Ballast questions

I have over the years heard that cinders were used as ballast on the San Juan extension. I am guessing these are cinders obtained from cleaning the fireboxes of the locomotives? Is there a different type of cinders that were used for ballast?

Was the narrow gauge ever in its history ever ballasted in a manner similar to the work taking place on the C&TS now and as was done on the Silverton branch?

How long should this work last before it will need to be redone?

How does ballast made from crushed rock work? Why is this seemingly the best material for this job?

Why does track that is ballasted in the current manner hold up better, or ride better, than if ballasted with another substance?

Why is well ballasted track easier on the running gear of the locomotives?

Thanks for answering my questions. I hope other readers would like to know these answers as well.