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Stuart Altmann

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  1. Colouring Porcelain

    Reply by Stuart Altmann: Iron oxides can produce a surprising range of colors, depending just on firing temperature and kiln atmosphere during firing alone. That is because iron has five(?) oxides, each with its own color. For example, a common, simple iron glaze recipe from Penland--I'm on the road now and don't have the formula memorized--starts with Fe2O3 (common 'red rust' ) and gives either a dark tomato red or jet black. In cone 10-11 in electric oxidation firing, this gives a dark tomato red, with various amounts of irregular black splotches.
  2. I would be as beautiful as the most glorious vase from the Ming dynasty, so that those who get great pleasure from looking at me will wonder how the potter could have sculptured anything so beautiful. And I would be as functional as the hands of that potter, so that those fortunate enough to watch me working at my wheel will be in awe of the beautiful, fluid dance of my two hands as I make this vase.
  3. Bowl proportions.

    Of course, a mixing bowl could be tipped over just from the force of mixing, particularly if the bowl's content are fairly rigid. But you probably want to know when it will tip over spontaneously. An empty bowl or any other object sitting on a horizontal plane surface, like a table top, will fall over spontaneously whenever the vertical projection of its center of mass falls outside its base, i.e. outside its horizontal foot ring in the case of a bowl. This is the basic rule. For a mixing bowl, this scenario is complicated by two aspects of its contents at any given time. First, the material being mixed affects where the center of mass is. If the center of mass of the bowl's contents is lower than that of the empty bowl and the contents are sufficiently 'stiff' --like, say, mashed potatoes--so that they do not shift position appreciably when the bowl is tipped, then the contents lower the center of mass of the bowl+contents, so unless the stiff ingredients are piled up so much off-center that the vertical projection of the combined center of mass falls outside the foot ring, the bowl will not spontaneously tip. Conversely, if the contents are sufficiently fluid, they may flow around the mixing spoon rapidly enough that the shifted positions of the center of mass of the bowl-plus-contents never gets out beyond (vertically projected) the base. Thus, a potter can easily influence a bowl's "tipability" in two ways. First, keep the center of mass low. For example, have a wall that is thicker in its lower parts (walls and base )and that does not flare outward very much, relative to its height. Second, make the base both wide and thick/heavy. One other factor is sheer mass and density. When my sister wanted flower vases that would not easily tip over when she put her very tall orchids in them, I loaded my standard clay with silica, which is about the densest of the common materials in my clay and I used it to make vases with walls that, while straight inside and out, are thickest where they meet the thick, wide base. The vases are narrowest at the top, giving the flower stalks minimal leverage against the vases' walls.