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Coe Study / Clay


glazenerd

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In going back over my flux tests in clay bodies, I began to notice a common COE pattern. So I decided to take a "closer" look. While COE issues caused by clay/glaze differentials are well documented: I wonder if COE issues caused by immature clay has been studied?

* all examples shown are under-fired / immature porcelain bodies.

 

Linear stress cracks

  PIC #1

 

I began noticing these linear stress cracks in nearly all of the under-fluxed/ or under-fired samples So I decided to run a sample that was half glazed, half unglazed sample; that was under-fired by 2 cones (estimated). The above picture is from the unglazed section of that bar, and the picture below is from the glazed area of the same bar. I used a monochrome filter to highlight the stress crack: hard to see in picture without it.

 

Clay stress under glaze

  PIC #2
 
A ultra pure clear glaze was applied at 0.15 grams per square inch, ( 1 second dip or so in the pottery world). Notice the crack follows a linear pattern which is normally associated with glaze COE issues.
 
I have since learned from extensive testing that the flux in the clay bodies moved from the bottom (shelf contact) to the top of the bars. I made a series of test cylinders in various thicknesses: 3/8 to 1" to follow that flux flow through a body. Then a series of hollowed out cylinders to replicate an ovoid piece cavity walls. In every test: flux moved from the bottom to the top, or from the center to the outside of ovoid forms. As this example shows:
 

Potassium flux test

  PIC #3    This image may appear upside down on some computers. *white glassy area should be on the top of the photo.

 

Sodium and potassium are in a gaseous state from cones 5-10; so simple physics explains the flux pushing upward or outward through the clay wall. You will notice the bottom of the sample shown is mature, while the top is not fully mature (some glassy matrix) but many voids.Should read: the flux moved from the bottom of the sample to the top: making the top mature faster. (white glassy matrix) However, when you look through that same body on a cross section, the voids become more evident.

 

bottom ( shelf contact

immature clay

  PIC#4   top
 
** I have to get a specimen holder--on my list.  Notice in this immature specimen, those linear stress cracks on the surface in the first two pictures, are likewise just below the surface.
 
An under-fired clay body can result in some COE issues of its own: regardless of how much care was taken to match the COE of the clay and glaze. The linear expansion shown in the second picture which was glazed: is not visible to the naked eye. However, they are prevalent all over this sample: only one was shown. ( my microscope has a very limited area of magnification)
 
Conclusion: Those doing functional ware should take extra precautions to ensure their clay bodies are fully mature. This study in my opinion in part explains why COE issues appear on pieces that are subjected to use. Firing a cone 10 body to cone 6 for example would magnify this problem, increasing the risk of COE issues later, and possible shivering of the glaze.
 
I will be continuing this study for months to come and adding to this thread when new results are ready. In addition, I am always logging flux data, maturity cycles: which should give me a precise guide to formulate firing cycles to ensure a mature clay body.
 
Nerd

 

 

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To further illustrate COE issues of under-fired clay. This body formula is the simple 50% kaolin, 25% silica, and 25% Nep Sy. A very common blend fired from cone 5 up to cone 10. This test bar was fired to cone 6:

 

 

COE Bar edge

 

Again using monochrome filters to highlight the stress crack caused by immature (under-fired) clay. This perticular example shows a U shaped stress crack due to the lack of flux. It begins as a large crack on the left, goes down the side of the test bar, and reappears as a smaller crack on the right. (follow the arrows). This piece would be a prime suspect for glaze shivering.

 

Note: 25% Nep Sy recipe weight. 3.78% alkali weight, 3.70% alkali molar. This is a commonly used cone 6 formula: not nearly enough to produce a vitrified body.

 

Nerd

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Ah glazenerd... you are truly a glaze nerd.

 

Very interesting experiment. I never realized that there was that much a difference from the top to the bottom of a piece. I guess I never looked that close.

 

Keep up the good work.

 

PS you also might want to post this over at cone 6...  http://cone6pots.ning.com/...this is what they like to discuss.

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Callie:

 

My stoneware study will start sometime at the end of October/ or so. Iron (Fe) has to be factored in as a flux in reduction, and/or high fire bodies. Just off the top of my head: particle size should (not there yet) play the big role in porosity of stoneware. Stoneware also has a bit less silica, and more alumina: which also effects melt: although the alumina + particle size is what gives stoneware its strength. The pitfall in stoneware is the molar flux levels: nearly 20% less than porcelain: which gives stoneware its natural clay look, and porcelain its smooth/white look. I do suspect however that stoneware could be blended with more molar flux, without altering the properties potters love about stoneware. Stoneware relies more on potassium than sodium in most cases.  Although potters like the "iron spots," that is just a simple addition of magnetite: so it is not necessary to blend around iron or magnesium other than for color. You could do that to porcelain if you wanted to: perhaps I should :)

 

I am currently collecting (proven) stoneware clay recipes: anyone who has them can pass them along in PM. I can assure you they will go no further, and deleted once I am done building a database.

 

Nerd

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I figured it out, the clay biz stuck the 9 on upside down. Should read cone 9-10.

 

Is 3.5-4% flux acceptable for a cone 9-10 body?

 

Not sure I am following along with the unfired and glassy matrix causing COE issues. Isn't that what clay is, a crystaly glassy mess? Melted and raw bits.

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Dirt: short answer-- I am a Nerd!!

 

Bob: yes I am truly a Nerd. I like the title, own the title, even have T-shirts with the title. Yes, shortly I will do that. Although I should let Tony do his own work...LOL

 

Nerd

 

 

No my friend, you are a dedicated researcher. ;) Makes me wonder if that "new wheel" money would have been better spent on a mass spectrometer. :lol:

 

 

I have some research experience and totally get where you're at.

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Joel said: Is 3.5-4% flux acceptable for a cone 9-10 body?

For porcelain that is the industry norm, and works pending firing schedule.

Not sure I am following along with the unfired and glassy matrix causing COE issues. Isn't that what clay is, a crystaly glassy mess? Melted and raw bits.

In pic #4 above, where it shows a mix of vitrified, and non vitrified- I will use an analogy. If you laid alternating strips of wood and steel together: then smacked it with a hammer: how the vibration moves through the steel and the wood is different. The steel would spread the vibration, the wood would absorb it. in clay, the vitrified transfers the heat, the raw area absorbs it: which creates an unequal contraction rate= equals COE issues.

Molarity measures saturation: or PPM (parts per million/ billions in our case) So as the available flux melts, and moves through the clay body: at some point saturation is complete: leaving no available flux to accomplish a complete melt/ matrix.As my tests have demonstrated and confirm: flux moves from the bottom up, or from the inside to the outside of the body. When the available flux becomes depleted: that means it is always the face (or right below) that is the weakest part of the piece. The face of the clay needs to be the strongest: because that is where the glaze is contracting: magnifying the problem.

>>> see picture/explanation below>>>>

Dirt said:No my friend, you are a dedicated researcher.

I have been coming to the conclusion that research seems to be my thing. I enjoy it more than any other aspect of clay.

 

Nerd

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  • 10 months later...

The contents removed from the porcelain survey thread to here, for a more accurate conversations on the topic.

"Cording" is being used as a general definition because the pottery dictionary of terms does not include this oddity.

 

As a frame of reference: this is what actual COE crazing issues look like under a microscope.

Crazing 200X

 

In this oddity case, the lines that appear on this piece: (Post 83 in the Porcelain Survey thread)

C6 Almond Porcelain

 

Look like this under a microscope

Cording 01

 
They do not break the glaze surface for the most part:

Cording 02

 
I believe them to be composed of titanium, that somehow ??? reacted to the clay??? I have no real answer at this point. Santa is bringing me a new 2500X microscope for Christmas, will take another look at these later.
 
Nerd
 

Cording 03

 

 

Microcrystals that formed all throughout this piece refracted light in such a way, making these cording lines impossible to see with the naked eye.

 

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I have since learned from extensive testing that the flux in the clay bodies moved from the bottom (shelf contact) to the top of the bars. I made a series of test cylinders in various thicknesses: 3/8 to 1" to follow that flux flow through a body. Then a series of hollowed out cylinders to replicate an ovoid piece cavity walls. In every test: flux moved from the bottom to the top, or from the center to the outside of ovoid forms. As this example shows:
 
 
  PIC #3    <top      bottom>

 

Sodium and potassium are in a gaseous state from cones 5-10; so simple physics explains the flux pushing upward or outward through the clay wall. You will notice the bottom of the sample shown is mature, while the top is not fully mature (some glassy matrix) but many voids.

 

 

 

If the flux were moving from the bottom to the top, shouldn't the top be at least as mature as the bottom?

 

Can you really be sure that the differences you're seeing aren't just due to differences in temperature? See the first picture in this post on digitalfire for an example where the base is less mature than the sides due to the shelf being cooler.

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Pieter:

Thanks for pointing this out, never realized my picture was upside down. Not exactly sure how that happened. Yes, I am fairly certain, that was one of several tests I ran on the topic.

Nerd

 

Something is wrong here: when I tap the picture, it displays on edge and in my gallery it shows it in the correct orientation. How do I fix this?

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Not going to be able to fix it DH, pic was on my laptop that crashed. Guess I will just put a edit note up there.

 

Have one final COE experiment on the table and results within a week. I have three test cylinders that run from 5.50 up to 7.18 in COE values: interested to see exactly how much spread there is between clay and glaze before crazing occurs. Still will not be exact, but it will be tighter than the usual generalizations.

 

Nerd

 

Edit note: now that is really interesting. On my Ipad the picture is upside down, on my table top (now) it is right side up.

Pieter: check my edit that you pointed out in your post. Read right now? Picture oriented correctly?

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On my computer, pic #3 shows the lighter area on top. However, in pic #4, where the voids are at the bottom, you've labelled "bottom (shelf contact " above the picture and "top" below it. Should this be the other way round?

 

Also, is pic #3 a cross-section, or a view of the top edge?

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