In my experience, sawdust is a fire
lighter, not a slow burner.
BH collects up the chips, shavings and dust when he's been doing woodwork, chopping or sawing, and it lives in a container next to the fire. When you want to lift a fire that's down to the embers, a shovel of sawdust does a great job.
A local woodworking company makes compressed sawdust rings - they have an water extractor thingy, bung in a bales' worth of sawdust and out comes a small sackful of rings. They're about the size of a large jamjar lid, 1/2" thick. I use two or three when starting a fire, or again they will lift a fire that's down to a deep red glow.
I do know of people who use compressed sawdust bricks - but I'd expect them to burn up pretty quickly, certainly quicker than an equivalent volume ash log, for instance.
Another good fire lifter is dried orange peel. We put our peel on top of the woodburner (on a trivet or it would burn on the top!
), then when you want to lift a fire, an orange-worth of peel will give you a firelighter effect.
There's a lot to know about different woods. A big thick ash log should keep most fires in all night. Larch won't, it's a fast burner. I think I remember that beech is at the fast end, oak at the longer slower burn end of the spectrum. If I'd been able to do another stint at Tinkers' Bubble, the characteristics of the different woods was on my list of things to learn about - they knew so much, those folks.
And yes, horse poo will keep a fire in overnight!
And help you restart it in the morning, too.