# Melted Wax Cooling With Conical Shape In Center

**URL:** <https://boards.straightdope.com/t/melted-wax-cooling-with-conical-shape-in-center/109508>\
**Category:** Factual Questions\
**Created:** [May 17, 2002, 6:58am UTC](https://boards.straightdope.com/t/melted-wax-cooling-with-conical-shape-in-center/109508 "2002-05-17T06:58:02Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Demo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/demo/32/8472_2.png) [@Demo](https://boards.straightdope.com/u/Demo)\
**Post date:** [May 17, 2002, 6:58am UTC](https://boards.straightdope.com/t/melted-wax-cooling-with-conical-shape-in-center/109508/1 "2002-05-17T06:58:02Z")

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I have this potpourri thing that has a tea light on the bottom and a glass bowl on top. Instead of the flavored water that is supposed to go in it, I put wax in it. When it cools, the center part is drawn down and, when it hardens, the center drops down like a cone. Anyone know what causes that? I assume surface tension and possibly capillary action draws the wax up the sides (as they would presumably cool faster than the center), but I may be way off. Anyone experience this phenomenon before or know why it happens?

I have it going right now. When it cools I’ll take a picture and post it in this thread.

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**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\
**Post date:** [May 17, 2002, 11:46am UTC](https://boards.straightdope.com/t/melted-wax-cooling-with-conical-shape-in-center/109508/2 "2002-05-17T11:46:59Z")

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It is because the wax cools, and therefore solidifies, faster at the edges. It will also contract as it cools. The net effect is that when the outer edge solidifies the volume and therefore level of the liquid is greatest. As it cools further the overall volume decreases and the level goes down. This is a common problem when casting metals and can be seen in ingots as a noticable dent on the underside.

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**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\
**Post date:** [May 17, 2002, 10:07pm UTC](https://boards.straightdope.com/t/melted-wax-cooling-with-conical-shape-in-center/109508/3 "2002-05-17T22:07:02Z")

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> [@](#):
>
> \*Originally posted by ticker \*  
> It is because the wax cools, and therefore solidifies, faster at the edges. It will also contract as it cools. The net effect is that when the outer edge solidifies the volume and therefore level of the liquid is greatest. As it cools further the overall volume decreases and the level goes down. This is a common problem when casting metals and can be seen in ingots as a noticable dent on the underside.

Yeah, and water does the opposite! Water expands, so it makes a spike instead of a funnel. If you place a drop of water on a sub-zero metal slab, it will freeze from the bottom upwards. Just before the finish, it makes a tiny spike. Try sticking a thick metal spoon in the freezer for awhile, then take it out and put a bit of cold water in the bowl (not too much, or it won’t freeze entirely.)

This is the origin of those mysterious “ice spires” which sometimes appear in ice cube trays, especially after power failures.