Eli5: Why do all my cups and glasses have rims on the bottom that collect water in the dishwasher?

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Eli5: Why do all my cups and glasses have rims on the bottom that collect water in the dishwasher?

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It helps the cup sit flat on the table, if you made a cup with a perfectly flat bottom it would be much more difficult to get the symmetry right, and with mass produced cups that would almost always result in cups that can’t sit straight on a table and would fall over easily.

Edit: I got an award lol I’m not sure what it means but thanks a lot!

Without a rim they form an even flat surface that could slide more easily if condensation forms beneath it

A perfectly flat bottom is harder to make and a convex bottom would make for a wobbly glass or cup.

Depending on the material, it also makes the cup more rigid. A plane is easily bent/folded/distorted because every direction can be a fold line. A convex shape doesn’t fold easily, so the shape is sturdier.

Do you use a drying aid? It can help prevent standing water in a dishwasher. A lot folks never use it but most dishwashers benefit from it.

If it’s a ceramic cup, it needs a “foot” that does not have a coat of glaze so that it can be removed from the kiln after firing. If you look at any regular ceramic cup, the ring at the bottom of the cup, the “foot,” is unglazed and rough. If it had a coat of glaze, the glaze would melt and fuze to the kiln as it cooled. There are cups that have unglazed flat feet (not hard to make unlike people here suggest) but that unglazed, fired clay can absorb liquids (thus making it not dishwasher friendly), be rough to handle, and besides cups and bowls and other ceramic objects have had that traditional shape for millenia. Don’t know about glasses, but most glasses I own look like they have flat bottoms.

Because if we tried to make them flat on the bottom they would always wobble, and they would slide around when there is even just a little moisture and, the clay ones would crack while being fired. But if they have a round rim or “foot” down there they fire nicely and are very stable and strong, and don’t wind up with more sloppy puddles on the bar counter.

Your dishwasher should have a drying cycle where it heats up everything in the dishwasher, causing any residual water to evaporate off of the cups, plates, etc.

Also make sure to fill up the rinse aid compartment – this is used to alter the surface tension of the water, preventing stains and aiding in drying.

Because you’re not tilting all of your mugs and glasses at 45 degrees (preferably so that the opening is facing the center of the washer). Do that, and none of you cups and glasses will have a rim on the bottom that collects water.

The problem is that you are putting them flat in the dishwasher. Put them on an angle so the water runs off.

If a perfectly flat cup sat upon a perfectly flat table and any kind of moisture was to develop between the cup and the table be in by condensation or spillage… the cup would stick to the table.

IIRC some cups (pretty sure I saw this on IKEA ones) have a small gap in the rim on the bottom so water can run off

Ikea makes cups that have a slit or two on the bottom exactly for this purpose, so the water flows out when upside down

The bottom rim helps cups not suction to surfaces under condensation and the air gap in the rim helps insulates your drink better. Some cups have gaps in the bottom rim to let water flow out, some don’t.

If it’s flat, there is a chance that if there is water at the bottom of it, the cup would slide.

Have you see before plastic cup with flat bottom? They slide when there are water at the base

Also, most cups from IKEA have a little chip taken out of the rim, to make way for water in the dishwasher!

Not an expert in any of those topics regarding cups, but Id speculate that for the case of pottery, its a product of the manufacturing process. In the production of pottery, a piece of clay is moulded wet, and then dried to set its shape. Due to the existence of gravity, the pottery will have to stand on something and dry that way. Then after drying, they fire the clay to make it water resistant. They will eventually have to scrape or get the dried clay off the thing it was sitting on while drying. If the bottom is flat, this would be problematic but if the surface in contact is a thin ring as opposed to a filled-in circle its easier to be released from the racks.

Plus, if its the same amount of material used, a dome is structurally stronger than a flat slab of material or feature of a design.

Because you have cups and glasses, which have rims on the bottom. So they collect water in the dishwasher.

So you, like me, can forget it’s full and tip water over yourself and the floor when you unpack it.