The Kester 48 solder wire series. 48 Activated Rosin Flux for cored solder wire was developed for lead-free applications to enable soldering of most common metals. Kester 48 has performance characteristics far exceeding standard RA fluxes. Kester 48 builds on the performance of its predecessor Kester 44 with "instant-action" wetting to provide fast and reliable solder joints.