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My dad bought an earthen pot and he kept it on our glass table. Worried that the glass could break on filling the pot with water. I kept a metal plate beneath it. At first, it seemed like a good idea , but on further thinking I was unsure if it would actually help in bringing down the pressure on the glass.

what if I put three coins beneath the pressure points instead of the plate, would it be any different than placing the plate (assuming the coins to be nearly as thick as the plate).

and if it won't be any different, am I right to think that I could further keep reducing the area of the coins until they start looking extensions of the stand on which the earthen pot rests.

pot

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  • $\begingroup$ FYI: That glass plate probably is plenty strong enough to withstand the stress of supporting that pot, even without the metal pan, and even if the pot was completely filled with water. I bet you could even file those metal legs down to sharp points, and the glass still would be plenty strong enough. $\endgroup$ – Solomon Slow May 31 at 17:59
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The arguments given here for pressure = force/area ignore one very important point, as follows: An earthenware pot in contact with a glass tabletop does not apply a uniform load to the glass because the bottom of the pot is not perfectly flat and smooth. It makes contact with the glass in only a few points corresponding to the tallest bumps on its bottom surface.

In a pot containing some sand in the clay, these asperities on the pot surface are likely to consist of sand particles: SiO2, which is harder than most glass and has extremely sharp edges on it.

For the case of a pot with sharp sand grains stuck to its bottom surface, the pressure multiplication will be huge, because the weight of the pot is applied to the glass over a small number of microscopic contact areas, and the glass will easily be scratched.

Once scratched, the glass's strength is significantly reduced because the scratch enables brittle fracture of the glass.

Putting a stiff piece of metal under the pot only works as a load distributor if the metal is ductile enough to conform to the glass surface- and if there is only a single sand grain between the metal and the glass, then for a flat piece of stiff metal the load concentration effect is made much worse.

For these reasons, wooden legs with flat surfaces on their bottoms are much preferred because they are far softer than the glass, and any sand grains that get into the interface will become embedded into the wood and not pushed into the glass with great pressure.

The best load distributor by far consists of a stiff metal plate with a layer of soft rubber on the side facing the glass tabletop. The metal plate distributes the load and the rubber eliminates stress concentrators by conforming to both the glass and the metal.

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Here's how to think about this case.

imagine a very small glass table, just wide enough so that the three legs of the pot fit inside the circumference of that small table.

For such a small table the force exerted by each of the three legs would not be a problem at all.

For comparison, imagine a table where the top is made of very soft wood. Then you do want to put coasters underneath each of the legs, otherwise you might get indentations in the wood.

Well, glass is in no danger at all of getting indented. I would not be surprised to see coasters that are made out of glass.

The relevant failure mode is the glass plate breaking as a whole. An instantaneous full length crack. For that failure mode the metal plate shown in the photo will not make much difference, if any.

To assess how much load the glass is subjected to I suggest looking at reflection in the glass as the earthen pot is filled with water. Whithout load the glass will be straight/flat, and the reflection will be like a mirror image. When the glass bends the view reflected in the glass is distorted accordingly. If you put some load on the table, and you see no distortion (or very little) then there is very little bending, and the table is presumably strong enough. If you do see bending of the glass as the pot is filled then that table is just not a good location for that pot.

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I don't think a metal plate is necessary for a thick glass like that, supported by an wood table.

Being transparent, glass looks like more fragile than it actually is.

I have a ceramic floor for example. Each ceramic plate has 60 x 60 cm, and much thinner than the glass of the picture. Moreover, the contact with the concrete behind it is far from perfect. But there are heavy sofas over it anyway. And ceramic is also a brittle material. It must be properly protected during transportation from the store to house, otherwise breaks easily.

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  • $\begingroup$ Usually ceramic tiles are set in grout, which improves the area of contact a lot. And also prevents bending. $\endgroup$ – mmesser314 May 31 at 18:00
  • $\begingroup$ Yes, of course. But after solidification there are voids from behind. I guess that the contact wood-glass of the picture is better. $\endgroup$ – Claudio Saspinski May 31 at 18:18
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Yes, definitely by putting a plate you are reducing the pressure the pot exerts on the table because $$p= f/a$$ where p is pressure, f is force and area is a . That basically is not a description but a definition of the word pressure itself. Though I'm not sure about this, I think the plate is better than the three coins. Think about it. Gravity is not like arrows from he pot shooting straight down (technically it is straight lines towards center of earth but I meant that it also has an impact on other objects on its way - how?- because gravity pulls the pot down which presses onto the plate and the electrostatic forces between them means that the plate is experiencing a force.)

Simply put: The pot exerts a downward force on the plate OR presses down onto the plate and the plate in turn presses onto the table but now with a larger area and hence with lesser pressure, so your table is much safer. The coins idea is not good as it does not change the surface area and simply acts as a mediator of the force of the pot above.

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It will reduce the pressure. It may help prevent breakage.

$pressure = weight/area$, where area is the area in contact with the surface below it.

Without the plate, the area is just the blue feet. The blue rubber helps. It is soft and deforms to be flat on the bottom. Without it, area would be whatever points of contact were resting on the table.

Glass is pretty incompressable. A softer material like wood might be dented by metal feet.

Assuming the plate is somewhat rigid and smooth, the area of contact is likely larger than the rubber feet. You could improve it with a rubber sheet or maybe a towel under the plate.

A coin does not have a flat surface. High points on the coin would rest on the surface.


But this may not have as much effect on breakage as you might think. The way glass breaks is different than what you might think.

Glass can bend. It is easier to see in large thin sheets. But smaller thicker sheets bend a little. When it does, the inside surface compresses and the outside surface stretches. Breakage occurs when the outside surface is stretched so hard that neighboring atoms are pulled apart.

In a perfect sheet of glass, it is hard to do. Sometimes you intentionally break glass. For example, to cut a sheet to the right size for a window. To do so, you scratch the surface, and bend it to pull apart the scratch. Once a scratch is started, it is a lot easier to pull it apart a little and make it deeper. It is like tearing open a plastic bag. It is much easier once the tear is started.

Also, water is a polar molecule. The O has a negative charge, and the H's have a positive charge. These charges can exert large forces on nearby molecules. This is why water dissolves more substances than any other liquid. Water normally has little effect on glass. But in the tip of a crack, the environment is such that water pulls apart molecules and makes the crack a little deeper. And then it does it again at the new tip. So often glass cutters scratch, and then wet the scratch.

So here are some ways to avoid breaking the table top.

  • Get a thick piece of glass that is harder to bend. Twice the thickness means 1/4 the bending. From the photo, it looks like the glass is 1/2" or thicker.

  • Don't scratch the bottom surface.

  • support the glass so it doesn't bend. There is a lip around the edge of the table that supports the glass. This is good.

  • Put the pot near the edge of the table, where it is supported. Not the middle, where bending will be greatest.

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