The type of fiber and flame-retardant compound plays an important role in the transfer of heat through clothing when exposed to flames or molten metals. The fiber/flame retardant should preferably form a well-developed char on exposure to heat, without softening or melting, to retain low thermal conductivity of the fabric and also prevent adhesion of molten metal onto the fabric, thereby avoiding excess heat transfer through clothing. Wool performed extremely well in practice and in the draft International Organization for Standardization (ISO) tests evaluated, because of its good char-forming property, which is further enhanced by the Zirpro flame-retardant treatments. Fabric construction parameters—weight, thickness, and density—are important for a given fiber type with regard to protection against flames and molten metal. Also, accumulation of moisture in clothing has a significant effect; in multilayer clothing assemblies, the accumulation of moisture in outer fabric layers as a result of exposure to water increases protection against flame by increasing fabric heat capacity, whereas the accumulation of moisture (body perspiration) in the inner layer when wearing clothing containing a vapor barrier significantly decreases protection against flames by increasing the fabric thermal conductivity. High moisture permeability of wool clothing demonstrably reduces wearer heat stress. Interlaboratory evaluations of two draft ISO test methods for molten metal splashes showed superior repeatability and reproducibility of the test method based on a polyvinyl chloride (PVC) skin simulant as an end-point detection system in comparison with a calorimeter heat sensor. The response and sensitivity of calorimeter heat sensors were significantly affected by the calorimeter construction, contamination, and damage during testing and by the location of the heat sensor vis-á-vis the poured metal.