Hardness of stone surface is affected by?
(A) Impression wax (B) Impression compound (C) Hydrocolloid impression material (D) Rubber based material
(C) Colloidal systems such as agar and alginate retard the setting of gypsum products. If these materials are in contact with CaSO4·½H2O during setting, a soft, easily abraded surface is obtained. Accelerators such as potassium sulfate are added to improve the surface quality of the set CaSO4·2H2O against agar or alginate.
These colloids do not retard the setting by altering the solubility ratio of the hemihydrate and dihydrate forms, but rather by being adsorbed on the hemihydrate and dihydrate nucleation sites, thus interfering in the hydration reaction. The adsorption of these materials on the nucleating sites retards the setting reaction more effectively than adsorption on the calcium sulfate hemihydrate.
Craig’s RESTORATIVE DENTAL MATERIALS E 14th P: 255