[Clayart] white clay leaks.
hmurrow at efn.org
Mon Jul 12 14:50:23 UTC 2021
One of my teachers was David Stannard, wo grew up in China as the son of two Baptist doctors, He eventually returned in’78 for a conference on Porcelain, going into the mines there to collect samples of the materials that made up the original Sung dynasty porcelains, These were analyzed at the U of Alaska for there constituents. The original
main component was hydrothermally altered Rhyolite, which had about 4% KNa and was white. As the mine was worked over the centuries, the potters had to add kaolin to the mix, to compensate for the loss of plasticity as the mine got deeper, then feldspar to compensate for the loss of KNa. The original material was volcanic ash which had weathered and through the hydrothermal alteration lost half the 7% alkali it started with.
Knowing what the original materials' genesis was, enabled David to find similar deposits of altered Rhyolite in Oregon and Alaska, which he made beautiful pottery from, before dying 14 years ago. I mine one of those deposits near Steamboat, OR.
Cheers, Hank in Eugene, OR
> On Jul 12, 2021, at 2:30 AM, Ric Swenson <ricswenson0823 at hotmail.com> wrote:
> There is kaolin based high fire (>cone ten...to 15) It contains no iron or other colorant oxides and fires pure white. The word porcelain comes from the Portugese for a pure white shell found on the beaches in Portugal "the porcelana". When they first shipped porcelains from Jingdezhen, China in the 16th century, they marveled at how white it was and called it porcelana.It was translucent and had zero water absorbency. It was fired in large clay saggars in wood fired kilns so no ash could contaminate the glaze surface. When asked where it came from they answered by mispronouncing Chang'nan as China which became the common European names for China and china. Chang'nan was the original town name for what we now call Jingdezhen. During the reign of the emperor at that time the dynasty: JINGDE and the word for 'town'....Zhen, gave us Jingdezhen. Most of the millions of wares exported during that time were blue and white dinnerwares. The emperor loved the wares so much that he had the name of the town changed. It is described in a Chinese saying : "Jingdezhen porcelain is white as pure jade, thin as paper, sounds as a bell and bright as a mirror."
> Porcelain can now be made in many countries because of mining expertise. In 1004 AD, the folks in Gao Ling (where we get the name Kaolin now), near present day Jingdezhen, discovered a mountain of pure primary clay which is the basis of porcelain. Secondary clays "travel" and pick up impurities like iron.
> There is white stoneware which fires at moderately high temps....cone 5-10 usually have 1-2 % absorption, usually toothy and opaque... and talc clay bodies which fire white but at lower temps, have little vitrification and thus have water absorbency of 2-5 % or more...unless glazed... and can leak.
> Your mileage may vary.
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