Well Jean, it seems you know the risks, but are not concerned about them.
I rest my case.
The blade is made of iron and steel and possibly nickel or other contrasting material. Only the steel is affected by heat treatment to harden during manufacture, and most blades are not hardened closer to the gonjo than about 6 or 7 cm. at most; often it is only the first few centis that are hardened, since the application of heat to remove a hilt is restricted to the sorsoran, the part of the blade that gets hot has not been hardened, which means that nothing short of red heat will affect that part of the blade --- of course, if you took it to red heat, the hardness further down the blade would be drawn because it would go into blue/yellow or higher, if it went higher than blue/yellow you would lose the hardness.
The way I work is to hold the hilt in one hand and the bare blade about 12 centis from the gonjo with the other hand. When the blade gets uncomfortably hot I take it away from the heat for a couple of minutes, then return it when it is cooler.
Working like this I can control the heat 100% and at the same time I can feel if the hilt itself is getting too hot, bear in mind that my hand is shielding the hilt from direct heat, any heat I can feel in the hilt is coming from the pesi.
When at home I use a propane torch with a relatively bushy flame that I hold in a vice and I pass the blade backwards and forth across the flame and alternate the sides, I can usually get 99% of hilts off in a few minutes, the very, very few that are stubborn get the heat/cool/heat/cool treatment over a number of days until they surrender.
When I'm in Solo I use either a little kerrosene lamp or a candle and play the flame directly and steadily onto the blade, when it comes time to try to separate blade and hilt I use an old bit of rag to grip the blade.
Using the lamp or the candle it is totally impossible to get the blade hot enough to do it any damage at all.
Using the propane torch it would be very easy for an inexperienced person to draw the blade hardness.
My concerns with hair dryers, ovens, boiling water , mirrors reflecting the sun's rays, geothermal heat, room heaters, etc, etc, etc is not that they will harm the blade material, but that because the heat cannot be localised and directed to a sufficiently small area, that heat will inevitably heat the hilt, and it will cause damage to some degree.
In respect of wood used for keris hilts.
It is recognised that it is not good for any wood to be exposed to direct sunlight, as the heat can cause cracking.However, the one wood which must never be exposed to heat or sunlight is tayuman. Another wood that is very susceptable to heat and moisture change is burl teak ( jati gembol). This wood moves and cracks can open up in it because of the movement.
Really, its just common sense:- nobody would consider for a moment that it was OK to place a piece of antique furniture in direct sunlight. Do this on a hot day and you could come back to something where all the joints had loosened and cracks had opened in the polished surfaces --- you might get away with it a couple times, but sooner or later you'll ruin something. So you just don't take the risk. No different with the wood used for keris dress. The caution should not be limited to only antique dress:- make a few enquiries of modern custom knifemakers and see how many will gaurantee natural materials in hilts and scabbards against cracking or distortion.
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