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So I have this pepper shaker made of glass with a print on it:

enter image description here

One fine dinner, it ran out of pepper, so I opened the lid to fill it up and noticed a peculiar thing – small particles of pepper dust were stuck to the inside glass walls but only in places where the print wasn’t present on the other side. Everywhere directly behind the print, the pepper particles almost didn’t stick to the glass.

(These photos also show vertical traces from a spoon, which are not relevant, I hope you’ll see the phenomenon anyway.)

enter image description here

enter image description here

I’ve been wondering about this for a while now and been thinking whether it’s got to do with light, or light and humidity, or different heat transfer, or electric charge… But as for now, I don’t have the final answer, and I'm looking for possible explanations of this phenomenon in the world of physics. This item is standing in an open space, so it is exposed to daylight, at times even a significant amount. Most of the time it is placed on a shelf above a gas kitchen stove, so it might be also getting some increased amount of humidity and heat.

Perhaps you know ways in which it could be explained, or you know a paper that studied similar effect experimentally? I would love to gather some clues to solve this mystery.

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    $\begingroup$ Can you see the effect of removing the sticker? As in remove some part and wait and see what happens. $\endgroup$
    – Superfast Jellyfish
    Feb 19, 2020 at 7:07
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    $\begingroup$ Oh I see! Then would that mean that the glass in that region is different in composition than the rest? Due to the difference in heat treatment. $\endgroup$
    – Superfast Jellyfish
    Feb 19, 2020 at 9:07
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    $\begingroup$ Can you add some info to your question about the environment of your pepper shaker? Eg, is it normally kept in a dark cupboard, or is it exposed to the light? And if it's in the light does it get much direct sunlight? My guess is that UV light may have more effect than visible light; sunlight & fluorescent lights have more UV than incandescent bulbs. Also, what sort of temperature range is in your house, and what's the humidity like? This info will help us to figure out if some kind of xerographic effect is operating. $\endgroup$
    – PM 2Ring
    Feb 19, 2020 at 9:12
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    $\begingroup$ Have you tried feeling the parts with print on the inside? Does the glass with print feel different compared to the glass without print? $\endgroup$
    – BioPhysicist
    Feb 19, 2020 at 16:39
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    $\begingroup$ A very interesting effect indeed !. There may be a lot of different causes, but my bet is that it is related with a static charge. In areas of print there may be a charge which with repulsion force pushes pepper apart. It may be a stupid idea, but you need to deny it by using your pepper shakers only with anti-static gloves on your hands. $\endgroup$
    – Agnius Vasiliauskas
    Feb 19, 2020 at 19:04

1 Answer 1

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The most likely reason for this phenomenon is that the figures on the shaker are painted with acrylics.

The acrylic paint binder is made of syntetic resin, an acrylic plastic polymer.

Due to the high electrical resistivity many of such polymers can hold an electric charge (and hence the electric field) last long.

The scientific name of such materials is, electret.

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  • $\begingroup$ But that is where there is no pepper. I was guessing the other way around, that the glass underneath the decorations should be more conductive, maybe because of glazing. $\endgroup$
    – user137289
    Feb 23, 2020 at 21:19
  • $\begingroup$ @Pieter, Indeed, that's interesting. So do you have any knowledge on whether pepper would charge negatively or positively? :) $\endgroup$
    – kamilazdybal
    Feb 23, 2020 at 22:11
  • $\begingroup$ @camillejr That would depend on where glass and pepper are in the triboelectric series. But I cannot really find pepper there, but vitreous charge is usually positive. $\endgroup$
    – user137289
    Feb 23, 2020 at 22:25

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