known as 34 carcass grades 34 a preferred but optional feature shown is the use of hollow bars 616 for the flame holder 606 so that a portion of the premix or of the gaseous fuel may be used to cool the hollow bars 616 of course this feature may be used in connection with any assembly of burner or burner and reactor according to this invention the hot gases of combustion from zone a entrain the droplets 622 of hydrocarbon feedstock so that the heat in the combustion gases causes the feedstock to be thermally converted to carbon black which is borne by the gases down the reactor to a quench zone b where atomized water 624 issuing from nozzle 626 to which it is supplied by supply pipe 628 reduces the temperature of the gases and carbon black to a level such that the reactions are stopped and the carbon black may be collected by any conventional collecting equipment not shown to which it passes through outlet 630 fig1 represents an alternate burner and reactor assembly 700 for the production of the finer or tread grades of carbon black the reactor shell 702 a steel cylinder is lined with refractory 704 a burner assembly 706 consisting of a premix plenum housed in shell 708 with entry pipe 710 closure means 712 and cylindrical flame holder 714 consisting of a series of flat annular shapes as non streamlined bodies 716 spaced apart so.