Why plates have a ridge on the base...

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Kero Rethwallen

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Why plates have a ridge on the base...
« on: 12/07/2009 00:30:02 »
Kero Rethwallen  asked the Naked Scientists:
I listen to your podcast while at work, and absolutely adore it. 

I was listening to some older podcasts, and noticed a slight error on one of your question answer bits.  In the show from 15.06.08, "Fire and Mud", the question was asked why plates have the ridge on the bottom.  The answer given was that it was to allow the plate to sit correctly, or to not slip on a slick surface.

Actually, this ridge is called the foot of the plate, and is there because of how the firing process of ceramic pieces works.  When a ceramic item is glazed (glaze is the shiny glass stuff that makes plates food-safe and easy to clean as well as gives them colour in many cases), it must be fired at a very high temperature.  Glaze is basically powdered glass with minerals mixed in for colour and effect.  The high temperature the ceramic piece is fired at melts the glaze, making it into the continuous covering that we see on the finished piece. 

If a piece has glaze on it where it touches the kiln shelf then when it cools the glaze bonds the piece permanently to the kiln shelf.  The only way to remove it would be to chisel or grind it off.  So when a ceramic piece is created, a thin unglazed ridge or other unglazed rest is created.  The shelf touches only the unglazed portion of this foot, and the piece can be removed without ruining it.  This way most of the bottom of the plate can be glazed, which discourages bacteria growth (unfinished ceramic is rough and porous) and makes the plate easy to clean. 

On some pieces stilts are used, little metal pins that the plate rests on instead of a foot.  These, leave small dimples in the glaze on the final piece.  If you turn your plates upside-down, you may see these dimples on some of them.

Thank you for your wonderful podcast, keep it up!


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