Find Orion. Starting at the foot on the right, there's a stream of stars making up the Eridanus River. The Furnace is located in the bottom curve, as shown below.
The Furnace is one of the BS constellations made by Nicolas-Louis de Lacaille, so there's not a lot to look at. Like you can see one star, maybe two if you're lucky.
So allegedly this is a chemical furnace. How I interpret that is that the brightest star is the fire of the furnace, and any extra stars you see nearby are like the little beakers and flasks and such that you boil chemicals in.
Source: link
