A method for retarding plaster setting and plaster preparations, relies upon a composition including a. tartaric acid in an amount of at least 0.01 to 1.0 weight % (based on plaster amount); b. sodium trimetaphosphate in an amount of 0.05 to 0.5 weight %, (plaster amount basis); and optionally c. sodium polyphosphate with an average chain length of at least 4 to 50 phosphate units.