|Monthly Tech-Tip |
In pottery, porcelain and ceramics the body-glaze interface must be well developed for functional ware. This bond is often a matter of consequence at higher temperatures but at lower ones it must not be taken for granted.
To do this procedure bisque fire a small tile of the body being tested (about 60 sq cm), apply the glaze at normal thickness, and then lay a line of 8mm square x 1/8" thick porcelain tiles on it. These tiles must be highly vitrified to be as strong as possible. We make the tiles using a 3D printed template, the process creates rounded tops and convex undersides (so they only contact the glaze around the edges). Black-stained porcelain is best for spotting subtleties in the breaking patterns. After firing to glaze temperature break them off with a hammer (the small size of the tiles is what enables break off).
The consistency of the test will be vindicated if they all break in the same pattern. A good bond is indicated if the failure does not occur at the glaze interface. When chunks of the underlying tile are taken and some remnants of the black porcelain tiles are left at the edges a good glaze-body bond is also indicated. Obviously, if repeated impacts are needed to break the tiles off this is also a positive indicator.
This is L4410P, a low fire white burning dolomite body. It has a high porosity and there was some doubt about whether it could develop an interface with glazes at cone 05. To do this test we bisque fired a small tile of the white body to cone 04 and applied two coats of Spectrum 700 clear glaze. Then we laid these 8mm square porcelain tiles on, they have a convex underside so only contact around the edges. After firing to cone 05 we broke a couple off using a metal tool. As you can see, the impact broke out chunks of the body on one side and some of the porcelain tile has been left on the other. The difficulty of breaking them off and the appearance of the break are good indicators that this transparent glaze is body well with this body.
We make these using a porcelain containing about 5% Mason 6666 stain. Originally they were made as pixels for making ceramic QRCodes, but they proved valuable for also doing glaze bonding tests. We fire them to vitrify well, this intensifies the black color. The 3D printed template stamps 9x9mm tiny square tiles. We roll the porcelain to 1/8" thickness for stamping. Pressing the cutter down into the clay rounds the top edges and creates a concave underside. An overnight drying provides enough shrinkage that they will drop out of the mold. Before drop-out we press them all further in, this trims excess around the bottom edges.
|By Tony Hansen|
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