I've never found a surtling core in a Troll cave.

You need 5 cores each for a Charcoal kiln and smelter. But you can make a lot of charcoal then beakdown the kiln to make the smelter so once you get your first 5 cores you can get smelting.

at your curret tech tier only the Crypts and Buirial Chambers in the Forest will provide cores. There are other sources beyond the Dark Forest but you'll need better gear.

Also a small tip. The Troll hide armor has the same armor values as bronze armor but is easier and cheaper to upgrade so save your bronze for crafting stations, weapons, and a couple boats

Good luck

EDIT: disregard what I said about the Troll armor, aparetly its been changed to be lower than the broze gear since I last really checked,