Sentence:  Polyphosphates gave renewed life to soap products at a <WSD>time</WSD> when surfactants were a threat though expensive , and these same polyphosphates spelled the decline of soap usage when the synergism between polyphosphates and synthetic detergent actives was recognized and exploited . 
Meaning of the word marked with <WSD> tags:  an indefinite period (usually marked by specific attributes or activities)