How I almost set fire to my garage...

[Jonathan]

You mention it getting hot enough to anneal the collet taper, forgive my ignorance but why would this be an issue?

The spindle shaft is machined from a grade of steel which can subsequently be hardened and ground. This is necessary to ensure that the wear caused by changing the tool/collet doesn't wear the taper out (much), which would lead to high run-out. If yours has been heated above a certain temperature (which depends on the grade of steel), it will be softened and will wear out much more quickly. The temperature range is quite wide though, and we've not really got much idea how hot it got, although the colour can be an indication of that. Either way it's hard to tell...