Lecture 7

Question 1

Describe the manufacture of gypsum crystals

Answer 1

Model plaster is produced by heating the gypsum (dihydrate) in an open kettle at 115oC yielding a porous hemihydrate powder of irregular crystals.
Exactly 115; not 114 and not 116; NOT a range
Regular-strength (?) dental stone is produced by heating the gypsum under pressure in the presence of steam at 125oC yielding larger, denser, and more regular crystals.
High-strength dental stone is produced by boiling the gypsum in a 30% solution of calcium chloride yielding the least porous, most regular, and largest crystals of all hemihydrates

Question 2

What are the effects of excess water?

Answer 2

Effects of Excessive water:
prolongs the setting time
reduces the strength
reduces the setting expansion of set gypsum materials

Question 3

Theoretically, 19 ml of water is needed to be combined with 100 gm of any hemihydrate to convert it to dihydrate. Mass needs more water to manipulate.
Do not want runny gypsum;
Amount of excess water needed to produce a mass that can be manipulated depends on porosity.
High-strength dental stone requires the least excess water to manipulate. Dental stone requires more water, and plaster requires the most water.

Answer 3

Ye

Question 4

Describe how temp and humidity affects gymspum setting

Answer 4

Setting reaction of gypsum is affected as follows:
Setting time decreases with temperature increase until 37.5oC
Above 37.5oC, setting time will increase because the dihydrate becomes more soluble in water
kinetics
Gypsum is hygroscopic (takes water from the environment/air), thus it is preferred to seal gypsum materials to avoid changes in the rate of setting reaction
May already have water contaminating itcan’t correct for