Naked Science Forum
General Science => General Science => Topic started by: Mike Vella on 21/06/2008 19:37:02
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Mike Vella asked the Naked Scientists:
Hi,
The other day you were talking about why plates have a ridge at the bottom (http://www.thenakedscientists.com/HTML/content/questions/question/2007/)and suggested that this increases friction, however static friction is only dependent on normal force (A surprising but true fact - It doesn't get easier to write with a pencil once it starts getting more blunt!).
I think the reason plates have that ridge is a much more innocent one, being that it's easier to make a nice flat surface which the plate wont wobble on from such a ridge than make the whole bottom of the plate perfectly flat.
Regards,
Mike Vella,
Malta
What do you think?
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Agreed.
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Thanks Mike!
Chris
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Stability.
The foot ring is a feature of earthenware in cultures that use tables. This hasn't always been so as most Inca ware is round-bottomed so it can be placed on soft ground and made to sit firmly.
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Your comment about a ring avoiding "aquaplaning" on a flat, wet surface (mentioned on the podcast, I recall) is probably true.
I also had the following thoughts this morning:
- Stability: by having a rim, you guarantee that the contact-points are around the rim, which makes the plate, bowl, or cup less likely to wobble badly (or even start rotating!) when it is in use
- thermal insulation: a rim results in a smaller contact-area than a flat base, so will conduct away less heat from the food or beverage to the table. (this would be more of an issue with a stone table than with wood!)
We should ask a pottery!
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I really like the stability argument. Same sort of reason as the three legged stool.
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Do you think they can make my exam desk with a ridge for stability? :P ...or perhaps they make them wobbly just to annoy me. There's a secret wobbly table factory out there somewhere ...possibly right next to the magic 'mum fridge' factory (the one's that always seem to contain food).
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I'm a potter, and from my understanding, there can be many reasons to have a ridge on the bottom of a plate or bowl, etc.
I agree with the stability argument primarily. Another good reason potters put ridges on plates is to retain structural strength while reducing the overall amount of material in the plate by removing the material that was originally within the ridge in the bottom center.
I'll briefly describe how a potter might throw a plate on a potting wheel. This is not the only way to do it, simply one technique. This might clarify things a bit.
You start with a lump of clay. The clay is centered and flattened progressively into a fat, flat cylinder, probably with a depression in the middle. You compress and strengthen the clay in the middle, and shape a "lip" around the edge--that raised part around the edge of the plate so your food doesn't spill out.
You remove the relatively thick flat disk from the wheel and let it slowly dry over days until the clay is still slightly damp but very firm.
You put the plate back on the wheel, up-side-down, re-center the plate and secure it to the wheel. Then you use trimming tools to carefully trim excess clay off of the bottom of the plate. You can create a ridge around the edge while you are trimming. Leaving a ridge allows you to trim more material from the center to make the plate thin and light, while leaving a strong edge to hold the plate round and flat while it fires.
When you fire ceramic, it cooks and shrinks and warps. Thicker parts warp less. How well you have aligned the flat plate-like clay particles while working the clay can play a large part in how and how much ceramics warp while firing. At least, in terms of the types of ceramic materials that people use to create pottery on a wheel.
I have a feeling thatthe reasons that modern, mass-produced plates have ridges on the bottom are limited to stability, looks and possibly durability. I imagine in the modern mass-production process, they have don't have to worry much about warping issues or trimming. They just don't make 'em like they used to.
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Thanks fallingleaves; very helpful.
Chris