The key point below is the absence of any mention of smelting iron ore. You need coke to do that. The mention of the Open Hearth Furnace is saying they took pig iron and by melting it and adding a bit of this and that changed the iron into a complex compound – steel. Obviously melting the iron requires a lot heat but that could be from any fuel. I’d put my money on natural gas for most of the 20th century (with the chance they used coal before the 1930’s) because there are natural gas wells in the area, including some just on the other side of the Sacramento River opposite Pittsburg.