Heat can have a significant impact on the color of objects. As temperature increases, colors tend to shift, fade, or deteriorate. Understanding these effects is important across many fields including manufacturing, art preservation, astronomy and more. This article will provide an overview of how heat impacts color.
How Color is Produced
To understand how heat impacts color, it’s helpful to first review how color is produced. The color we perceive an object to be is determined by the wavelengths of visible light it absorbs and reflects.
When white light shines on an object, some wavelengths are absorbed while others are reflected. The reflected wavelengths determine what color our eyes perceive. For example:
| Object | Absorbs | Reflects | Appears |
| Red apple | Green, blue, etc. | Red | Red |
| Green leaf | Red, blue, etc. | Green | Green |
The absorption and reflection of light is determined by the atomic and molecular structure of the object. Organic pigments and dyes also impact what wavelengths are reflected.
When heat is applied, it can cause changes to color in several ways:
– Altering the object’s atomic/molecular structure
– Breaking down pigments or dyes
– Oxidation or other chemical reactions
These changes affect how the object interacts with light, leading to shifts in its apparent color. Next we’ll look at some specific examples of how heat impacts certain materials.
Effects of Heat on Organic Pigments
Many colored objects contain organic pigments such as chlorophyll, carotenoids, anthocyanins and more. These pigments reflect certain wavelengths that give objects their characteristic colors. However, when heated, organic pigments can break down or volatilize.
For example, anthocyanins are water-soluble pigments that give many flowers, fruits and vegetables their red, purple and blue colors. However, they are highly heat-sensitive. Exposure to high temperatures causes anthocyanins to break down, resulting in fading or color shifts.
This effect can be seen when purple or blue flowers fade under intense sunlight. It also explains why blueberries turn red when cooked. The breakdown of anthocyanins reveals the underlying carotenoid pigments that produce red and yellow tones.
Similarly, carotenoids like beta-carotene give many plants their orange color. But when heated, these pigments can oxidize and shift toward dull brown and olive green hues. This explains why cooked carrots and squash lose their bright orange tones.
Effects of Heat on Inorganic Pigments
Inorganic pigments based on metals, metal oxides and minerals are also impacted by heat. For example:
– Cadmium pigments used in paints and plastics tend to darken under heat due to oxidation.
– Vermilion, a red mercury sulfide, turns black when heated due to the formation of metacinnabar.
– Heat can alter the particle size and crystalline structure of pigments, shifting their reflective properties. For instance, hematite (red iron oxide) becomes more purple when heated as its particle size decreases.
– Lead pigments such as lead white and chrome yellow darken under heat due to the formation of lead oxide and lead sulfate.
Artists and paint manufacturers must take these effects into account when selecting and preparing pigments. Understanding a pigment’s thermal degradation mechanisms allows creation of more stable, lightfast paints.
Effects of Heat on Dyes
Fabric dyes also exhibit shifts when exposed to high temperatures. For instance:
– Red azo dyes tend to fade and shift toward yellow under sustained heating.
– Anthraquinone dyes like turquoise blue fade to pale pink when heated.
– Indigo dyes, used to produce blue denim, become paler and shift toward red-orange when washed in hot water.
As with pigments, these shifts are caused by chemical changes to the dye molecules. The high vibrational energy of heat accelerates destructive chemical reactions. These changes alter how light is absorbed and reflected by the dye.
| Dye Type | Heating Effect |
| Azo dyes | Fading, yellowing |
| Anthraquinone dyes | Fading to pale pink |
| Indigo dyes | Shifts red-orange |
Understanding dye degradation allows textile manufacturers to select and apply colorants appropriately for a garment’s intended use.
Effects of Heat on Minerals and Gems
While organic pigments are vulnerable to heat damage, inorganic minerals and gemstones tend to be more resistant. However, some interesting color shifts can still occur.
For instance, amethyst (purple quartz) will fade and turn yellow when heated, owing to the loss of trace iron impurities. Synthetic amethyst is especially prone to heat damage.
Tanzanite can also exhibit heat-induced color shifts. This rare blue-violet gemstone has trace amounts of vanadium that alter its color when heated. Jewelers must be careful to avoid exposing tanzanite to high temperatures during manufacturing.
Interestingly, some gemstones are deliberately heated during processing to improve color. Sapphire is heated to intensify blue tones, while aquamarine is heated to reduce unwanted yellow hues. So for some minerals, heat is used advantageously.
Effects of Heat on Metals
When metals are heated, they can undergo oxidation reactions that drastically alter their surface colors. For example:
– Copper turns from orange-red to black as cupric oxide forms. With extreme heating, it can appear green from copper chloride.
– Brass yellows under heat as zinc is lost from its alloyed structure. Prolonged high temperatures turn it reddish.
– Silver tarnishes and becomes gray or black due to sulfur compounds in air.
– Stainless steel can turn blue, brown or grey under very high temperatures from surface oxidation.
These color shifts primarily occur on the metal’s surface. The underlying bulk metal retains its original reflective properties.
| Metal | Heating Effect |
| Copper | Black, green |
| Brass | Yellow, reddish |
| Silver | Gray, black |
| Stainless Steel | Blue, brown, gray |
Understanding thermal oxidation allows metalsmiths to anticipate and control color changes during annealing, soldering, brazing and other processes.
Effects of Heat in Space
The effects of intense heat on color have also been observed on astronomical bodies. For example:
– Spacecraft have observed surface darkening on asteroids as their minerals experience thermal degradation from the Sun’s heat. The darkening occurs over days or weeks of solar exposure.
– Infrared studies show that minerals on Mercury’s surface have been altered by the extreme daytime temperatures, which can reach 430°C. Thermal fracturing and reduction of iron oxide likely contribute to its dark gray color.
– The heat of comet tails as they pass close to the Sun can be enough to char and blacken dust particles as they are ejected from the nucleus. This may be caused by a process called “sublimation” where dust transitions directly from solid to gas.
Understanding these color changes helps planetary scientists study surface properties and compositions. High surface temperatures tend to darken most rocky materials through dehydration, reducing atmospheres, and destruction of light pigments. Only a few minerals like silica remain bright.
Conclusion
In summary, heat can induce a wide range of color changes across many materials. Organic pigments and dyes tend to fade, shift hue, or darken due to decomposition. Inorganic pigments experience changes in oxidation state and crystal structure that alter their light absorption. Metals form surface oxides that transform their appearance. Even gemstones and astronomical bodies exhibit color shifts from extreme temperatures.
By studying thermal color degradation mechanisms, researchers can better preserve artwork, select durable pigments and dyes, process minerals and metals appropriately, and interpret the geology of distant worlds. So while high temperatures often have detrimental effects on color, these effects give us insights into the atomic nature of matter. With careful study, heat’s impact on color reveals many secrets.