[Melanie]

Melanie asked the Naked Scientists:

Hey guys,

I love the show.  I was listening to the 6/15 podcast and there was a question about ridges on plates and glasses.  

I throw pottery, and the term for this ridge on the bottom is a foot.  The foot of the plate/bowl/cup is important because it is an unglazed portion of the item.  The glaze, which melts into the glassy covering, would fuse to the bottom of the kiln or oven if there wasn't a foot tooled onto the bottom of these dishes.  I assume, even with modern techniques of manufacturing, that the foot remains on our dishes because of this reason.

Keep up the good work!

Melanie

What do you think?

[graham.d]

Hi Melanie. I will start by saying I know nothing about this subject but as nobody has ventured an answer I thought I would speculate. I had a look at various plates and pots I have. Some are old, some are new, some are cheap mass produced and other hand made. In every case there is a circular ridge under the plate, bowl, cup etc. Sometimes this is glazed too, especially on new mass produced items so I'm not sure how they do that. Mostly though it is not glazed and, in the case of one early victorian piece, none of the bottom inside the ridge is glazed either.

I would guess that the main reason is not just to do with the manufacturing process but to do with practicality of use. A ridge is much easier to planarise that the whole bottom surface so the finished plate will sit on a flat surface without wobbling about on a bulge which may have occurred during firing. But also, if as you say the glaze can stick, it would be good to reduce the area of this to a minimum. I guess there may also be advantage in insulation of the bottom of the plate (with an air-gap) from a surface to reduce heat transfer during usage. All these problems could be solved  by having just three "feet" on the bottom of the plate too but maybe aesthetics comes into it as well.

Any potters out there??

[LeeE]

That insulation factor is a good point, but if you only had three 'feet' instead, of a continuous ridge, you'd get air circulation beneath the plate.  Having a continuous ridge traps the air and for hot food especialy, probably helps keep it hot.