This narrow passage allows hot gases to flow from the primary melting furnace into an adjacent chamber, such as an annealing arch or caulker. The restricted diameter controls the draft and heat transfer, keeping the secondary space at a lower, stable temperature. Its name comes from the traditional comparison that the opening is only wide enough for a small bird to fly through.
Heard on the job
Check the draft on the caulker to make sure the linnet hole isn't choked with slag.
Insider tip
Because of their narrow diameter, these flues are prone to clogging with soot or volatilized materials and require regular inspection to maintain proper draft.