Retardation of Gypsum Plaster by Calcium Tartrate

¹ Commonwealth Bureau of Dental Standards, Melbourne, Australia

From a consideration of the change from normal of the thermal behavior, in particular the deviation from linearity of log T for the extremely retarded mixtures, and also the microscopic examination of the crystals, it was obvious that the retardation was due to a modification of the crystal habit of the dehydrate. Calcium citrate is known to be an effective retarder, but the relatively much lower solubility of calcium tartrate precludes its use in the form of an aqueous solution. However, calcium tartrate dissolves in dilute alkalis, with possibly the formation of the complex ion [Ca(C₄H₄O₆)₂]⁼. Such solutions are effective retarders. The retarding effect produced by Rochelle salt in alkaline solution was probably due to the formation and subsequent solution in the alkali of calcium tartrate, while the same effect was produced when a tartrate-modified material was mixed with dilute alkali.