Balanced stone is dental stone:
A stone with a setting time established by the addition of proper quantities of both accelerator and retarder is called ‘balanced stone’. 
Typical accelerators are potassium sulfate and potassium sodium tartrate (Rochelle Salts).
Typical retarders are sodium citrate and sodium tetraborate decahydrate (Borax).
Manappalil pg-313
Which undergoes rapid expansion
In which, accelerators or retarders have been added according to need
In which, amount of water of hydration is controlled
In which, the crystals are all of uniform size