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:

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. In the "flux Limits for porcelain" thread: for cone 6 I recommend a minimum of 5.0% total flux weight.

Tom

This is an interesting topic. Sodium carbonate melts at 1,564 F and potassium carbonate at 1,636 F with both exhibiting significant levels of sublimation (turning from a solid into directly into vapor) just below these temperatures. As with the vaporization of lead oxide from leaded glazes, this loss is also a function of time. Lithium carbonate may be more temperature stable, melting and sublimating at 1,333 F but not boiling until 2,390 F.

http://www.sciencedirect.com/science/article/pii/S0040603198002895

Sodium, potassium and lithium carbonates vaporize before reaching a temperature where they lose the carbon dioxide and are reduced to oxides. These metal carbonates are very different from "high temperature fluxes" magnesium and calcium carbonates which reduce to carbon dioxide and the metal oxide when heated in a kiln. The calcium and magnesium oxides don't vaporize until well above kiln temperatures.

I suspect most people dealing with ceramics assume the calcium / magnesium model for these other metal carbonate fluxes.

Fortunately for Cone 6 firings, boron oxide does not sublimate or vaporize until 2,732 F. As a result I've found boron oxide an essential ingredient when creating a cone 6 porcelain body.

While phosphorous oxides sublimate at fairly low temperatures, calcium and iron phosphates do not, remaining a flux in porcelain and bone china.

The volatile carbonates of sodium, potassium and lithium reduce the temperature and calories of heat required to melt / bond silica and alumina, but they can only perform this function while they remain in the clay or glaze.

Although they can form compounds with alumina and silica, I don't know if they can act as a non-consumed catalyst in these reactions leaving a densified ceramic after the metal carbonates have boiled off. I would assume this is unlikely.

Norm: once again TY for the reply and info. These tests are all results of pin pointing the flux limits for a cone 6 porcelain body. They were raised incrementally from 20% (recipe), all the way up to 32% (recipe). I sent these findings to Ron, and he is digesting them as well. Actually his initial response was "very interesting." Which for Ron means he is more than curious about it.

For cone 6 porcelain bodies: I have found 4.35 to 4.50% total fluxes (molar) to produce the best vitrified body. I have long known that KnaO are both in a gaseous state in c6 firings. What I have problems explaining is the flux flow through the body. When molar levels hang around 4.0%, I see the mason/dixon line almost every time as shown in pic 4. There is a distinct line where vitrification ends, and un-vitrified begins.

I can see the gas vapors moving up my test cylinders when I look at them close up. I know there is relatively no pressure in an electric kiln: but there has to be a small amount inside the clay walls. However slight, I see evidence where it is being forced to the surface. I have yet to find accurate information about the total loss of KnaO from vaporization: although I know it is occurring. I have pictures of a fully vitrified body: and the clay/glaze interface. Just having a hard time explaining the mason/dixon line in pic 4: I have seen that alot in underfired and or under fluxed bodies. 

Clay/Glaze interface:

This is what a fully vitrified cone 6 piece is suppose to look like. The clay and glaze are interlocked at the face.

Tom

Norm: another example of flux movement through a clay body.

The yellow line is the edge of a test bar: below it is the side of the bar, and above it is the face of the clay. The red area is pointing to an un-vitrified body: full of pin holes and gas pockets. Below it is the green area, indicating a vitrified area. It was taken with a monochrome filter to help highlight these issues. In every example: vitrification moved from the bottom of the bar to the top ( in maturity). The last 1/16th inch of so of this bar is littered with gas pin holes: so that sorta speaks to how much is gassing off.

Tom

Sorry.. forgot the pic for the post below--- need lots more coffee.. lol

Tom - Without adding the flux / glass formers boric acid and aluminum phosphate to a porcelain body I've only seen full vitrification in cone 6 or cone 2 porcelain when the pieces are thick.  Now I know why, the thickness must trap enough of the carbonates of sodium, potassium or lithium long enough to react with the aluminum silicates.

In addition to the study I linked showing direct sublimation of sodium carbonate without decomposition into carbon dioxide and sodium oxide, a number of answers by chemists online have been quite assertive that decomposition of these three Group 1 "alkaline metal" carbonates cannot occur in an oxidizing atmosphere, unlike Group 2 "alkaline earth metal" carbonates like calcium and magnesium.

Yahoo - Does sodium carbonate have a decomp temp?

Incidentally, aluminum phosphate is "isoelectric" with SiO2 silica, (the exterior charges and shape are the same, in spite of having a different interior molecular structure.) so can directly replace a silica molecule in ceramic structures.

Norm:

Probably one of the most informative research docs I have read. The metes and bounds of feldspars are hashed through very well IMO. I also like they made distinctions between how sodium and potassium work in a clay body.

The Properties of Feldspars - PDF file

Tom

Norm:

Some technical info from the Brindley & Ougland researcg doc: "Quantitative Studies of High Temperature Reactions ofQuartz-Kaolin-Feldspar Mixtures." Trans. Brit. Ceramic society Soc. 61:599 (1962)

Cited and confirmed in Ceramic Science for the Potter  Lawrence & West 1982 (2nd Edition)

Also confirmed in the University of Illinois/ Campaign..Study below.

In direct relation to cone 6 firings- critical temps we need to be mindful of:

980C (1796F) Metakaolin changes to spinel with ejection of finely divided silica.

1050-1100C (1922-2012F) Spinel formed from metakaolin starts changing to mullite. The peak mullite conversion temps being 2012-2192F). Rapid decrease in feldspar and the appearance of the glassy phase.

1200C (2192F) All feldspar has melted and is no longer detected on X-ray diffraction. Porosity of the body decreases rapidly, and the pores of the body close tightly.

2282F-- cristabolite begins to form in high kaolin bodies. and 2192F for high ball clay bodies.

In reviewing all of this information, I question the long held firing schedules for cone 6 bodies; more so porcelain bodies. Very common for potters to ramp to 2180 +/- with a long extended hold for a cone 6 melt. However, that blows past the most productive range for development and maturity in a clay body. For several years I have been firing to 2100F, and drop to 120F an hour to 2230F with a 10 min. hold. Test samples shown, are the result of this firing schedule. After review, I am going to ramp to 2050F, and climb 125F to 2230F, with 10 min. hold.

I cannot help to think the rapid fire to 2180, with and extended hold is blowing past these critical development temps for clay: which results is an immature body. At 2180F, the critical formation period (temp) has been over-shot in a hi ramp schedule.  Just a thought: but the ramp I use also produces this result in porcelain:

A completely vitrified body: who needs glaze?  From my private porcelain formulation.

Tom

Another c6 porcelain body fired with the below schedule:

Tom

The hold at peak temperature on Cone 6 firings, we use 20 minutes, are generally intended to let glazes finish reacting and lay down prior to cooling. Until now I've not considered what effect each portion of the firing has on body maturity and densification.

Particularly making sculptural pieces with paper clay, the body never really matures, but is simply a porous scaffolding to hold glaze. Prior to glazing I sometimes dip the bisqued sculptures in a solution of water with soda ash, lithium carbonate and boric acid and let them dry prior to glazing because the paper clay he have access to is fluxed to cone 10 rather than Cone 6. The flux dip helps densify and add glass to the spongey body.

The process in making commercial bone china is changed around. The porcelain is fired to maturity with the use of plate sitters as the porcelain does not have the strength to maintain its shape once hot enough to vitrify. Once cool, the dry glaze powder is then electrically charged and sprayed onto the positively charged ware and fired to a much lower temperature. Embellishments are either painted or transferred prior to the dry glaze spray, or gilded on after the glaze firing.

The bone china recipes I've experimented with were exactly the same with a very narrow temperature difference between being:

A.) a partially vitrified self-supporting porcelain.

B.) a fully vitrified glass which slumps into a blob without the use of a refractory bowl setter.

Synthetic Calcium Phosphate is a powerful flux, having a higher phosphorous to calcium ratio than natural bone ash.

The porcelains we purchase from Laguna Clay support themselves as they never become as vitreous as bone china.

Norm:

I understand the peak hold, and why it is used. I am suggesting that instead of the fast ramp to 2180 +/_ commonly used with a hold: ramp to 2050F, drop to 125F an hour to 2225 / 30: then do the glaze hold. I think fast ramping up to 2180 with a hold is blowing by the crucial temps for body maturity. In part, why I put up this blog: because I believe most pin hole problems are caused by off-gassing in the clay: not the glaze. The only time off-gassing in a glaze would be a problem: would be high formula limits of KnaO.

Ramping to 2050F, with a slow cycle to 2225/30F gives the clay more time to develop mullite and a glassy body: which in turn would eject gases from the body. As the pics below demonstrate" clay can develop COE issues of its own if it is under-fluxed, and or under-fired. I would estimate that fast ramping to 2170 =/- would create an immature body: as your comments about paper clay suggest. Of course the only way to verify that is to test it with your paper clay.

All I have ever done is crystalline glaze on porcelain: started out in the pottery world doing it. Which explains why I am cross-eyed, irritable, moody ; and suffer from manic episodes. Thankfully, my big jar of lithium is nearby for emergency tantrums when I open the kiln... LOL

Tom

Looking at the pre-programmed "Medium-Speed" firing on our Cress, once the temperature rises above 1,100 F it ramps at rapid 400 F per hour until 250 F below the set-point then drops to a 120 F per hour ramp so the temperature doesn't overshoot.

So, probably not by intent to mature the body, it is slowly firing between roughly 1,939 to 2,189.

It's always been my impression that most clay body off-gassing occurs during the bisquing process which is why we fire to Cone 04.

While  most glazes foam as they melt, we have only rarely had a problem with a chemical interaction between a glaze and the clay body resulting in pin-holes. As an example, glazes containing fluorine frits like Ferro 5301 often interact badly with Laguna Clay Half and Half clay. Refiring them to Cone 6 again doesn't seem to help much, so there must be sort of a catalytic reaction where it continues as it reached deeper into the clay body.

There's always something new. I'm very glad to have taught others at our studio to maintain the kiln and make glazes, but we had a kiln problem over the past few weeks I'd never seen before. The kiln was increasingly err-ing out halting the firing with a warning that the electronics board had exceeded the maximum temperature, something like 165 F.

I had replaced the rusted ventilating grill on the top of the control box with a new heavy gauge punched stainless sheet, so I assumed this thicker metal must be conducting more heat from the side of the kiln. But upon opening the controller box to readjust the stainless plate, I discovered one of the stainless machine screw connections between power cable and the double-twist element end had obviously experienced huge amounts of heat - finally to the point where the stainless machine screw, washers and nut had melted-off.  So probably one of the two wires in the element end had broken turning the stainless screw connection in the control box into a heating element - which would quickly become a problem for the CPU board.

Rather than replace the element I temporarily crimped an aluminum O terminator onto the stub of the element wire and cut a new asbestos covered lead and connected it with new washers screw and nut.

If that jury-rigged fix survived the subsequent Cone 6 firing I'll be very surprised. That was two days ago and I won't know if it worked until Saturday when we go back to the studio. At least no one has reported a fire in our absence.

***************

Clamping that O-Ring onto the kiln element and attaching the 240 volt wire has continued to work well, in spite of my misgivings. I never would have believed it.

Tom, please thank your friend for putting you up to this.  This post, and subsequent comments, are fantastic.  Thanks to you and Norm for sharing your insights and hard work.  

YW Erik: some additional close ups of pin holes, as relative to clay off-gassing. This pin hole is only an 1/8th of an inch; the magnification makes it look much larger than it really is. Enough off gassing from the clay was present to literally push the glaze out of the site. If you notice, bare clay is visible in the center of the hole: so enough gas was escaping with enough force to push the glaze out.

In this example, the pin hole is less than a 1/16th in diameter. The curious thing about this close up; is the large dark area around the pin hole.  A small crater (raised area) around the hole again speaks to the amount of gas escaping. If you look at the 3rd picture in my original post: where there is a distinct color difference between the top and bottom of the test cylinder: you see that same color as in this pinhole. So the dark area is from off gassing of the KnaO in the clay body.

In both examples, the glaze is applied at 0.15-0.20 grams per square inch: ( a two second dip in the pottery world.) So the glaze represents a very thin veneer, which would exclude the possibility of off gassing due to a heavy glaze application. The industry norm is to suggest a longer hold at peak to heal glaze pin hole issues. My question: is that longer hold actually healing glaze, or is it simply maturing the clay body which results in no off gassing? Which is why I am suggesting that thought needs to be given to our ramp cycles. Currently they are intended to melt glaze: perhaps changing them to mature the clay would result in much less incidents of glaze issues.

Tom