I guess it was last summer I brought my kiln home from the gallery basement after finding things like it to be a bigger chore than I anticipated to have to run over there at times like 2 AM to make sure the kiln shut off properly.
I haven't done any firing since January in it, but I have several clay models that were finished months ago and longer that were on display at the gallery, of course I needed to bring them all back home after selling the building, but I had 2 sitting in the back of the car left to find a storage spot for and I decided to fire the one that happened to be just small enough to fit in the kiln.
I put it on at noon yesterday, it will take about 37 hours to complete the cycle, so it has a ways to go yet being its about 575º now.
Here it is setting in the kiln and the ramp schedule, I cut R1 down to about 7-1/2 hours since this model was done and air dry a year ago so I know it's dry that far completely through.
R1: 90º/hour to 195º, hold for 8-3/4 hours
R2: 60º/hour to 1200º
R3: 90º/hour to 1700º
R4: 80º/hour to 1950º
R5: 60º/hour to 2050º hold for 5 minutes.
The back, hollowed out to about 1" to 1-1/4" wall thickness. That's not a sag on the top, which is actually the bottom, it is curved that way.
I have the sculpture out of the kiln now, while it went through the process fine with no warping etc it came out like this, which is how another piece came out about a year ago that stumped everyone as to how it happened.
Same clay, same firing schedule, same kiln.
At first with the previous small model that looked like this I thought maybe having some other pieces in the kiln that had a small galvanized hangar wire in their backs might have outgassed zinc or something and caused it, or something in the mold- those were pressed clay in plaster molds, this piece today was not made in a mold it was directly modelled.
I suspect that I got a bad batch of clay back then, or a couple of bad boxes, because I've fired several other pressed pieces in the last year from the same type red clay from the same supplier, same firing schedule, and they look gorgeous.
It could be a contaminate in the clay or in the water used to mix it, it's almost like the residue you see with hard water, but that lime disolves off with most any acids or even vinegar, nothing removes this stuff.
Interesting thing is, this wierd coating is not just on the front surface it's also on the inside, all except for the very top of the cap which I seem to remember I had to do a fair amount of reshaping of when it was still pliable.
Also, on the back edges where the greenware was chipped a little, it clearly shows the correct red, so this white "wash" stuff which even acid and a wire brush does nothing to- appears to be just on the surface but it's definitely burned into the surface if even acid doesn't remove it.
The kiln is vented, it was the only piece in the kiln and I don't do glazes or decals, so I can't think of anything that could possibly contaminate the surface, this model never had a mold made of it so it wasn't even sprayed with lacquer to do that, it wasn't pressed in a plaster mold so there was no possibility of some residue from plaster or a release of any kind getting on it.
It almost looks like the color is "burned out" but this course red clay is good from cone 05 through cone 4 and I fire it at what would be about cone 0 if there was one, just a little under cone 1 or 2050º The cone 01 and cone 1 cones I used both tipped normally, and the Bartlett temperature display also jived with my separate pyrometer meter display at the final 2050º temperature.
This clay turns dark brown at cone 4, so it's not overfired.
It doesn't matter for this sculpture it's just a master to use for mold making, but it sure would be nice to figure out what the devil the cause is!
This is what the clay SHOULD fire up like, and this piece is even larger at about 24" tall:
have you tried posting on a forum specifically for pottery. Im pretty sure someone firing clay on a regular basis would be familiar with this
Going to try the supplier first Mantrid.
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