A lamp furnace starts from an oil lamp, a small, steady flame fed by a wick drawing fuel from a reservoir, chosen precisely because it burns more predictably and controllably than an open fire built from wood or coal. A blowpipe is what turns that gentle lamp flame into a working heat source: held to the operator's mouth at one end and aimed at the lamp's flame at the other, a blowpipe lets a steady stream of breath bend and concentrate the flame into a narrow, hot point aimed exactly where the worker needs it, rather than letting the flame simply rise straight up and spread its heat over a wide, unfocused area. That combination, a small controllable flame plus a directed stream of air, gave a goldsmith or enameller a heat source precise enough to solder a tiny joint, fuse enamel onto metal, or work fine detail on a small object without heating, and risking damage to, anything around it.
Because the whole apparatus was small and portable, a lamp furnace could sit directly on a bench in front of the worker rather than requiring the worker to bring materials to a large fixed furnace, putting the heat source exactly where close, careful handwork actually happened. Several workers could each keep their own lamp furnace burning at once on a shared bench, since the apparatus needed so little space or fuel compared with a single large central furnace.