# Heat coloring of metals

**Heat coloring of metals** (also called heat tinting or temper coloring) is the formation of interference colors on a metal surface that has been heated in air so that a thin, transparent oxide film grows on it. The color is produced by thin-film [interference of light](https://www.edgepedia.org/interference-of-light) reflected from the top and bottom of the oxide layer, so it reports the film's thickness rather than any pigment, and on steel it has long served as a shop-floor indicator of tempering temperature.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup><sup> • </sup><sup>[2](https://pubs.usgs.gov/bul/27/report.pdf)</sup>

| Key fact | Detail |
|---|---|
| Mechanism | A thin transparent oxide film grown by heating produces color by light interference at the metal-oxide-air interfaces; color varies with film thickness<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup><sup> • </sup><sup>[3](https://onlinelibrary.wiley.com/doi/10.1002/col.20403)</sup> |
| Steel temper scale (AISI 304, air) | Pale yellow 290 °C, straw yellow 340 °C, dark yellow 370 °C, brown 390 °C, purple brown 420 °C, dark purple 450 °C, blue 540 °C, dark blue 600 °C<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup> |
| Time dependence | Published charts are usually based on one hour of heating; longer exposure deepens the colors, making the steel appear to have been heated hotter<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup> |
| Film thickness | Interference colors on heat-treated steel samples correspond to an oxide film of about 200 nm<sup>[4](https://www.phase-trans.msm.cam.ac.uk/2008/Oxide/Oxide.html)</sup> |
| Titanium by anodizing | Anodizing titanium at 10–100 V for 10–100 s grows a TiO2 film that yields purple, blue, cyan, green, yellow, orange, and red, plus angle-dependent (goniochromatic) effects<sup>[5](https://www.sciencedirect.com/science/article/pii/S0013468625020948)</sup> |
| Corrosion trade-off | Heat tint on stainless steel prevents formation of the usual protective passive film and decreases corrosion resistance<sup>[6](https://www.imoa.info/stainless-solutions/archive/20/Black-grey-colored-stainless-steel.php)</sup> |
| Practical uses | Temperature estimation in tempering, weld heat-tint assessment, and color metallography of microstructures<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup><sup> • </sup><sup>[7](https://materialsdata.nist.gov/bitstream/handle/11115/160/Color%20Metallography.pdf)</sup> |

## How the color is produced

When a bright metal surface is heated in air, oxygen reacts with the surface and an oxide film grows progressively thicker. Light reflected from the air-oxide interface and light reflected from the metal beneath the film travel different path lengths, and the two beams interfere. Wavelengths for which the interference is constructive are reflected strongly; those eliminated by destructive interference leave the complementary color visible. The color therefore shifts in a fixed sequence as the film thickens, and the same sequence appears whether the film was grown by a furnace, a torch, a welding arc, or an electrochemical cell.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup><sup> • </sup><sup>[8](https://doi.org/10.1016/j.jmrt.2025.02.216)</sup>

The interference condition follows thin-film theory. For a beam striking the film, the path difference between the two reflected beams is

\[ \delta = 2d\sqrt{n_1^{2} - n_0^{2}\sin^{2} i_0} \]

where \( d \) is the oxide film thickness, \( n_0 = 1 \) is the refractive index of air, \( n_1 = 2.3\text{–}2.7 \) is the refractive index of the TiO\(_{x}\) film on titanium, and \( i_0 \) is the incidence angle; constructive interference occurs when \( \delta \) equals a whole number of wavelengths and destructive interference at odd half-wavelengths.<sup>[8](https://doi.org/10.1016/j.jmrt.2025.02.216)</sup> Because the condition depends on the viewing angle, the colors are gonioapparent, shifting as the part is turned. On steel, a surface that appears blue has eliminated the red component (about 725 nm), implying an oxide layer roughly 0.25 × 725 nm thick; measured interference colors on heat-treated samples correspond to films of about 200 nm.<sup>[4](https://www.phase-trans.msm.cam.ac.uk/2008/Oxide/Oxide.html)</sup> Once the film grows thick enough to become opaque, interference through reflection from the metal surface is no longer possible and the color sequence ends.<sup>[4](https://www.phase-trans.msm.cam.ac.uk/2008/Oxide/Oxide.html)</sup>

## Temper colors on steel

Steel heated in contact with air at temperatures in the tempering range takes on temper colors from the thin oxide film that forms.<sup>[9](https://dl.asminternational.org/technical-books/edited-volume/115/chapter-abstract/2203668/Temper-Colors-for-Steels)</sup> A United States Geological Survey bulletin describes the classical observation: a bright steel surface heated in air between 200° and 300 °C is soon covered with an oxide film whose color, and hence inferred thickness, depends primarily on the temperature of exposure, and technical treatises list the sequence from very pale yellow onward.<sup>[2](https://pubs.usgs.gov/bul/27/report.pdf)</sup> For stainless steel type 1.4301 (AISI 304) heated in air, the British Stainless Steel Association chart runs from pale yellow at about 290 °C through brown at 390 °C and dark purple at 450 °C to blue at 540 °C and dark blue at 600 °C.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>

**The color is not a thermometer.** Several factors shift the color-temperature relationship, so no single table represents all cases and the colors indicate temperature only approximately.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>

- Composition: of all the elements, chromium has the greatest individual influence on a steel's resistance to oxidation; as its percentage rises, the steel withstands heat better and the colors appear at progressively later stages. Carbon-steel charts should not be read across to stainless steels.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>
- Time: the film keeps thickening while the piece stays hot, so colors deepen with exposure and can mimic a higher temperature.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>
- [Atmosphere](https://www.edgepedia.org/atmosphere): the oxygen level matters; heating in air assumes about 20% oxygen, and in welding the shielding gas effectiveness and welding speed change the heat tint around the bead.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>
- [Surface finish](https://www.edgepedia.org/surface-finish): rougher surfaces may oxidize faster and appear as deeper colors, and because the colors arise by light interference, surface smoothness also changes their appearance.<sup>[1](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)</sup>

Where temperature-measuring equipment is unavailable, reading the oxide film color on a cleaned surface remains an inexpensive, fairly accurate way to estimate the temperature of a piece in the tempering range.<sup>[10](https://www.govinfo.gov/content/pkg/GOVPUB-C13-9d3193e93202905da6c546702b1a9d73/pdf/GOVPUB-C13-9d3193e93202905da6c546702b1a9d73.pdf)</sup>

## Titanium and controlled coloring

Titanium shows the effect especially strongly because its oxide is transparent and its refractive index is high. Above roughly 500 °C titanium reacts intensely with oxygen, and the oxide thickness sets the interference color, so welders use heat-tint colors to judge contamination of titanium welds.<sup>[11](https://beiyutitanium.com/titanium-weld-color-heat-tint-contamination/)</sup> Heating with a torch produces the same color sequence as [anodizing](https://www.edgepedia.org/anodizing), but it is difficult to control precisely; anodizing instead grows the oxide electrochemically, with voltage setting the thickness.<sup>[12](https://www.danco.net/PDF-Downloads/Titanium-Color-Anod.pdf)</sup>

In a controlled study, titanium Grade 4 anodized in an ethylene glycol electrolyte containing NH4F and H2O, at voltages from 10 V to 100 V in 10 V steps and times from 10 to 100 s, produced a wide spectrum including purple, blue, cyan, green, yellow, orange, and red, with angle-dependent effects, as the TiO2 film grew thicker and denser.<sup>[5](https://www.sciencedirect.com/science/article/pii/S0013468625020948)</sup> The same thickening that produced the colors also enhanced corrosion resistance, by reducing the corrosion current and increasing the charge-transfer resistance, which supports paint-free color coding for aerospace, defense, and other industrial applications.<sup>[5](https://www.sciencedirect.com/science/article/pii/S0013468625020948)</sup> Because the colors are designable, interference-colored titanium is used architecturally; [Nippon Steel](https://www.edgepedia.org/nippon-steel) developed computer-graphics simulation of the colors, achieving an average color difference within 5.0 ΔE and Grade 4 in the color tolerance classification, sufficient for practical design work.<sup>[13](https://www.nipponsteel.com/en/tech/report/pdf/128-13.pdf)</sup> The film is not inert over decades: on titanium building products the oxide film has grown over about 10 years, shifting the color from yellow to purple.<sup>[14](https://www.nipponsteel.com/product/catalog_download/pdf/T003en.pdf)</sup>

## Uses and limitations

**Color metallography.** Heating a mounted-free, polished specimen in air until a light oxide film forms is a standard metallographic technique for revealing microstructure. A red-violet color generally works well, though thinner films are often useful, and the temperature must be kept low so the tint stays selective rather than coloring everything; published temperature lists exist for different metals and alloys, and the method has been applied to commercially pure titanium (ASTM F 67, grade 4) and as-cast Ti-6Al-4V.<sup>[7](https://materialsdata.nist.gov/bitstream/handle/11115/160/Color%20Metallography.pdf)</sup> Restrictions apply: unmounted specimens are required, low-melting-point alloys are unsuitable, and the method should not be used if the tinting temperature would alter the microstructure.<sup>[7](https://materialsdata.nist.gov/bitstream/handle/11115/160/Color%20Metallography.pdf)</sup>

**Corrosion consequences.** On stainless steel the heat tint is a liability as well as a signal. Heat blackening methods all decrease corrosion resistance by leaving a heat tint or scale that prevents formation of the usual protective passive film, and none produce a consistently uniform dark appearance. Using a blowtorch or similar uncontrolled heat source to blacken stainless steel can significantly harm both corrosion resistance and mechanical properties.<sup>[6](https://www.imoa.info/stainless-solutions/archive/20/Black-grey-colored-stainless-steel.php)</sup> Where a colored finish is wanted on stainless steel for architecture, electrochemical and PVD coloring are the most corrosion-resistant of the non-paint methods, and heat-colored material in severe environments may require a much more corrosion-resistant alloy.<sup>[6](https://www.imoa.info/stainless-solutions/archive/20/Black-grey-colored-stainless-steel.php)</sup>

## References

1. [Heat Tint (Temper) Colours on Stainless Steel Surface Heated in Air, British Stainless Steel Association](https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/)
2. [Bulletin No. 27, United States Geological Survey](https://pubs.usgs.gov/bul/27/report.pdf)
3. [Diamanti, M. V., Del Curto, B., Pedeferri, M. P. (2008). Interference colors of thin oxide layers on titanium. Color Research & Application 33(3), 221–228](https://onlinelibrary.wiley.com/doi/10.1002/col.20403)
4. [Oxide on Steel and Tempering Colours, Phase Transformations Group, University of Cambridge](https://www.phase-trans.msm.cam.ac.uk/2008/Oxide/Oxide.html)
5. [Choi, Y., Jeong, C. Correlation of oxide film thickness with interference coloration and corrosion resistance in anodized titanium. Electrochimica Acta 545, 147737 (2026)](https://www.sciencedirect.com/science/article/pii/S0013468625020948)
6. [Black-grey colored stainless steel, IMOA Stainless Solutions](https://www.imoa.info/stainless-solutions/archive/20/Black-grey-colored-stainless-steel.php)
7. [Color Metallography, NIST Materials Data](https://materialsdata.nist.gov/bitstream/handle/11115/160/Color%20Metallography.pdf)
8. [A new understanding on the electrochemical coloration mechanism of titanium surface, Journal of Materials Research and Technology (2025)](https://doi.org/10.1016/j.jmrt.2025.02.216)
9. [Temper Colors for Steels, in Failure Analysis of Heat Treated Steel Components, ASM International (2008), p 539–540](https://dl.asminternational.org/technical-books/edited-volume/115/chapter-abstract/2203668/Temper-Colors-for-Steels)
10. [Heat treatment and properties of iron and steel, US government publication](https://www.govinfo.gov/content/pkg/GOVPUB-C13-9d3193e93202905da6c546702b1a9d73/pdf/GOVPUB-C13-9d3193e93202905da6c546702b1a9d73.pdf)
11. [Titanium Weld Color Chart: Heat Tint, Oxygen Pickup and Acceptance, Beiyu](https://beiyutitanium.com/titanium-weld-color-heat-tint-contamination/)
12. [Titanium Color Anodizing, Danco](https://www.danco.net/PDF-Downloads/Titanium-Color-Anod.pdf)
13. [Simulation Technology for Titanium Oxide Layer Interference Color, Nippon Steel Technical Report No. 128 (2022)](https://www.nipponsteel.com/en/tech/report/pdf/128-13.pdf)
14. [Features of Titanium Building Materials, Nippon Steel](https://www.nipponsteel.com/product/catalog_download/pdf/T003en.pdf)

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*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Manufacturing processes and fabrication*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 11, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*

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