The word saltpetre is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 13
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (39 words)
elaters, leapers, letters, palette, palters, patters, peltate, pelters, petrels, plaster, platers, platter, pleaser, prattle, prelate, presale, pretest, rattles, realest, relapse, relates, repaste, repeals, repeats, replate, repleat, respelt, restate, retapes, retaste, septate, settler, spatter, stapler, starlet, startle, stealer, tapster, trestle