Hello,

A thread( any simple thread) when brought near a candle flame, burns. But, when this thread is wound over a coin and then brought near the candle flame, it does not burn. Why???

I never saw that personally, but if it does occur as you say I would guess that the coin conducts and radiates heats so quickly that the thread can not reach its kindling temperature. I friend once told me a story of using cutting torches on large lumps of naturally occurring copper. The heat is conducted away so fast by the copper that the torch was inefficient.

As far as I know the same happens with a plastic bottle filled up with water. It won't burn/melt in a bonfire.

I that case, the water must boil off at 212F before the temperature can rise enough for the plastic to melt and burn. The trick to walking on hot coals barefooted is to have the bottom of your feet sweat. This will create a protective water layer on the bottom of the feet that keeps the temperature down.

The outer core of a candle flame has ~ 1500 K

I don't know the autoignition temperature of thread is
lets say it is about as high as the autoignition temperature of paper

The autoignition temperature of paper is 450°C
the average temperature of a room is about 20°C
Δt=430 K

The temperature conductivity a is divided the warmth conductivity "lambda" through the product out of density "rho" and "specific Warmth capacity" c:
a = lambda / (rho * c)

how much is rho of thread and "what the coin is made of"
how much is c for thread ...

anyway the result will show that a(thread)is lower than a(coin)

so the heat is "absorbed" by the coin and the coin is isolated by the thread by being wound around the coin.
The thread wont ignite because the temperature does not reach the "autoignition temperature of thread" unless the coin has nearly that temperature.

If anyone has an hour spare please calculate
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please ignore grammar

Metal (the coin) absorbs a lot of heat. This is why metal is the hottest thing in a hot place, and the coldest in a cold place e.g. outdoors. The thread doesnt want to get very hot, but the coin does, so thread most the heat for itself and therefore it does not burn.

Isn't the AIT of paper 451 Farenheit (hence the Ray Bradbury book) = 233C?

this is hard work