Which of the following is NOT a heat source for accelerated curing?

Natural sunlight is not typically considered a heat source for accelerated curing, especially in controlled environments where curing processes are carefully managed. Unlike the other options that provide consistent, direct heat for increasing the curing efficiency of materials (such as concrete or adhesives), natural sunlight can be unpredictable. Factors such as weather conditions, time of day, and location can greatly influence the intensity and duration of sunlight exposure, making it less reliable for achieving the desired curing effects.

In contrast, space heaters, live steam, and hot-water or oil steam lines provide controlled and stable sources of heat, which are essential for ensuring uniform curing rates that can significantly enhance the properties of the materials being cured. These sources allow for precise temperature management crucial in industrial and construction applications, where consistent results are paramount.