When glass is heated intensely, the color it displays is a direct visual measurement of its temperature. The term “red hot” is a universally recognized descriptor for extreme heat, but it represents a specific, measurable range on the temperature spectrum. This visible glow is a phenomenon rooted in the laws of physics, providing glassworkers and scientists with a precise thermal indicator. Understanding the temperatures associated with this red coloration is key to comprehending the science of thermal radiation and the practical art of glass manipulation.
The Science of Incandescence
The physical process that causes glass to glow when heated is called incandescence, which is the emission of light from a hot body. All matter emits thermal radiation because of the movement of its constituent particles, but most of this energy is invisible to the human eye, existing in the infrared spectrum. As the temperature of the glass increases, the energy of the particle movement also rises, causing the peak wavelength of the emitted radiation to shift. This shift is governed by Wien’s displacement law, which dictates that hotter objects emit light at shorter wavelengths. The first wavelength to become visible to the naked eye is the longest visible wavelength, which is red. This initial, faint glow typically appears once the glass reaches approximately 977 degrees Fahrenheit (525 degrees Celsius).
Temperature Ranges for Red Hot Glass
The “red hot” phase is a thermal range divided by the shade of red emitted, a scale often used by metallurgists and glassworkers. These distinct color changes provide a simple, immediate way to estimate temperature without a physical thermometer.
- Faint Red: Visible around 930°F (500°C or 773 Kelvin).
- Dark Cherry Red: Approximately 1,175°F (635°C or 910 Kelvin).
- Medium Cherry Red: Near 1,275°F (690°C or 965 Kelvin).
- Bright Cherry Red: About 1,450°F (790°C or 1,060 Kelvin).
The correlation between color and temperature is constant because it relies on the fundamental physics of light emission, not the material itself.
Working Temperatures and Glass Behavior
The temperatures within the red-hot range directly correspond to the mechanical state of the glass, making the glow a practical guide for artists and manufacturers. Glass is an amorphous solid, meaning it transitions gradually from a hard, brittle state to a soft, pliable one, rather than having a distinct melting point. The red-hot glow signals the material’s entry into the practical working range.
Once glass reaches the cherry red range, its viscosity—its resistance to flow—drops dramatically. This reduction in viscosity allows the material to be shaped, blown, and manipulated without fracturing. The working range for many common types of glass often falls between 1,000°F and 1,700°F (538°C and 927°C), covering the red and orange spectrum. At these temperatures, the glass is soft enough to be deformed by tools or air pressure, yet still viscous enough to hold its shape.
The Full Spectrum of Heat
As the glass continues to heat beyond the red-hot stage, the color progresses through the visible spectrum toward shorter wavelengths, indicating substantially higher temperatures.
- Dark Orange: Appears around 1,630°F (890°C or 1,160 Kelvin).
- Orange: Approximately 1,725°F (940°C or 1,215 Kelvin).
- Lemon and Light Yellow: Reaching temperatures near 1,975°F (1,080°C or 1,355 Kelvin).
- White (White Hot): Typically seen at 2,200°F (1,205°C or 1,480 Kelvin) and above.
At white-hot temperatures, the glass is highly fluid, resembling a thick liquid that flows easily. This entire color progression serves as an accurate thermal map for anyone observing the process.