He'd be cutting for the rest of his life with a carbide chain, they're very slow and dead ash can be very hard, although some is rotten as can be.
OP:
Flush cutting with a semi-chisel is the quickest and easiest way to deal with these stumps.
When cutting wood with dirt or stumps, do everything you can to avoid pulling dirt thru the cut. If the tip of the bar comes out the opposite side of the stump, make sure you are using the top side of the bar so the chain throws the dirt away, this will keep the chain sharp much longer than if you pull any dirt through the cut.
When I have large stumps or many to do, I rent a large 55hp diesel grinder that's about 600 a day with rental fee, tax, and fuel. I use it on customers stumps, then bring it home and do whatever stumps I've made at.the house and neighbors. If I have very large stumps I'm doing for a client(36 or larger), I will normally do over 20 stumps average, but most times I'm doing around 30 and ha e done over 40. That number includes driving 25 min either way to pick up the grinder, then add two or three stops with unchaining/unloading, reloading and chaining, filling it with fuel, and doing a great job blowing it off with my backpack blower. When I grind stumps I'm typically grinding them very deep and chase roots ,then remove the grindings(Usually the next day), then add topsoil and seed. Yours will go much quicker if you are just flushing them to get over with a mower, when I'm at a client's that doesn't want to spend the extra on grinding I just upcharge for sharpening time and my chain and cut them very close, it works just fine, but some people want to plant grass or a new tree.
Hope this helps.
Here's a cherry stump I ground, you'd be there for over an hr with a small grinder to achieve similar results, my time is more valuable than saving a few bucks with a little grinder, but I'm sure there's a place for them.
[MEDIA=youtube]5yHlqxdY1tc[/MEDIA]
[USER=19757]@pioneerguy600[/USER] could we get this moved to the appropriate forum. Thanks 
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