walls of the duct are made in steel alloy which is highly resistant to high temperatures application_number US 16215588 A description with reference to fig1 a furnace for firing the insulating plastics resin cladding of electrically conductive copper wires is generally indicated with the reference numeral 1 which furnace comprises in a known way a principal chamber 2 provided with an inlet opening 3 and an outlet opening 4 in which the wire 5 advances longitudinally in a continuous manner this chamber 2 has an elongate form and comprises a first portion 6 the cross section of which increases towards the inlet opening 3 and a second portion 7 of substantially constant cross section along the side walls 8 of the first portion 6 are disposed heating elements 9 conveniently constituted by electrical resistors a a first temperature sensor 10 housed in the said first portion 6 and a second temperature sensor 11 housed in the second portion 7 close to the outlet opening 4 detect the temperature in these portions the furnace 1 further includes an auxiliary unit 14 disposed alongside the principal chamber 2 this auxiliary unit 14 includes a first fan 15 facing an aspiration opening 16 communicating with the principal chamber 2 close to the junction between the two portions 6 7 of the chamber 2 itself and operable to draw in a first stream 17 of a mixture of air and solvent vapours produced in the first portion 6 a first heat exchanger 19 and an electrical resistance heater 20 housed in a first duct 22 a first primary circuit 19a of the heat.