In my usual habit I'll reply off the bottom of the que... Lots of good
comments and suggestions...
In no particular order:
I suspect the surface of the black tire had cooled below ambient by
convective radiation... Happens to my windshield on mornings above
freezing ambient whilst the windshield will be a skim of ice...
Though the thought of what the tire might read under non-convective
conditions occurred and was duly filed away in that great container in
my head for further inspection... Actually I use the Rubble Method for
filing information.. That is everything in my head is tossed on a
great heap of rubble, and as I stir through the pile looking for one
item, interesting bits of flotsam and jetsam flow back to the top of
the heap for inspection, "oooh ya, I remember that!"

Yes, the thought that there might be unwanted resistance in the
connections to the condenser plates occurred and remains to be
investigated... The connecting wire is the #10 magnet wire forming the
coil and is firmly bolted with #8 brass hardware and flat washers to a
1"x1" tab formed on the edge of the plate when I cut the plates from a
sheet...

For Yuri, congratulations on the CQ160 win(s), nice job my friend...
Drop me a description of his antenna setup... Wanna be's like me are
always looking for an edge...
On the IR gun, it was in the ~$65 range on sale, as their economy
model, I don't remember who the internet vendor is, I can get you the
make and model if you need it... But just do a search, lots of sources
and prices... The little bit I have used it I have been satisfied with
the readings - nothing rings my 'no-way, Jose!' meter, so far... The
35' long radiant tube heater in my shop shows ~375-385 F at the mid
point from 8 feet away, which is about right as it will not ignite
paper on contact I tried though it makes it brown - paper has an
ignition temp of ~454 F per Ray Bradbury at least... The gun has a
laser spot so you can see where the detector is aimed and the size of
the spot is proportional to the area being measured at that distance...

I suspect that dielectric loss in the soda glass is the prime
contributor to the temperature rise on the plates... There is a
significant amount of joules passing through the glass as strain in the
dielectric..

Oh yeah, the question on the type of glass... Bad habits as an old lab
rat surfaced when I wrote that... Common window glass is float formed
on molten tin, and that is what my old chemical stained brain popped
out as 'stannous'...

On differential IR readings from the surface of various materials -
well I can't say for sure as my credentials in thermodynamics are shaky
at best (picture me shuddering in pain at the memories of Schrodinger
equations and those IBM punch cards we used to program the computer in
the physics lab 40 years ago - 'the horror, the horror'..... But, all
the materials in the tuner were within a few tenths when at ambient, so
I have no reason to suspect a gotcha when midly warmed...

denny / k8do - often confused but never in doubt...