Below are 2 pictures of the same set of models; What are the main differences between these 2 sets, from a materials point of view, after you have reviewed the Gypsum Products chapter in Phillips dental Materials Science.
1. Identify the types of gypsum
2. How these products are manufactured
3. What is the W/P ratio for each?
4. How does changing the W/P change how the gypsum reacts in relation to the setting time
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| Trimed set of Final models |
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| Trimed set of Primary models |
1. Identify the types of gypsum
ReplyDeleteBoth have the chemical formula of CaSO4•0.5H2O, yet differ due to different manufacturing processes and the resulting crystalline properties of each process. The yellow stone models are made with Alpha-hemihydrate, while the plaster of Paris is made with Beta-hemihydrate.
2. How these products are manufactured
When gypsum – chemical formula CaSO4•2H2O – is heated to between 110 and 130°C in an open vessel, the result is Beta-hemihydrate, or rather, plaster of Paris.
Yet, when gypsum is heated in an autoclave under steam pressure to a temperature of 130°C, the resulting product would be dental stone, or Alpha-hemihydrate.
3. What is the W/P ratio for each?
Although exact Water/Powder ratios would be dependent on the manufacturer’s specifications, plaster of Paris would usually have a W/P ratio of 0.5 and yellow stone a W/P ratio of 0.3.
4. How does changing the W/P change how the gypsum reacts in relation to the setting time
Increasing the W/P ratio would mean that more water has to be added. This would mean a slower reaction time as well as longer setting time for both the yellow stone and plaster of Paris, because there are fewer nuclei per unit volume.
The opposite happens when less water is added to plaster or stone – a lower W/P ratio results in a faster reaction time (as there are more nuclei per unit volume).
Correction: The yellow stone models are made with Alpha-hemihydrate, while the plaster of Paris [*models are] made with Beta-hemihydrate.
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