When working to control the setting time of plaster, the old plaster masters used vinegar to delay the setting time and everyday table salt to accelerate the setting of gypsum based cements  including Plaster of Paris. Though table salt does indeed speed the setting time, it will appreciablely reduce the strength of the plaster.

There is a much better way through the magic of Terra Alba (white earth)

Terra Alba is calcium sulfate dehydrate made by finely grinding and air separating a select, high purity, white gypsum containing about 20% water of crystallization.

Terra Alba has a number of food and pharmaceutical uses. In commercial baking, it supplies calcium to enriched flour, cereals, baking powder, yeast, bread conditioners, baking powder, and cake icing. It is also incorporated as a thickening agent in canned vegetables and artificially sweetened jellies and preserves.

When used in brewing, Terra Alba provides a smoother tasting beer while improving shelf life and stability. 

In pharmaceutical applications, it is used as a diluent. In that capacity, it has the additional benefit of being a dietary calcium supplement.

With all these wonderful applications, it has the added benefit of speeding up the setting time of plasters. It doesn’t take very much of it either up to 2% by weight. Above that ratio it will not accelerate setting any quicker. See the graph above.

Terra Alba is premixed in dental plasters at a ratio of only .05-1% to speed the setting of dental plaster.

To use it remember that Terra Alba should be added to the dry plaster powder before you add water, as it in not very effective in water.

 So if you need to accelerate your plaster, a pinch of this magic pixie dust is all you need.