I'm thinking about getting my cast manifold coated and descided to do a little research. I've noticed quite a few of the coatings are black. Now I know heat (energy) can be transferred via 3 methods: conduction, convection, and EM radiation. The coatings regardless of color can be effective conductive barriers, but anyone who's taken any thermo or physical chem will tell you black is transparent to blackbody (EM) radiation. We want something optically reflective like "silver" in color that gives a reflection back. I knw there are some silver coatings out there, but why are the higher temp coatings black when a more effective barrier would be optically reflective? Is the EM heat proportion insignificant?

 

One reason black and gray are common is that some silver coatings get their color from aluminum which melts at a lower temp than the other colors. Does the color even matter when you are trying to prevent heat transfer from the base material to the air and not the other way around?

 

Reflection of radiant heat is not a concern because the item that is coated is the item radiating the heat. Heat conducts through the metal, so a barrier to conduction to the surface of the material where radiation occurs is the only concern. A silver layer would simply reflect radiant heat that had already been reflected elsewhere.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beepy &raquo;</TD></TR><TR><TD CLASS="quote">Reflection of radiant heat is not a concern because the item that is coated is the item radiating the heat. Heat conducts through the metal, so a barrier to conduction to the surface of the material where radiation occurs is the only concern. A silver layer would simply reflect radiant heat that had already been reflected elsewhere.</TD></TR></TABLE>

Thats what I was thinking, wasnt sure though. thanks

 

I had thought of that explanation, but I wasn't sure of it. I'm still not exactly convinced though. For example if we have a red glowing piece of cast iron and surround it (with a small air layer in between) with an insulating optically reflective material. In this instance the we are insulating for convection and EM. If we put this insulative material ON the hot cast iron surface, how are the optically reflective properties changes. Just 'cause it's in contact with the hot surface doesn't change it's properties. Sure the insulator will also get hot and emit EM, but it's not the original source of the EM radiation. The cast iron is.

The distance between the two materials should be immaterial with respect to optical properties. Whether I lay a mirror directly on a stove eye or 1 mm above it, it will still reflect or more appropriately whether the thermal coating is directly in contact of not it's properties should not change. I not sure if I'm being articulate enough. Material science was not my thing in undergrad. Let me know if I'm looking at this all wrong.