Abstract: When preparing a settable slurry such as a cement slurry for cementing an oil well, cement or other solid powder is mixed with water and a set retarder. In order to be able to run a convenient and rapid check on the concentration of retarder after dilution at the site of use, a tracer material is mixed with the retarder in known amount during manufacture. The tracer is chosen to enable its concentration to be determined analytically after dilution at the site of use, thereby providing a way to determine the concentration of set retarder after such dilution. The tracer may be a redox-active material and its concentration may be determined by voltammetry. Tracer may likewise be mixed with additives other than set retarder.