the temperature of the tunnel the unit 18 is carried on a support 11 which is provided with rollers 13 to facilitate separation of the halves of the unit during certain parts of the curing cycle to prevent melting of the coated wire the casing essentially has a central bore 24 when in the closed position and on the surface of this bore there is a coating of a metal oxide such as titanium dioxide to which ferrous oxide has been added to change the hue of the material to substantially black the refractory or ceramic material with this coating functions in accordance with the black body principle and radiates electromagnetic energy at a frequency proportional to the absolute temperature of the body essentially it is important to have a coating material that has good emissivity particularly at elevated temperatures after extensive tests a material was found that is sold under the tradename ilmenite chemically it is essentially fetio 3 and has an emissivity at 1000 u00b0 f 538 u00b0 c of 0 90 that rose to 0 97 at 3000 u00b0 f 1649 u00b0 c zirconium oxide zro 2 colored to a black hue is a further alternate other coatings have been tried for example pyromax which has a similar emissivity and is rated to 1370 u00b0 c but it burns off at that temperature mica fiber has an emissivity of approximately 0 7 up to 1200 u00b0 c only and uncoated ceramic bodies have an emissivity of 0 79 at 540 u00b0 c that drop to 0 38 at 950 u00b0 c thertefore with the proper coating the emission can be maximized until the wire line is running.