[WalterBright]

Just stuff the tree into the furnace. It'll burn just fine.

(The only reason the industrial revolution switched to coal was because the landscape was denuded of trees. The Jamestown colony in America was tasked with making glass, glass needs lots of fuel, and England ran out of trees. "Connections" by James Burke)

[blacksmythe]

You have to convert the wood to charcoal to burn hot enough for a blacksmith forge: https://en.wikipedia.org/wiki/Charcoal

I highly recommend a visit to Plimouth Plantation to anyone interested in 1700's era technology: https://www.plimoth.org/

Or Old Sturbridge Village for 1800's era technology: https://www.osv.org/