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

[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.

 

[Tom.G]

@hutchphd
The emissitivity graph below is for Carbon Steel, other graphs in the book for Stainless Steel are essentially flat across temperature. Not too surprising for me, a SS keg may cost more than its contents!

"Steel emissitivity at high temperatures" by Tuomas Paloposki & Tuomas Paloposki, 2005.
ISBN 951386717X (soft back ed.)
ISSN 12350605 (soft back ed.)
page 79

(downloaded from:
https://www.vttresearch.com/sites/default/files/pdf/tiedotteet/2005/T2299.pdf)

Cheers,
Tom