a) The water content of the hydro colloid impressions inhibits the setting of the gypsum at the surface.
b) A known gypsum retarder, such as borax, is used as a filler in agar impression material, but it can cause the surfaces of gypsum casts prepared from an agar impression to be too soft for use as dies.
c) The gelation process of alginate impression produces not only insoluble calcium alginate but also sodium sulfate.
d) Sodium sulfate is a gypsum accelerator at low concentration, but it becomes a gypsum retarder at higher concentration.
e) The amount of sodium salt used in the alginate impression material produces enough sodium sulfate to retard the setting of gypsum poured against the alginate.
f) These deficiencies can be overcome in two ways:
(1) by immersing the impression in a solution containing a gypsum accelerator, such as 2% potassium sulfate solution, prior to pouring the impression with the gypsum-forming product; or
(2) by using products that incorporate a gypsum hardener or accelerator m the material.
g) The sulfate in the formulation for agar and the potassium titanium fluoride in alginate satisfy the second condition.