I know that this is a very OLD topic, but it really bears restating CORRECTLY!

Platinum, iridium and rhodium are all members of what are called the platinum group of metals; each of them are effective in initiating chemical reactions, either alone or in combination (alloys).

Platinum alone is best but is usually alloyed with one of the others for mechanical reasons (strength, durability), and for COST!.

YES, A HOT ENGINE CAN MOST CERTAINLY START WITHOUT A VOLTAGE BEING APPLIED TO THE GLOW PLUG: most every modeler of my age has seen a hot early model Fox .35 start this way. those early, sand cast jobs had a much higher compression ratio than the later models, 8.0:1 vs. 5.25:1 for the more recent ones, and were very prone to such behavior.

Sorry to be so blunt, but any other opinion is just foolish jawflap: there's democracy, to be sure, just don't ever lose sight of the important thing which is - what is, IS!, regardless of what we might wish for, or wish things to be.

I don't imagine that anyone will ever pay attention to this, but it really needed to be said, once and for all time.

Dear Lord: thank you for allowing me to be an engineer...

Morty.