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Posts I've Made
Posted 20 May 2013I just saw stuff when I was over in Yixing, China that blew me away as to the technical execution..... amazing. Many of which I pondered, "How is this done"?
What if the whole bowl was a very thin bone china, fired to maturity in the ususal slump mold. Then over the translucent body a resist stencil of the pattern was put on and then a opaque white glaze fired to a lower temperature was applied over the resist. So the body only is showing through the area that the stencil protected.
Really...... haven't got a clue. Amazing feat no matter HOW it was done.
Posted 20 May 2013Jim,
I don't know if any of the earthenware suppliers for handcraft type potters are doing this, but an industrial trick to get the glaze on earthenware to go into slight compression (to prevent any crazing) is to deliberately add some of the cristoboalite form of silica into the body formulation. Because it has a very high COE... it helps the body shrink slightly MORE than the glaze (which without lead is typically dominated by hiogh COE alkaline fluxes) and keeps it from crazing. So if the glazing application is uniform and covers all of the clay........ no leaks.
This does not stop the absorbtion of moisture through unglazed areas like the rings of feet. Ot into things like pinholes and other such glaze defects. So microwave use after getting them wet is still potentially an issue.
Posted 20 May 2013I'm with Mea's thought train on tracking this issue down. If the change is sudden.... look for thngs that have changed related to the kiln and the building since the time that it did not happen.
Local pickup kiln vents do not deal with the heat energy transfered to the room air from the exchange with the hot surface of the exterior of the kiln. They are designed to get oxygen into the chamner to complete reactions and to pick up noxious fumes and get them out of the kiln room.
While most electric kiln lids do have the issue discussed.... they trypically have this happen slowly over time. Not as a sudden thing. (BTW........ this issue can easily be fixed by routing out the pivot holes for the lid hinge to elongate them so that the back side lifts as the kiln expands.)
If you (or someone else) fixed the vent system....... is it possible that the fan is now running "the wrong way"? Meaning that rather than pulling fumes (and hot air) OUT of the kiln, it is now pumping fresh air INTO the kiln.... and hence causing the positive pressure created inside the kiln to cause the hot kiln gases to vent into the room thru the cracks in the kiln and the peep holes?
Or when the vent was "fixed".... maybe the connections to the output ducting (leading outdoors) after the fan is not tight.... or was forgotten to hook up again? Then the gases pulled from the kliln are getting partially or fully dumped into the room.
Have you chacked the outside termination of the vent duct? Did a bird or bees or rodent build a nest in there?
Or maybe did you add insulation to the room that the kiln is in since it was last fired?
Or maybe did you do some other weatherproofing to the room.... and cut down the naturla air turnover in the space?
Something drastic like this does not just "happen" naturally without a significant cause. Something is up.
Posted 19 May 2013OK
now i feel like a 2nd grader in a calculus class. i will go to my corner and sit.
Body formulations with nothing supplying additional sources of fluxing materials to cause more of the SiO2 in the clay components (both as trace admixtures and from the kaolin/metakolin silica ejection) to convert to a glassy phase. Depending on the firing range, if you don't see an addition of feldspar, neph. sy., frit, sometimes spodumene, or the like in the formulation..... there there is the possibility of the issues I mentioned.
That (in red) is the key phrase in there..... any of the recipes that don't have those materials listed are the ones I am talking about.
Does that help?
Posted 19 May 2013John, would you consider the iron content in some of the above clays an adequate flux if firing reduction?
The iron in the reduced FeO state is an active flux on silica (SiO2), for sure, and it begins its action at quite low temperatures. So yes, it will add some fluxing of the available silica and keep that silica from potentialy converting to the chrstobolite phase form. Unfortunately, iron fluxed silica glass is very brittle... so the body's glassy phase in that case is not a very optimal one to use to "glue things together".
Plus the iron/silica melt has a low melting temperature and also a low volitilization temperature... possibly accounting for some additional bloating issues.
A particulary bad combo is a somewhat high iron body with inadequate sourcing of other fluxes. You end up with a brittle body that is getting "loaded" upon uneven cooling (or reheating in food service) by excessive COE factor from the cristobolite... and BANG..... dunting.
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- July 19
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