Several times over the last year I have been asked to comment on a photo of one of the Trade Center Towers. The photo shows a molten flow from one of the windows. The flow falls down along the building. It appears orange and turns to a gray color as it cools.
The questions usually want me to address "Is this photo a fake?" and "Is the flow steel or aluminum?" "Is this situation possible?"
First, I will address the temperature range, then the color of the flow.
I am working in imperial units and temperature in degrees F [To convert to C use this link]
Metals lose about 50% of their strength at 60% of their melting temperature. This is common knowledge and may be found in any undergraduate text regarding "Fracture and Deformation of Materials."
If the approximate melting temperature of steel is 2750 F the the material would be plastic at 1650 F. Even assuming a safety factor of 3, you would expect the bolts or other structural members to deform and fail near this temperature, especially with the additional weight if a jet air liner. I would assume that the live load calculations did not include the typical office equipment and an airliner plus a factor of 3. THEREFORE I assume that the flow is not steel and that the temperature of the steel members at the time of the photo is less than 1650 F.
Assuming that the flow would be molten aluminum from the airliner and the color of molten aluminum is silver then why is the flow orange?
The color of pure molten aluminum is silver, It has an emissivity of .12. Steel has an emissivity of .4 and appears orange in the temperature range of molten aluminum.
The emissivity of aluminum oxide is .44 and also appears orange in the melting temperature range of molten aluminum.
The emissivity of plate glass is .937 It begins to soften at 1000 F and flows around 1350 F. Silica has an emissivity of .8
Copper oxide also has an emissivity of .8. however I will assume that their effect is negligible.
Aluminum oxidizes readily in the foundry under ideal melting conditions. Large surface area relative to thickness, turbulence, the presence of water or oil greatly increases the oxidation of aluminum. A jet airliner is made of thin aluminum sheet and most probably suffered considerable oxidation especially in contact with an open flame and being in contact with jet fuel. If you don't believe this, try melting a few soda cans over coals or open flame. If you are lucky you will end up with only 50% aluminum oxide. However, the cans may completely burn up.
The specific gravity of aluminum is 2.7. The specific gravity of aluminum oxide (Al2O3-3H2O) is 2.42 the specific gravity of Si = 2.40 and Glass is 2.65 these are all very similar and likely to be entrained in a molten aluminum flow. Don't believe it? lightly stir the dross into molten aluminum. The surface tension is so high is is almost impossible to separate them.
THEREFORE assuming that the flow consist of molten aluminum and considerable oxides, and assuming that the windows in the trade center were plate glass and also in a plastic state and that they were also likely entrained in the molten aluminum. I would expect the flow to appear to be orange in color. Especially since both the entrained materials have emissivities equal to or more than twice that of iron.
Also since dross cools to a gray color and glass with impurities also turns dark. I would expect that the flow would darken upon cooling.
I would also suggest that not only is the photo possible, but entirely likely.
Summary: The flow is not steel because the structural steel would fail well below the melting temperature. The flow is likely to be a mixture of aluminum, aluminum oxides, molten glass and coals of whatever trash the aluminum flowed over as it reached the open window. Such a flow would appear orange and cool to a dark color.
Stephen D. Chastain
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