What Causes the Darkening of Metal Bucket Color at High Temperatures?

[Awwtumn]

This is the color of the metal bucket when it's not hot. But whenever it is hot, the red changes to dark, and then back to red again when it is not hot. See the following youtube where I just threw the fake money at the bucket (see background below). Can you explain how the phenomenon work? Maybe related to Blackbody Radiation or something?



In the bucket, I'm burning fake money at the cemetery which our family does a few times a year. In this particular time, it's middle of so called Ghost month. The Chinese tradition believes that during certain period in a year, there is loosening of barrier between our world and the dark world (although they didn't say dark matter world).

Of course, we Newtonians can just scoff at the idea, we can prove with our nut and bolts machine that there is no dark world or something.

 

[Drakkith]

Looks like thermochromism of the coating of the bucket, but I couldn't say exactly what's happening at the molecular level or anything.

 

[Baluncore]

Thermochromic pigments indicate temperature by changing their structure at a particular temperature. The different structures have different optical properties and so have different colours.
https://en.wikipedia.org/wiki/Thermochromism

Low temperature thermochromic pigments use organic molecules.
This example changes at 31°C, when warmed by the body.
https://www.ebay.com.au/itm/125358953763?

High temperature thermochromic pigments are usually inorganic minerals.
https://en.wikipedia.org/wiki/Thermochromism#Inorganic_materials

 

[Awwtumn]

Thermochromic pigments indicate temperature by changing their structure at a particular temperature. The different structures have different optical properties and so have different colours.
https://en.wikipedia.org/wiki/Thermochromism

Low temperature thermochromic pigments use organic molecules.
This example changes at 31°C, when warmed by the body.
https://www.ebay.com.au/itm/125358953763?

High temperature thermochromic pigments are usually inorganic minerals.
https://en.wikipedia.org/wiki/Thermochromism#Inorganic_materials

The metal bucket is just cheap. Did they intentionally put thermochromic pigments in the paint or is it usually un-intentional?

 

[Baluncore]

Did they intentionally put thermochromic pigments in the paint or is it usually un-intentional?

The fact that the change is high temperature and reversible suggests it was intentional.

The material goes black when hot, which is unusual. It may be that the pigment becomes transparent when hot, which allows a black base to show through. If you scratch the outer paint, you will be able to see the base layer. What is that colour?

Red haematite, Fe₂O₃, is rust red and changes to black magnetite, Fe₃O₄, when heated. The recovery takes weeks when cooled, because it requires oxygen from the atmosphere.

 

[Awwtumn]

The fact that the change is high temperature and reversible suggests it was intentional.

The material goes black when hot, which is unusual. It may be that the pigment becomes transparent when hot, which allows a black base to show through. If you scratch the outer paint, you will be able to see the base layer. What is that colour?

Red haematite, Fe₂O₃, is rust red and changes to black magnetite, Fe₃O₄, when heated. The recovery takes weeks when cooled, because it requires oxygen from the atmosphere.

The base material is black (see above photo). The red paint turns transparent when hot? What particular pigment is that? Haven't encountered it before.

I was initially thinking of blackbody radiation that interacts with the paint. But the metal could melt if it reaches the temperature that could interact with the red to produce black, isn't it.

 

[hmmm27]

While black body radiation is typified by a substance "glowing red hot", red paint not so much. A quick google turned up a chroma-paint which has the attributes you're looking for (also available in blue and green), but the change was at 31C, somewhat lower than what you want.

 

[Awwtumn]

The fact that the change is high temperature and reversible suggests it was intentional.

The material goes black when hot, which is unusual. It may be that the pigment becomes transparent when hot, which allows a black base to show through. If you scratch the outer paint, you will be able to see the base layer. What is that colour?

Red haematite, Fe₂O₃, is rust red and changes to black magnetite, Fe₃O₄, when heated. The recovery takes weeks when cooled, because it requires oxygen from the atmosphere.

In the above video. I lit a lighter just below the red cover. The coating changed to dark hue. But notice it didn't become transparent because you can clearly distinguish the coating and black base underneath. So what material or coating is it which changes to dark color when heated and return to original red color when cooled as you can clearly see at the end of the video? Is it still thermochromic pigments? But notice the pigments didn't turn invisible.

What happens to the atomic levels in the above demonstration?

 

[bob012345]

The color of heated steel is in the table here;

https://en.wikipedia.org/wiki/Red_heat#cite_note-2

 

[Awwtumn]

In the above video. I poured boiling water over the alleged thermochromic paint. It didn't change color or become transparent. Such paints are supposed to change color instantly with hot water applied.

So it's not a thermochromic paint.

When you applied lighter flame from below the metal. The metal was not supposed to glow because it was not heated to melted molten. So how did the red color change to dark when heat was applied? What other phenomenon could explain?

 

[Drakkith]

In the above video. I poured boiling water over the alleged thermochromic paint. It didn't change color or become transparent. Such paints are supposed to change color instantly with hot water applied.

So it's not a thermochromic paint.

Perhaps its a thermochromatic paint that doesn't change color until well above the boiling point of water?

 

[bob012345]

Could the effect be an optical illusion due to the index of refraction of air at very high temperatures? Perhaps something akin to a mirage?

 

[Drakkith]

Could the effect be an optical illusion due to the index of refraction of air at very high temperatures? Perhaps something akin to a mirage?

No, mirages don't work like that. They won't change the color of an object in this manner, and certainly wouldn't turn the bucket brown/black from red.

 

[bob012345]

No, mirages don't work like that. They won't change the color of an object in this manner, and certainly wouldn't turn the bucket brown/black from red.

I didn't mean exactly a mirage but some optical effect especially the index of refraction at very high temperatures?

 

[Drakkith]

I didn't mean exactly a mirage but some optical effect especially the index of refraction at very high temperatures?

Nope. Any such effect would severely distort the image of the bucket, which isn't seen.

 

[bob012345]

What brand is the burner? Have you asked the maker?

 

[Awwtumn]

What brand is the burner? Have you asked the maker?

It's just a cheap generic metal bucket from Chinatown. No brand, manufacturer unknown.

After knowing about the existence of thermochromic stuff. I bought a so called magic mug (see above video )where you can put any pictures or images on it, and once you pour hot water, the images will come out.

It's cool. This seems to be activated below boiling point of water. But for those thermochromic metal that can be activated only with temperature of flame. Isn't there a product where this is used as some kind of fire detector (imagine a laser shining on the black paint, and when it becomes transparent from heat from flame, it will reflect back full blast and sound off the alarm).

Also it seems in high temperature thermochromism. Some form of phase transition or charge transfer mechanism occurs.

 

[bob012345]

Nope. Any such effect would severely distort the image of the bucket, which isn't seen.

Light is normally scattered at the incidence angles but in a mirage it is bent like in this image so if you were looking from above you might see less incident light being reflected and perhaps notice a slightly dimmed surface. For the bucket if the hot spot refracts incident light along the bucket surface then an outside observer might see a dark spot.

 

[Drakkith]

Light is normally scattered at the incidence angles but in a mirage it is bent like in this image so if you were looking from above you might see less incident light being reflected and perhaps notice a slightly dimmed surface. For the bucket if the hot spot refracts incident light along the bucket surface then an outside observer might see a dark spot.

There's no reason for a dark spot to appear, as light from somewhere else would be deflected into your eye and you would see a distorted image of whatever object that light came from.

 

[bob012345]

Maybe it is just the enamel coating then. I found this reference on Wiki;

• Chromium(III) oxide and aluminium(III) oxide in a 1:9 ratio is red at room temperature and grey at 400 °C, due to changes in its crystal field.[1]

1. Bamfield, Peter & Hutchings, Michael G. (2010). Chromic Phenomena: Technological Applications of Colour Chemistry. Royal Society of Chemistry. pp. 48–. ISBN 978-1-84755-868-8.

 

[Awwtumn]

I gave my friend a thermochromic mug but she refused to drink from it. I can't find reference what would happen if the thermochromic dye in the mug got hot and interaction with skin. My friend said she was afraid the invisible dye may slowly sip into her lips and she saw Hollow Man once and didn't want her lips to turn become transparent showing her teeth.

In mugs, how do they exactly print it, won't an ordinary ink seep into your lips when the mug is hot with coffee?

Lastly, would ordinary paint and thermochromic paint have same toxicity profile? Thousands of thermochromic mugs exist worldwide so we ought to this aspect of it. Thank you.

 

[Drakkith]

In mugs, how do they exactly print it, won't an ordinary ink seep into your lips when the mug is hot with coffee?

No, the coating shouldn't wash off. And even if it did, it wouldn't turn your lips transparent. It's the coating itself that changes colors or becomes transparent, not the underlying material, so that even if you coated your own lips with a fresh layer, all someone would see is your regular lips once the coating got hot enough to change. Just imagine wearing lipstick that turned transparent. It would just look like you weren't wearing any lipstick, you wouldn't be able to see through your lips.

Lastly, would ordinary paint and thermochromic paint have same toxicity profile? Thousands of thermochromic mugs exist worldwide so we ought to this aspect of it. Thank you.

I can't say. There are thousands of different types of paint. Some are harmful, some are not. I expect the same is true of thermochromic paints as well. Presumably the ones used to make mugs are not harmful.

 

[Baluncore]

The web marketplace is now flooded with low-temperature, polymer-based thermochromic pigments. Those pigments have been selected to change for body heat, or hot water. The pigments are bound in a clear paint, so they will not flow like a wet ink.

Unfortunately, the plethora of gimmicks available on the web, makes it hard to search and find the higher temperature pigments on the web.

 

[Awwtumn]

When thermochromic paint got heated and become invisible. What part of the spectrum is it hiding? In the infrared maybe?

Why are there no natural physical objects that only exist in the infrared or other frequencies and hence invisible to the naked eye? Or since I remember infrared is heat. Why do objects only occur in visible wavelength?

 

[hmmm27]

Why are there no natural physical objects that only exist in the infrared

You mean like glass ?

 

[Drakkith]

Why are there no natural physical objects that only exist in the infrared or other frequencies and hence invisible to the naked eye? Or since I remember infrared is heat. Why do objects only occur in visible wavelength?

There are plenty of objects that are transparent to visible light. Glass, some types of plastic, wings of insects, water, air, etc. Similarly, there are objects that are transparent in infrared but not in visible light, such as plastic trash bags.

 

[Awwtumn]

I don't see the following explanation anywhere.

Can we say the covalent and ionic bonds of atoms correspond to energy levels of the visible wavelength of photons? (except those of glasses, some types of plastic, wings of insects, etc.).

And to change those normally visible object to transparent, you have to adjust the covalent and ionic bonds of those solids to say that of glass? And this is not possible without shredding the objects electrons?

 

[Drakkith]

And to change those normally visible object to transparent, you have to adjust the covalent and ionic bonds of those solids to say that of glass?

Something like that, yes. For a coating that turns transparent, the electronic energy levels of the molecules are altered by the increased temperature such that they no longer correspond to visible wavelengths, much like glass.

And this is not possible without shredding the objects electrons?

Electrons cannot be shredded, for they are fundamental particles and can't be subdivided into anything else.

 

[bob012345]

And to change those normally visible object to transparent, you have to adjust the covalent and ionic bonds of those solids to say that of glass? And this is not possible without shredding the objects electrons?

Did you mean to say shedding instead of shredding?

 

[Baluncore]

There are very many different mechanisms that result in thermochromism.
Find a copy of this 220 page book: Lötzsch, Detlef; Seeboth, Arno; Thermochromic and Thermotropic Materials - Pan Stanford (2013).
On page 204.
“Energy efficiency calculations were also carried out for thermochromic windows based on vanadium dioxide [11]. … … The investigated vanadium dioxide based coated thermochromic films possesses switching temperatures of 38.5°C (VO2 + TOAB), 43°C (VO2 + gold), 45.5 (VO2 + gold + TOAB) and 59°C (VO2). As an outcome of the calculations, the energy-saving performance of the vanadium dioxide based thermochromic films mainly depends on the switching temperature”.
[ TOAB = surfactant, tetraoctylammonium bromide ]

Zinc Oxide: New Insights into a Material for All Ages - By Amir Moezzi.
PhD Dissertation. University of Technology. Sydney. 2012
https://opus.lib.uts.edu.au/bitstream/10453/20439/2/02Whole.pdf
“Crystalline ZnO is thermochromic, changing from white to yellow when the temperature is increased to >300°C [24] and then from yellow to white upon cooling. This is probably because of the formation of crystal lattice defects due to a loss of oxygen and the formation of the non-stoichiometric Zn₁+xO, with x increasing with temperature”.

 

[Baluncore]

Some idea of the complexity of the subject can be gained from the contents of the book: Lötzsch, Detlef; Seeboth, Arno; Thermochromic and Thermotropic Materials - Pan Stanford (2013).
Contents.
1. Thermochromic Materials Based on Reflection 1
1.1 Light Reflection 1
1.2 Liquid Crystals 3
1.2.1 Cholesteric Liquid Crystals 3
1.2.1.1 Cholesteric phase of rod-like molecules 6
1.2.1.2 Discotic cholesteric phase 7
1.2.1.3 Induced cholesteric phases 8
1.2.1.4 Lyotropic cholesteric phases 13
1.2.1.5 Temperature dependence of the helical pitch 15
1.2.1.6 Cholesteric polymers 17
1.2.1.7 Cholesteric polymer networks 21
1.2.2 Smectic Liquid Crystals 23
1.2.3 Blue Phases and TGB Phases of Chiral Liquid Crystals 24
1.3 Crystalline Colloidal Arrays 27
1.3.1 Crystalline Colloidal Arrays Embedded in Gel Networks 27
1.4 Semiconductor to Metal Transition of Inorganic Materials 31
2. Thermochromic Materials Based on Light Absorption 39
2.1 Light Absorption 39
2.2 Inorganic Thermochromic Materials 41
2.2.1 Solid-State Thermochromism of Inorganic Materials 41
2.2.2 Thermochromism in Solution of Inorganic Materials 44
2.3 Metal Organic Thermochromic Material 45
2.3.1 Thermochromism of Metal Organic Compounds in Solid State or at the Melting Point 45
2.3.2 Thermochromism of Metal Organic Compounds in Solution 47
2.3.3 Thermochromism of Metal Organic Gel Networks 49
2.4 Spiroheterocyclic Compounds 51
2.5 Salicyl-Schiff Bases 55
2.6 Overcrowded Ethenes 56
2.7 Conjugated Polymers 58
2.7.1 Thermochromism in Polythiophenes 58
2.7.2 Thermochromism in Poly(Phenylene Vinylenes) 60
2.7.3 Thermochromism in Polydiacetylenes 61
2.7.3.1 Thermochromism of polydiacetylenes in solid state 61
2.7.3.2 Thermochromism of polydiacetylenes in liquid crystalline state 64
2.7.3.3 Thermochromism of polydiacetylenes in solution 65
2.7.3.4 Thermochromism of polydiacetylene gels 66
2.7.4 Thermochromism in Other Conjugated Polymers 66
2.8 Thermochromism by Leuco Dyes 67
2.8.1 Thermochromism by Leuco Dye–Developer Systems 69
2.8.2 Thermochromism by Leuco Dye–Developer–Solvent Systems 72
2.9 Thermochromism by Charge Transfer Complexes 77
2.9.1 Thermochromism of Charge Transfer Complexes in Solid State 77
2.9.2 Thermochromism of Multilayer Films Due to a Ligand to Metal Charge Transfer 79
2.9.3 Thermochromism of Charge Transfer Complexes in Solution 80
2.10 Indicator Dyes Incorporated into Hydrogel Networks 80
2.11 Thermochromism of Cyano-Substituted Oligo (p-Phenylene Vinylene) Dyes in Polymer Matrices 85
2.12 Thermochromism Based on Surface Plasmon Absorption 87
2.13 Miscellaneous Thermochromic Composites 94
2.13.1 Thermochromism on Silica Gel Based on pH Indicator Dyes 94
2.13.2 Thermochromism of pH Indicator Dyes Incorporated in Polymer Matrices 94
2.13.3 Thermochromic Sol–Gel Material 96
3. Thermochromic and Thermotropic Materials Based on Light Scattering 105
3.1 Light Scattering 105
3.2 Thermochromic Gel Networks Based on Light Scattering 109
3.3 Thermotropic Polymer Blends 111
3.4 Thermotropic Gel Networks 112
3.4.1 Thermotropic Gel Networks Based on the Appearance of Liquid Crystalline Phases 113
3.4.2 Thermotropic Gel Networks Based on Phase Separation 115
3.4.2.1 Phase separation in chemically cross-linked polymer networks 116
3.4.2.2 Phase separation in physically cross-linked polymer networks 116
3.4.3 Hybrid Thermotropic and Thermochromic Gel Networks 127
3.5 Aggregation in Aqueous Polymer Systems 131
3.6 Thermotropic Casting Resins 133
4. Application of Thermochromic and Thermotropic Materials 139
4.1 Thermometers and Temperature-Indicating Labels 140
4.1.1 Thermography 143
4.2 Thermo-Sensitive Paper 143
4.2.1 Thermochromic Inks 144
4.3 Thermochromic Thermoplastic Polymers 146
4.4 Thermochromic Thermosetting Polymers 159
4.5 Sun-Protecting Glazing 166
4.5.1 Sun-Protecting Glazing Based on Thermotropic Materials 167
4.5.1.1 Glazings containing a layer of a thermotropic hydrogel 168
4.5.1.2 Glazings containing a layer of a thermotropic polymer blend 171
4.5.1.3 Glazings containing a layer of a thermotropic casting resin 172
4.5.2 Sun-Protecting Glazing Based on Thermochromic Materials 183
5. Active Triggering and Energetic Characterization of Thermotropic and of Thermochromic Materials 193
5.1 Active Triggering of Thermotropic and of Thermochromic Materials 193
5.2 Energetic Characterization of Thermotropic and of Thermochromic Materials 199
6. Concluding Remarks 207
Index 209

 

[Awwtumn]

There were days when radium inks (radioactive ink) or asbestos were used by the public. And the danger aspects of them came out later.

Something concerns me. Although thermochromic dye like Leuco dyes are well characterized and safe and these can be found in mugs and dozens of other commercial items.. The transition temperature being 87.8 F or 31 C. These comprise the organic thermochormic dyes. But inorganic dyes are not common. I read that "Inorganic thermochromic materials are the main potential substitutes due to their thermal stability and durability in a wide range of temperatures, such as chromium compounds, mercury compounds, and some other metal halides [8]. However, these compounds are either toxic or carcinogenic in many routes of transmission." (see complete reference below)

Can you give other commercial product where the metal changes color to gray at 400 C? Or is the red bucket I bought like radium ink, an experimental product, Remember these are made in china and unregulated. When the bucket burns with fake money, can the chromium content (or sorta) created fumes that can be toxic or carcinogenic? If there is a possibility. I may just throw it away and get regular bucket without this feature. I need to know.

Reference: https://www.sciencedirect.com/science/article/abs/pii/S0143720817303042

"Introduction
The concept of “inorganic thermochromism” has been found in solids, liquids, solutions and gases for thousands of years, however, the applications of this phenomenon are limited [1]. Thermochromic materials are important family of colour change materials with potential applications in many fields, such as thermometers for cooking tools and hotplates, temperature sensors, laser marks, and thermal warning signals [2], [3]. Compounds that exhibit thermochromism phenomena are mostly found in conjugated organic molecules due to the feasibility for electron transfer excited by external photons [4], such as in liquid crystals [5], dyes [6], and conjugated polymers [7]. However, these organic molecule based materials could only be used under 400 K (127 °C) due to their poor thermal stability. Inorganic thermochromic materials are the main potential substitutes due to their thermal stability and durability in a wide range of temperatures, such as chromium compounds, mercury compounds, and some other metal halides [8]. However, these compounds are either toxic or carcinogenic in many routes of transmission. Besides, most of inorganic thermocrhomic materials are irreversible due to the partially decomposition or phase transition, such as NH4VO3 and Mn(NH4)P2O7 [9]. For example, Co2+-doped Zn3(PO4)2·4H2O shows a color tuneable change of pink → blue → violet due to the thermal-induced H2O release from the compound [10]. Thus, the synthesis and producing safe inorganic thermochromic pigments with reversible color change dependent on temperature is a pressing need in industry. Until now, reversible inorganic thermochromic materials are still limited to two categories: vanadium dioxide (VO2) [11] and tungsten- or molybdenum-based oxides (AW1−xMoxO4, A = Mg, Co, Ni, Cu, or Zn) [12], [13]. SrMnO3 also showed thermochromic behaviour at liquid nitrogen temperature due to the structural distortion of corner-sharing Mn2O9 units [14]. There are two mechanisms for reversible inorganic thermochromic materials: first order phase transition (e.g. vanadium oxides [11]) and charge transfer due to the bond change of ligands and center metal cations (e.g. tungsten and manganite oxides [12], [13], [14]). The ligand field around the chromophore or the crystallographic phase transition occurs in reversible thermochromism in oxides, which gradually reduced the band-gap with increasing temperature."

 

[bob012345]

It is doubtful the chemicals in the enamel paint come off when the bucket is heated. Enamel paint is like a glass layer. Just wash your hands after handling the bucket if you are that concerned or use gloves or get one that is unpainted.

 

[Drakkith]

Can you give other commercial product where the metal changes color to gray at 400 C? Or is the red bucket I bought like radium ink, an experimental product, Remember these are made in china and unregulated. When the bucket burns with fake money, can the chromium content (or sorta) created fumes that can be toxic or carcinogenic? If there is a possibility. I may just throw it away and get regular bucket without this feature. I need to know.

There's little possibility that we can answer this. You can try contacting the manufacturer and seeing if they can tell you what the coating is made out of. As for safety, there's no real way to know without testing the bucket or finding someone who has already tested it.

 

[hutchphd]

As a a college student I built a wood stove from a shiny stainlees steel beer keg. With a raging fire inside, it gave off almost no heat until it started to glow (orange-red perhaps 1000C ??) and suddenly heat would exude abundantly I took it to be some plasma Temperature of the metal but I never figured it out in detail...I just sprayed it black with stove paint and it behaved like normal woodstove. The transition (with tempeature) before it was painted was pretty impressive.