I CONDUCTED A TEST --- I got together some materials for a rudimentary test of the charcoal in clay radiant heat barrier.. Two equal quantities of clay were mixed up. One completely raw chunk was formed into a quarter inch thick disk about the size of a hockey puck.
The other glob of clay was mixed with a pile of powdered charcoal about 20% as large as the glob of clay. It was also formed into a disk near identical to the other one.
Both disks were allowed to dry for a week. After 2 days they were pretty much dry. Once dry, the one containing charcoal was only slightly darker than the other one.
Today I did a test. Each disk was propped on edge and a
light bulb was focused on each of them for five minutes. The bulb was exactly 6 inches from the disk on each test. The bulb was positioned across from and not below the disks so that any convective current rising from the hot bulb would not strike the disk. We're testing for radiant heat resistance only.
RESULTS --- The disk made of raw clay got quite a bit hotter than the one containing charcoal. Although it was a crude test, the result is promising. It appears that some heat was reflected away from the disk containing charcoal. Therefore, I will build two equally sized “igloos” from the two mixes and run the hot brick test.
Once finished with the disks, I coated both with vegetable oil since I didn't have a drying oil like linseed. It readily absorbed and seriously darkened both disks. The one with charcoal went almost black. After 6 hours, both took on a lighter colour and much of the contrast between them faded. If the right oil were used it might be possible to retain this wet look.
WATER TEST --- I ran water over both disks and was surprised to find that vegetable oil doesn't seem to offer any rain protection. Both disks took on water and with a light rub of my finger it was easy to erode the surface into a muddy mess. Drying oils like linseed leave a surface residue and shine which makes it more difficult for water to soak through. . I won't make the mistake of using any untested oil finish on a real building. I'll probably go for a time tested lime wash.
SNAP TEST --- Nothing fancy. I snapped both disks with a twist of the wrist. The one containing charcoal broke a little easier but both were surprisingly strong.
I'll post more once more scientific results are in. --- Dale