Porcelain Clay/COE Issues

[Tom Anderson]

I have been testing porcelain bodies for nearly two years. Recently I purchased a 1600X lab scope to help me see what I had long suspected.

This first image was taken of the clay/glaze interface: the dark area is glaze (cobalt.) The body has a strong glassy matrix and is fully vitrified: so I had to look further.

In going back over my flux tests in clay bodies, I began to notice a common COE pattern in pieces that were under-fluxed, or under fired.

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.

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:

                                            < top of cylinder    >>> bottom of test cylinder

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. However, when you look through that same body on a cross section, the voids become more evident.

^^ top of bar   ( bar is upside down in this view.

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)

Friend asked me to post it.

Tom

[Tom Anderson]

The Kiln Song    (For Christmas)

The Kiln Song

Twas the night of the firing,

And all through the studio,

Crystals were growing,

Even though they are pseudo.

 

When up through the controller,

The temps they did climb,

Even though my old elements,

Laid over like dimes.

 

On copper, on cobalt, on ZNO,

For tomorrow after is the art show,

On titania, on mango, and SiO2,

No time for reduction on this load.

 

If only I had fired the day before,

With joy then would I open this door,

Even then a strike firing at best,

Alas, but this load will sell for less.

 

If only somehow the copper bled,

Giving me one piece turned to red,

Then with joy I would affix,

Thrice the price for best pick.

[Norm Stuart]

Tom - Six years ago someone bought some boxes of Cone 02 porcelain, a short-lived product from Laguna.

If fired to Cone 6 to mature a glaze, the porcelain never created pinholes but instead would bloat - creating puffy areas in the body which is a certain sign of a clay taken beyond its firing range. I can't imagine what that was fluxed with.

[Tom Anderson]

 Norm, a follow up to your post, where you stated:

"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."

1922-2012F ( 1050-1100C) is when metakaolin starts changing to mullite: which coincides with the feldspar beginning to melt. At 2192 (1200C), all the feldspar is completely melted and no longer visible in X-ray diffraction. These are critical temperatures for a cone 6 firing: and blowing through them creates problems in the clay body. (my theory).  At 1800-1880F, the usual bisque temps, the silica has changed from alpha to beta quartz and is high expanded and very porous. The carbons have burnt off, perhaps that is what you were referring to?  A slip of the font?... done it myself many times.

If we fast ramp to 2180F with and extended hold: we create a problem by trapping gas in the body. The vitrification begins around 1950-2025F range as the feldspar begins to melt. The outer walls of the body have absorbed enough heat: but the interior has not. Vitrification also means compaction: the porous body created by expanded silica begins to close up.

There is a general tendency to view the ramp cycle only in regards to glaze melt. Certainly the melt would start earlier because it is directly exposed to heat: and would take far less time to absorb through a thin layer. Clay however is a 1/4 to 3/8" thick slab, which requires much more time for that heat to penetrate the inner core. As the test cylinder above confirms: the bottom half is fully vitrified, and there is a very distinct line where that ended.  So again, for these very reasons I believe the vast majority of pin holes are created from immature clay still off gassing.

The other issue then becomes porcelain verses stoneware at cone 6 temps. Porcelain body flux is primarily a blend of potassium and sodium (KnaO), most of the time heavy on sodium. Stoneware bodies tend to be very heavy on the potassium side. At cone 6, potassium is an off gassing cow, much more so than sodium. If you follow the stats: pin hole problems at cone 6 stoneware are much more prevelant thanpin hole problems in porcelain at the same temps.

Tom

[Tom Anderson]

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

[Erik Evans]

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.  

[Norm Stuart]

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 Anderson]

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

[Norm Stuart]

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.

[Tom Anderson]

Another c6 porcelain body fired with the below schedule:

Tom

[Tom Anderson]

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