Why Does Black Glass Melt Fastest?

It is not uncommon for glass fusers to notice that black glass comes out of a kiln processed more than clear glass. Some have also noticed that the use of iridised or dichroic glass seems to exacerbate uneven heating, commenting that it has something to do with heat energy being reflected. All these observations about how glass behaves when heated are entirely correct, although aspects relating to glass formulation also have a bearing.

My intention for this posting is to introduce the some of the physics to explain the causes behind the observations without getting ridiculously technical. I also hope that glass artists will notice how useful science can be for their own work.

The underlying issues for this posting can all be related to the physics of black body radiation and how light energy is transmitted and reflected by glass. It also means the exact same principles actually apply to glass when cooling, though I’ve not noticed any glass fusers mention this.

You may remember from science lessons, or through literature on energy savings measures in the home, that heat energy is moved around by conduction, convection and radiation. Although they all have a part to play, it is radiation that is most important when heating glass in a kiln.

Thermal energy is what we commonly refer to as heat energy and in turn can also be referred to as infra-red radiation. The terminology differs because they originate in different branches of physics. But it turns out they are all one and the same thing really. Heat energy, or infra-red radiation energy, is supplied by kiln elements (whether by red-hot wires or infra-red lamps) and ultimately gets absorbed by everything inside the kiln, including the kiln interior surfaces. Why one kind of glass should absorb this energy in a different way than another kind of glass is the key theme for this posting.

Infra-red radiation is a portion of electromagnetic spectrum with wavelengths that are adjacent to red light, the only difference being that infra-red has longer wavelengths and therefore less energetic energy than red light of the same intensity. In this sense infra-red radiation is nothing more than a form of light that is similar to red light but invisible to the human eye and a little less energetic. We can’t see infra-red light but we can certainly feel it as heat. Infra-red radiation is very important for glass fusers because it is the main method by which energy used to melt glass in a kiln, and as glass cools it is the main method by which energy is dissipated.

By contrast, ultra-violet radiation is a portion of electromagnetic spectrum with wavelengths adjacent to violet light, the only difference being that ultra-violet has shorter wavelengths and is therefore more energetic energy than comes from violet light of the same intensity. In this sense ultra-violet radiation is nothing more than a form of light that is similar to violet light but invisible to the human eye and a little more energetic. But, interestingly, insect pollinated plants often exploit the ability of insect to see ultra-violet light radiation by making their flowers particularly attractive in the ultra violet part of the spectrum. We don’t see ultra-violet light but insects can. Ultra-violet light is only important to glass fusers who choose to open hot kilns for reasons that are revealed at the end of this posting.

For our purposes, even though not strictly correct, we can interchangeably substitute the term “light energy” (optionally including infra-red and ultra-violet) whenever we see the term “electromagnetic radiation” because the former is just a sub-set of the latter.

We now need to consider the concept of black bodies (and black body radiation later). It is a theoretical concept that is useful to know about because it does have implications for how energy is absorbed and emitted by real-world materials, including glass.

All materials we normally encounter absorb electromagnetic radiation to some degree. A theoretical perfect object that absorbs all radiation falling on it at all wavelengths is called a black body and any matt black object we encounter in real life is a fair approximation.

Now that we have some understanding of ideal theoretical black body from physics, we can now consider the real world. Unfortunately most objects are less than perfect examples of a black body. Red objects are reflecting red light, not absorbing it. Shiny objects reflect light because they are shiny. They are not behaving like a theoretical black body yet they are the key to understanding why some colours and surfaces of glass behave differently.

To address one of my opening comments, the ability of visible light to pass through clear glass as opposed to black glass needs considering in the context of what we know about black bodies.

Visible light energy that is not reflected by the upper surface any piece of glass will pass into the body of that glass so has the potential to be absorbed. The same applies to infra-red light energy and therefore to the rate by which a piece of glass will get heated. But as we know from observing visible light passing through a window pane, much of the light energy is likely pass straight through clear glass, so in our kiln it will reach the kiln shelf. But we can also guess see that no light passes straight through to the kiln shelf for black glass.

In our kilns, black glass must be absorbing most of the infra-red radiation energy, because it is not letting it through to the kiln shelf and only a little of the incident energy is being lost by reflection off the top shiny surface.  By contrast, clear glass can not be absorbing much of the infra-red radiation energy because it is reflecting a little of the incident energy off the top shiny surface and then letting most of the remaining energy pass through to the kiln shelf.

The black glass seems to be a reasonable approximation to the theoretical black body but the clear glass is not behaving remotely like a black body. This leads us to an understanding that darker colours that are opaque will heat up more quickly like a black body rather than paler and lighter colours that are transparent which are less like a black body.

Now that we have an understanding of why black glass and clear glass behave differently, it’s time to consider how different colours of glass might behave.

Consider a red apple. It looks red because it reflects lots of red light energy and must therefore be absorbing the light energy of other colours so it is not a perfect black body. Similarly, a blue ball must reflect a lot of blue light but absorb all other colours. But we are in the real world with real materials that do not absorb or reflect colours very precisely. In truth, the red apple is reflecting mainly red light but also some orange and a little yellow light. And the blue ball predominantly reflects blue light but will also be reflecting some greens and purples. With these real-world facts about our imperfect black bodies, that are being heated by infra-red energy, we begin to understand that colours of glass that tend to reflect light at the red end of the spectrum are likely to heat up more slowly than colours that tend to reflect light at the blue end of the spectrum simply because the source of energy to heat the glass comes from the red end of the spectrum.

The intensity of a colour should also have a bearing on how fast the glass will heat up. A pale pink opalescent glass must be reflecting all colours of light energy to a significant extent but with slightly more of the red light. A deep red opalescent glass by contrast is reflecting lots of  red light but only a little of other colours. Because the source of heating is infra-red light we expect the deep ref opalescent glass to heat up more quickly. Although neither of these colours of glass are remotely like a black body, the pink is less so because it is reflecting most of the incident light spectrum.

Our final consideration relating to heating glass is the very top surface of the glass. To be a perfect black body requires a surface that does not reflect any of the incident light energies. This means a perfect black body has to have a matt surface which in turn means that a shiny surface is not ideal. We can therefore predict that iridised glass and dichroic glass will tend to heat up more slowly than glass that has no shiny metallic coating because they are reflecting more of the incident light. But as glass fusers you will know that scuffing-up the surface of your glass is going to provoke devitrification – so stick to the lesser evil of glass that heats up just a little bit slower!

To finish my story, I should address the cooling process as it relates to black bodies and to real-world glass. There are some interesting observations that relate to glass fusing as much as they do to astrophysics.

All materials we normally encounter, including glass, emit electromagnetic radiation at all times. A black body will emit electromagnetic radiation that has a characteristic frequency (or wavelength) distribution that depends on the temperature and is called black-body radiation. At normal temperatures the characteristic wavelength distribution turns out to be nothing but infra-red light energy so we don’t see it (but can feel it as heat).

Black body radiation becomes useful for glass fusers when temperatures are in excess of about 500°C. At around this temperature any substance, including glass, will emit energy in the visible spectrum as a dull red glow. As the temperature rises the light being emitted will have increasing amounts of energy which in turn causes it to have shorter wavelengths which means the colour will move up the spectrum from red to violet in accordance with what we should expect from black body radiation theory. This is very important because it means you can tell the temperature of a hot object by the colour of light being emitted.

You have probably noticed that glass in a kiln appears red when it is hot and can be an orange colour when it is even hotter, and why you can not see these colours at normal temperatures (because it is being emitted in the infra-red part of the spectrum). Again, we are assuming our glass behaves like a theoretical black body but it is useful to know that you can tell the temperature of hot glass by the colour of light it emits.

Another consequence of black body radiation theory is that glass at around 500°C will be a dull red colour disappearing into the infra-red as the temperature drops. By chance this happens to be around the same temperature range as you should be annealing. If you open your kiln and see the dull red colour disappear then you have crash cooled where you should have been carefully annealing!

Black body radiation also explains why ultra-violet light is a hazard when opening a hot kiln. The interior of a hot kiln is emitting ultra-violet light as a direct consequence of its temperature, higher temperatures causing more ultra-violet. Now you know why you need those special sunglasses – it’s all down to black body radiation.

A final teaser is for you to think about how the principles of black body radiation might affect the cooling behaviour of glass. Will black glass cool faster than clear glass? Will an iridescent coating slow down the cooling? How might these issues affect the annealing and final cooling of your next masterpiece?

A visit to http://en.wikipedia.org/wiki/Black-body_radiation is a good starting point to learn more about black body radiation and http://en.wikipedia.org/wiki/Pyrometry for information about how black body radiation is used to measure high measuring temperatures.

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About chatterglass

Maker of stained glass frippery.
This entry was posted in Annealing, kiln schedule, Melting Glass and tagged , , , , , , , , , , . Bookmark the permalink.

One Response to Why Does Black Glass Melt Fastest?

  1. Pingback: Which Solder Should I Use? | Chatter Glass

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