The specimens and slug plate were surrounded by a 63.5 mm (total thickness), 25.4 mm wide guard of the fumed-silica insulation board and mounted between two Inconel frame plates (see Figure 3). The entire assembly was held together by a set of eight retaining screws (two on each of the four edges of the plates). The assembled sandwich was either suspended from two wires or simply placed on the bottom center of the box furnace and the thermocouples were mounted; three Type N thermocouples were placed at the various depths in the center slug plate and one each was placed on the two exterior surfaces of the specimens being evaluated, between the specimens and the retaining Inconel plates. In addition to these thermocouples, the temperatures of the zone box and the furnace were also monitored and recorded. In most tests, the furnace temperature was programmed to follow a curve similar to the standard ASTM E119 temperature-time curve1, but with a less rapid initial temperature rise, due to limitations on the heating rate of the furnace. The actual temperature-time curve employed will be shown in the results to follow. The tests were generally aborted by turning off the furnace power when the central steel slug reached an average temperature of 550 ºC. However, the thermocouples still monitored temperatures during cooling. Thus, while the heating portion of the tests generally occurred over a period of 1 h to 2 h, the cooling portion could take as long as 24 h to 48 h, when the furnace was not opened to accelerate the cooling. Additional tests were conducted at slower heating rates to determine the sensitivity of the computed thermal conductivities to this parameter. Typically, the furnace temperature was linearly raised to 600 ºC during the course of either 4 h or 16 h and then held there until the center slug plate also attained nearly this temperature, at which point the furnace was turned off and a cooling curve monitored.