HAYS COUNTY, Tex. — In a former machine shop here in the Texas Hill Country, Jeffrey Stuffings, a founder of Jester King Brewery, led a tour group up a narrow staircase, to a loft where dozens of oak barrels surrounded what looked like an enormous copper sheet pan. Stained-glass windows bathed the pan in a pink and orange glow.

“This vessel right here is called a coolship,” Mr. Stuffings said.

It’s a specialized piece of equipment, he explained in reverent tones, used in Belgium to brew the sour style of beer known as lambic, which is made through a traditional process that Jester King has been following since 2013. On cool winter nights, the brewers open the loft to the elements and pump in a lambic-style recipe made with raw wheat and aged hops.

“There’s so much steam that you would not be able to see me,” Mr. Stuffings said. “The coolship kind of creaks and groans.”

The miracle of the lambic process — which yields some of the world’s most complex and captivating beers, yet has caught on in America only recently — is what happens next: The beer starts to ferment spontaneously, no starter required.