Deteriorating High watt resistors.

I am working on a guitar amp that has been setting in the attic for years. It a repair job that I'v been meaning to get to but couldn't fing the time.

So while cleaning off the years of dust and funk, I noticed that the powder in a coupla 180 ohm 10 watt resistors has begun to break apart.

I will just replace the resistors but I've never seen this breakdown of a resistor of this type.

I was wondering if any of you have seen this and what would cause it? Heat in the attic? BTW the amp is an ampeg SS140C which when working sounded great.


Thanks

Denny