at the bottom of the hole. So this pin hole is caused by off gassing.of the clay.

 

Clay pinhole

 
 
600x of pin hole caused by glaze immaturity. (notice immaturity). The hole is less than 1/16th inch in normal view. You can see a dark splotch around the hole: immature/ partially melted perhaps?
 

pinholeglaze

 

 

 

Open topic on pin holes: join in.

 

Nerd

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Oh nerd now you've gone and done it. You do know your new toy is dangerous right? Looking at every piece of clay and glaze with it means we shall have to send in the nerd rescue team when we find you babbling incoherently about a 1/16th of 16th a 1/16th of a 1/16th. Just kidding!

 

That is amazing! What details you can see! I bet you are going to learn so much. I am enjoying and learning all kinds of stuff myself through your Nerd-I-ness, so keep up the good work.

 

I am still in awe of your glaze crystals and can hardly wait to see what you think up next.

 

T

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we find you babbling incoherently

Been doing that for the last two years--- too late!!

 

Just trying to prove a long held belief: clay and glaze should not have that many variables. Pin holes are commonly viewed as glaze formulation issues primarily: one issue I have long disagreed with. Glaze is a very thin layer, and it would take a very short time for the fluxes to off gas. In those instances that it is truly a glaze defect: the answer is to do a peak to "heal" the glaze. So does that extended peak heal, or does it simply complete an immature melt? The dark splotch is in the glaze, not the clay below.

 

Nerd

 

Suspect I will be here less in the near future. Suppose to be semi retired, and find myself working more than ever.

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The answer is also simple: an ill conceived formula that exceeds formula limits. It would also likewise heal/mature; given enough time at peak, or a slow cool from peak: they both increase heat work done.

 

The recent history of glazes is also telling in regards to glaze formulation. Ron Roy, John Britt, and a few others all started putting out books in the early 90's on glaze formulation and formula limits. I can tell when someone post a recipe in the forum if it was done before or after that period. Just as I can tell who uses strict Seger unity because you see recipes given in 33.15 or 57.12 etc. They are adjusting to the 100th decimal place trying to reconcile molecular unity. Just as I can tell recipes pre 80's because of 70-80 recipe % of Nep Sy, Custer, or G-200. Any recipe with that much potassium or sodium is going to burp gas for some time. The formula limits placed on KnaO (combined sodium and potassium levels) are done so in part to prevent pin holing due to excessive off gassing. Anytime I see a recipe with more than 50% Nep Sy, Custer, G200, or any other feldspar/potash I immediately check it: often it violates limits.

 

Nerd

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I think pin holes are a complex issue. Might want to check out Digitalfire on this issue, and Hamer and Hamer for good measure.

 

Below is an excerpt from a great thread we had last year called " Bubble Bubble Toil and Trouble". It was about the causes of bubbles in glaze, but since pinholes are so closely related to bubbles I think it is relevant here. The people in glass-land (where most of this information comes from) have done a lot of work on this issue, because even one pinhole is too many for them...

 

"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

 

B) 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....

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How much of this silica melting and bubbles do you think applies to clay bodies? If it is happening in glazes it must also sort of be happening in clay just to a different ratio.

 

Pinholes are complicated  :blink:

 

I have a few more bubble tests to run from seeing what looks like unmelted silica way back. There is some Imsil A25 waiting to be melted but with all the studio troubles and moving around I am yet to get the test through a kiln and see if swapping for a much finer grade silica changes anything.

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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.

Agreed- which comes back to formula limits. Some of the older recipes floating around having 70-80% feldspar-will be a spewing Louie. (You had to be at my graduation party in 74 to know what Louie looked like. )

 

 

Kind of makes you wonder how clean those glaze ingredients really are....

The diapers off my nieces kid after he eats beans is cleaner...

 

 

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,

Imsil A-25 rocks!!!  I said rocks,, I feel "cool and hip" now. Ultra pure and ultra fine... and it does make a difference. The pics of the two pinholes are the only two pinholes I have ever gotten since using it: and that was solely due to glaze experimentation.

 

 

but research shows that whether you use potassium, sodium or lithium matters far less than the amount of whichever one you use.

Amen brother!

 

 

Conclusions:

 

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

 

2. Use the cleanest glaze ingredients you can get to reduce the chance of bubbles   I have gotten a forum reputation for being "Mr/ Clean" when it comes to glaze ingredients: but it extends far beyond just crystalline: better ingredients make a better pizza.

 

3. Use glaze ingredients with small particle sizes  Imsil A-25

Curt : I concur-- we express ourselves differently at times, but have reached the same conclusions.

 

 

Joel said-How much of this silica melting and bubbles do you think applies to clay bodies? If it is happening in glazes it must also sort of be happening in clay just to a different ratio.

A lot more than most realize. Point I was trying to make at the beginning: the immediate response to pin holes is most usually a glaze problem. After doing much research: I find it to be a clay problem often.

 

Dick >>> more info for you

 

Joel, in doing my flux limit/s testing on porcelain (which would also effect any clay) I have found fluxes in clay can be very problematic for several reasons. In America, clay issues on many levels are the fault of the potter, not the maker/s. 1. Potters often buy the cheapest clay, but yet expect BMW level results: not going to happen. Clay quality has grades just like any other products. Clay makers rely heavily on Nep SY, because it is cheap, and are moving toward imported blended fluxes because they are cheap.

Sodium and Potassium are the primary fluxes in all commercial bodies: and most in house bodies as well. Which I will repeat myself for the 50th time: both are in a gaseous state at c5-10. So they build in the interior walls of a piece, and move outward. Simple physics: pressure on the inside of the wall pushed the gas to the outside of the wall. When testing formula limits, rather I purposely under/over fluxed a body: the available  flux pushed to the surface/s. That also means that decomposing gas was pushing to the surface as well. Back to clay body formulation sorry: because it is a competitive market: consumer demands produces consumer costs, which in turn dictates formulation more than specific uses.

(Here we go Dick) I have fired most c6 and c10 porcelains available to me locally (not all nationally). In my testing, clays with a c6-10 claim is an absolute farce. A c10 clay by formulation (porcelain primarily) must have less flux and silica to avoid dunting and bloating issues: which means when fired to cone 6 they do not come close to maturity. (absorption over 10% tested). C6 clays should have a minimum of 10% more fluxes than c10 to reach maturity. However, that formulation criteria also creates another problem: off gassing. If  a forum survey was done: I would expect to see way more pin hole problems at c6 than c10.why? Because at c6 sodium and potassium are at the (beginning) of their peak melt temp, and at c10: they are at the (end) of their peak melt temp. Reason c10 pieces are stronger than c6 firings... right Mark? As Curt aptly pointed out: c6 firings almost require an extended hold ramp at peak just to produce a mature clay body: irrespective of the glaze problems/issues.

 

Nerd

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Here is a picture of the bottom half of a porcelain test cylinder. (Cone 10 recipe)

 

C10 msture

 

Beautiful!!  All glassy and shiny- who needs a glaze?

 

Here is the top half of the same test cylinder (Cone 10 recipe)

 

<top        bottom>

 

Oh yeah, forgot I only fired it to cone 6.. The flux raised from the bottom (right side) to the top (left side): bottom is mature, top is not. To the point of making a distinct line. Notice the little voids in the top (immature) ?.......potassium/sodium gas being pushed upwards to the top.

 

Nerd

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Yes, agree Nerd with the general thrust of your comments.  Crystalline glazing is a hard task master and we are all benefitting from the fact that that is where you chose to start (I think) your ceramic journey.  

 

A good potter friend of mine with an active business in high quality studio ceramics (much wiser and more experienced than me, probably a bit in Mark's category) likes to remind me we are in the silica business, not the clay business.  I stopped laughing when I looked at the chemical composition of my fired pots.  As we dive deeper in to the porcelain rabbit hole, we get closer and closer to pure silica glass.  In fact we could dispense with all this fluffing about if we just cranked our kilns up to 2200 C (4000 F).  OK, there might be some forming issues and other practical challenges :lol: but we could just load the boat on that rockin' Imsil A25 and head for the mountains.  The ultimate in self glazing ceramics!  Bubbles and pinholes be gone!

 

Meanwhile, back in the real world, evidence of impurities is everywhere in our clay and glazes, and further upstream in almost every one of our ingredients (except for Nerd's maybe). If we accept that as a starting point, then the challenge is how to get them out of our clay and glazes before they get too baked in and create flaws like pihholing. 

 

I think this is where we need to get very particular about the material composition of both our clays and glazes, particularly regarding the temperature of where EACH of those materials melt, both on their own, and in potentially eutectic combinations, and how this is impacting the presence of impurities discussed above.

 

For example, in some glazes, some materials melt early, even on their own (eg, a high boron frit).  If they are prevalent enough, these early melting materials suck other nearby materials into their melt process, accelerating the overall melt dynamic, and this then cascades into a more general overall melt.  If this whole cascading melt process happens before the various impurity-carry materials in your clay or glazes have started to decompose, off-gas and generally spew out their CO2 and sulphur related impurities, that means this second-stage impurity-releasing decomposition happens when the glaze is already liquid, or near liquid.  

 

And then those impurities, rather than leave freely as gases, have to be forced out of a liquid structure as bubbles, which then need to heal over, etc.  Some never escape, and get frozen in the matrix.  Some just break on the surface and form pinholes. Industry uses antimony and other fining agents, and I guess that is one way to go.  

 

However, I guess what I am arguing here is that unless your materials are already as clean as Nerd's or the glass people (the best solution), it would be good to keep the glaze and clay UNMELTED for as long as possible, to facilitate impurities exiting.  You would probably like them to all melt suddenly together nicely near the end of the firing process.

 

It is also one of the reasons I personally tend to avoid frits in my high-fire glazes if I can, even though I acknowledge that they often make a glaze look and feel better.  Frit 3134 is already running like water at bisque temperatures.  Always a compromise somewhere I this game.

 

As an interesting exercise, sit down and go through the list of materials in your favourite glaze and make a note of where each one decomposes and or melts.  And once you do that, if the laundry is still not done, start looking for eutectics between pairs and groups of those materials.  Does anything jump out at you?  I think what we need to ask is: Is there some advantage to having one of the materials in my glaze melting 100s of degrees before any other?  This is a genuine question from me, because there may be.  Or should we adjust chemistry to source the right mix of fluxes from other materials that behave more similarly?

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Nerd said "(Here we go Dick)...In my testing, clays with a c6-10 claim is an absolute farce..." You are preaching to the choir, my friend. I can take of myself on this matter by choosing an appropriate body for what I need, but I simply can't seem to get it through to other potters and studio operators who would rather believe a vendor's absurd claims that 'oh no, this ^6-10 body is perfectly functional and food safe at ^6. See..., it says so right there on the box...' Buuulll (well, let's not go there, this is a family-friendly place, eh?) But even if I can't cause a change out there, I am totally fascinated by your new pictures. The picture truly is worth 1000 words.

 

dw

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