Electric Mid&Hi Fired Ceramics
Porcelain Clay/COE Issues
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.
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
Oct 6, 2016
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.
Oct 6, 2016
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.
Dec 17, 2016