A slurry disperses starch in a little cold liquid before it enters a hot sauce. This prevents dry powder from forming lumps on contact. Cornflour, called cornstarch in the US, is a purified starch, not whole maize flour.
Step by step
Check the starch and amount
Use the thickener specified in the recipe. Cornflour, potato starch and other starches behave differently, so do not assume the same quantity and boiling time work for all of them.
Mix with cold liquid until smooth
Put the measured starch in a small bowl and stir in the recipe’s cold water or other cold liquid. Break up dry pockets until it is a smooth, pourable mixture. Do not stir dry starch directly into boiling sauce.
Stir again and add gradually
Starch settles quickly, so remix just before using. Stir the hot sauce as you pour in a little slurry. Allow it to disperse before deciding whether to add the rest.
Heat enough to develop the thickness
Follow the recipe or packet’s heating instructions. A cornflour sauce generally needs to reach a simmer to thicken properly. Check the texture before adding more, and avoid prolonged hard boiling once the intended result is reached.
Lumps, settling or a jelly-like sauce
Whisk small lumps promptly; strain if needed. If the sauce becomes too thick, stir in a compatible liquid a little at a time. A sauce that has not yet heated through may look thin temporarily, so adding repeated spoonfuls of slurry too soon can make it gummy.
