Modifying the setting time of gypsum plasters
Plasters used in building construction should set in a short period of time and the setting process should take a low time to complete. In high-quality plasters the setting should not start in less than 8 or more than 25 minutes and it should not take less than 20 minutes or more than an hour for the process to complete.
Since more time is needed for molding plaster in decorative works, we can use additives to slow down or speed up the setting time of the plaster. The type and amount of these additives are as follows:
Salt
If we add 0.5 percent of salt to the plaster, we can reduce its setting time to 5 minutes. If we add 1 to 4 percent of salt to the plaster, we can reduce its setting time to 4 minutes. However, adding more salt causes the plaster to harden slowly, such that, adding 10 percent of salt to the plaster changes its setting time to 12 minutes. Adding more than this amount will slow down the setting time of the plaster.
Serish (eremurus) and serishom (gelatin)
Two other substances we can add to the mortar are serish paste (made from eremurus plant) and serishom paste (made from animal fat and skin). These two additives slow down the drying rate of the mortar and are used when more time is needed for applying the plaster or increasing the viscosity of the additional decorations attached on the plaster surface such as in mirror work.
If we add 1 to 6 percent of serish paste to the mortar, it will start setting in more than 12 minutes. It should be noted that serish is a type of plant-based paste which is widely used in book binding industry. If we add 0.5 percent of serishom paste to the gypsum mortar, it will start setting in 10 minutes to an hour.
Borax
If we add 1 percent of Na2B4O7. H2O to the gypsum mortar, we can increase the setting time by 15 minutes to an hour.
Clay
If we add 10 to 50 percent of clay to the gypsum mortar (clay-gypsum mortar), we can slow down the setting time by 12 minutes.
Alum
Another substance we can add to the plaster is alum which has an opposite effect compared to salt. In other words, we can slow the setting time by adding less (1 percent) alum to the plaster, and speed up the setting time by adding more (5 percent) alum to the plaster. If we add 1 percent of alum to the gypsum mortar, the setting time will increase by 15 minutes, whereas if we add 5 percent of alum to the plaster, the setting time will decrease by 7 minutes.
